The Potential for Biological Control of Eurasian Watermilfoil ( Myriophyllum spicatuin) :
Results of the Research Programs Conducted in 1992.
Year 3
Interim Progress Report
April 1. 1993
US EPA REGION I LIBRARY
JFK FEDERAL BLDG
BOSTON, MA 02203-2211
by
Robert P. Creed Jr., Sallie P. Sheldon and L. M. O’Bryan
Department of Biology
Middlebury College
Middlebury, Vermont 05753 USA
Prepared for
Region I
U.S. Environmental Protection Agency
Boston, Massachusetts
Warren Howard
Project Officer
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The Potential for Biological Control of Eurasian Watermilfoil ( Myriophvllum spicaturn) :
Results of the Research Programs Conducted in 1992.
Year 3
Interim Progress Report
April I, 1993
by
Robert P. Creed Jr., Sallie P. Sheldon and L. M. O’Bryan
Department of Biology
Middlebury College
Middlebury, Vermont 05753 USA
Prepared for
Region I
U.S. Environmental Protection Agency
Boston. Massachusetts
Wau ren Howard
Project Officer
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i’able of Contents
Introduction 4
Research at Brown ington Pond 6
luitroduction 6
Study Site 6
Materials and Methods 7
Survcys 7
Experiments 13
Results 21
Surveys 21
Experiments 28
Discussion 31
Research at Lake Bonioseen and Middlebury 37
Research at Lake Bomoseen 37
Introduction 37
Study Sites 37
Materials arid Methods 38
Results 40
Discussion 43
Research at Mmddlebury 46
Culture and Life History of E. lecontei 46
Materials and Methods 46
ResuIt and Discussion 48
The effect of E. Iecontei adults on native plants 53
Introduction 53
Materials and Methods 53
Results 55
Discussion 59
Weevil Introductions 62
Noiton Brook Pond 62
Site Description 62
Materials and Methods 62
Results and Discussion 63
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Van V leck’s Pond .66
Site Description 66
Materials and Methods 66
Results 67
Discussion 68
Betourney’s Pond 70
Site Description 70
Materials and Methods 70
Results 71
Discussion 71
Communication of Research Results 72
Summaiy Discussion 74
Literature Cited 77
Tables and Figures 79
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INTR( )I)UCTI( )N
Eurasian watermilfoil ( Mvriophyllum spicaturn L.) was accidentally introduced into
North America sometime between the late l U0’s and the 1940’s (Bayley et al. 1968,
Reed 1977. Aiken et al. 1979, Couch and Nelson 1986). Since its introduction it has
spread over much of North America (Aiken et al. 1979, Couch and Nelson I 986, Nichols
and Shaw 1986. Painter and McCabe 1988). It was first reported in Vermont in 1962 in
Lake Champlain (Holly Crosson, Vermont Agency of Natural Resources (VtANR), pers.
comm.). A number of methods. many of them quite costly (Anonymous 1990), have
been employed to contiol watermilfoil in Vermont and elsewhere, including use of
drawdowns, herbicides, bottom barriers, and mechanical harvesting. In general, while
these control methods may result in short-term reductions in watermilfoil abundance
(Bayley et at. 1968, Nichols and Cottarn 1972, Aiken et al. 1979) they do not appear to
have pioven satisfactory for long-term control of thus introduced, aquatic weed (Bayley
et at. 1968, Spencer and Lekic 1974, Aiken et at. 1979).
Rcceiitly. attention has focused on the potential for biological control of
Mvriophvllum spicatum Aquatic herbivores such as the caterpillar Acentria nivea
( =Acentropus nivcus)(Lepidoptera ; Pyralidue) and the weevil Euhrychiopsis Iecontei
(Coleoptera; Curculionidue), have been found associated with declining populations of
wateiniilfoil in noutheastern North Ameu-ica (Painter and McCabe 1988, Sheldon and
Creed. pcrs. obs.), including Brownington Pond, Vermont. However, the exact role
played by these herbivores in bringing about these declines remains undetermined.
We are currently evaluating the potential for insect heibivores to act us biological
contiol ugeilts for Euiasian wateimilfoil. There arc six main obJcLtives to this i-esearch:
I) Determine the probable cause(s) of the Eurasian watermilfoil decline in
Brownington Pond (see Figures 1-3. Creed and Sheldon 1991a).
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2) Examine thc grazing/boring effects of all major herbivores omi Eurasian watermilfoil
and native aquatic plain species.
3) Determine the feasibility of herbivore introductions into other inilfoil-infested lakes
in Vermont.
4) Detem mine if Lake Bomoseen is a suitable site for herbivore introductions/collect
pie-introduction baseline data.
5) If deterii’iincd to be feasible and appropriate based on previous research (a high-
likelihood of success and relatively free from causing negative imiip icts 10 non-target
species). use herbivorous insects to control Eurasian watermilfoil in Lake Bomoseen.
6) Develop a public education program to keep Vermont’s citizens abreast of the
results of the research.
The research described in this document is from the 1992 field season. This is the
thud progress report from this five year study.
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RESEARCH AT BROWNIN( ;‘l’( )N POND
Introduction
One of the few declines in a watermilfoil population in North America occurred at
Browniiigton Poiicl in noitheastern Vermont. While the cause of this decline has yet to
be determined, initial samples of the watermilfoil found three insect herbivores
associated with this pest macrophyte. The goal of the Brownington Pond research is to
determine the cause of the waterinilfoil decline and ascertain the role the insect
heihivores may have played in the decline. In 1990, we initiated mesearch at
Bro nington Pond. We monitored the abundance of watermilfoil and its associated
iiivertebrates. We also conducted field and laboratory experiments. In 1991, we
continued to monitor the atcrmilfoil population and the abundances of the associated
herbivores. We also conducted additional experiments which evaluated the effects of
various herbivores on watermilfoil in both lab and field settings. In 1992. we continued
to monitor the watermilfoil and herbivore populations in the poiid and conducted
additional lab iiid pond experiments.
Study Site
Brownington Pond is a small, mesotropliic lake in northeastern Vermont (Bro nington
and Dci by Townships. 44°53’N, 72°09 ’W). Total surface area of the pond is 64
hectares, maximum depth is 10.7 m with an average depth of 5.5 m (Figure I). There are
two inlets, one oii the north shore and one on the east side, and a single outlet. Day
Brook. Lcss than one quarter of the thoreliuie has been developed with summer camps.
most of which arc located along the northeastern shore. There is a public boating access
on the ‘cst side of the pond.
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Malerials and Methods
Surveys
Pond Survey
Since the first summer of this pi-oject we have been qualitatively mapping the positions
of any watermilfoil beds in Browningion Pond (Creed and Sheldon 1991u&b, 1992).
The information for these maps has been gathered by snorkeling and boat surveys (Creed
and Sheldon 199 la&b). We surveyed the pond in a similar fashion in the summcr of
1992.
Water Tcinpcratiiie
Tv o stations, located in the South and West watermilfoil beds, were established in the
pond at which weekly measurements of temperature were made. Temperature was read
from pairs of maximum/minimum thermometers suspended from buoys. One
thcrmometcr was 0.5 m below the surface and the second was 0.5 iii above the bottom.
Thermometers were reset after each weekly reading.
Watem Chemistry
Two surveys of nutrients (nitrate, nitrite and orthophosphate) in the water column
were made on 3(1 June and 27 August. Samples were collected from the east side of the
pond (an aica wlieie Potamogeton amplifolius and Heteranthera dubia are Lhe common
macrophytes) amid from inside the two wateimilfoil beds (in the case of the South bed, in
the area where the bed uscd to be). Instead of sampling a fixed point, three or more
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locations were chosen to sample a broader array of potential microhabitats within a site.
Water samples were collected using a Keminerer sampler. Pairs of samples, one shallow
and one deep were taken at each point. Five pairs of samples were taken at each site in
June: thrcc pairs were taken at each site in August. Upon finishing a collection, samples
were placed on ice and transported to the lab of the Vermont Department of
Environmental Conservation for analysis.
Sediment Chemistry
Sediment samples were taken in Brownington Pond on II August 1992. Samples
were taken in: I) the West Bed. 2) a watermilfoil-free aiea adjacent to the West Bed
(West Shallow), 3) in the South Bed, 4) in a watermilfoil-free area adjacent to the South
Bed (South Shallow) and 5) on the east side of the pond in an area dominated by .
dubia and . amplifolius . Pond sediment was collected by a SCUBA diver. A 3.8 I
sealable plastic bag was filled with sediment below the water-sediment interface. The
bag was sealed and then returned to the surface. Atl samples were kept cool and sent to
the Ai my Corp of Engineers Waterways Experiment Station (Vicksburg, Mississippi) for
analysis. Samples were sent to Mississippi within 4 hrs of collection.
Plant Transects
In 1990 watermulfoil appeared to be restricted to water between 2.0 - 3.5 rn deep
(Creed and Sheldon 1991 a&b). To see if this distribution pattern persisted in 1991 we
established thiee permanent transects thiough both of the main beds. An attempt was
made to ‘ pacc the transects across the beth. Along each transect, locations were selected
at hall meter depth inter uls ranging from 0.5 m - 3.5 in deep, for a total of twenty one
sample points for each bed. At each sample point two PVC pipe rs were pushed into the
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sediment at right angles to one another to form a cross. The four ends of the T’s were
numbered fiom one to four.
The permanent transects established in 1991 were sampled again on three dates during
the 1992 giot ing season. To ensure that the areas sampled in 1992 were not affected by
the 1991 sampling, each transect was shifted 4.5 in: the direction that the transect was
‘ hiftecl was randomly determined. Samples were collected on three pairs of dates in
1992 (West Bed samples taken on the first date of each pair): 10 & II June, 8 & 9 July
and 12 & 13 August. For each point to be sampled, one of the four numbers from the T’s
was selected at random from the remaining possible numbers prior to sampling. The
saiiiples were collected by SCUBA divers. The divers inserted a 2 in long piece of PVC
pipe into the appropi mate numbered opening (sampling a quadrat 2 iii from the Ts
minimized the disturbance of the area to be sampled by the diver when reading the
numbeis on the PVC T’s). A 0.25 x 0.25 m quadrat was then placed on the bottom at the
end of the pipe and the sample taken. All above sediment plant biomass was clipped and
placed into a numbeied, plastic bag. Upon returning to the lab, plants from each sample
weic soited to species and diied in a drying oven at 80° C. Plants were weighed after
drying to a constant weight. For clarity of data presentation, di y weights for native
species were lumped together in the category “Other.”
Pem mailent Grids
In addition to delem mining the location of wutermilfoil beds in the littoral zone, we
initialed a program to record finer scale expansions and contractions of M. spicatum beds
using permanent grids. Four grids were established in the pond in 1990, two in each bed.
The grids covem- all area of 8 x 6 in with buoys placed every 2 m in a 4 x 5 array. Percent
covei- of watermilfoil was determined by a diver using a 0.5 x 0.5 ni quadrat subdivided
into 25 subunits. Placement of the quadrat across the bed was determined using a
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transect “line” made of PVC pipe with openings placed every 0.5 rn into which the
tiuadiat was inserted. Percent cover was evaluated along four transects for each grid.
The position of the transects corresponds to the four lines of buoys that run along the
longer dimension of each grid, i.e. A - E. The number of quadrat subunits lying over
watermilfoil plants was then recorded. This technique generates percent cover values
ranging from 0-100%. For clarity of data preseiltation, we grouped the percent cover
valucs into five categories - I) 0% 2)1-25%, 3) 25-50%, 4) 50-75% and 5) greater than
7 5 k (note: in I 99(1 and 1991, we grouped the percent cover values into four categories -
I) less than 25%. 2) 25-50%. 3) 50-75% and 4) greater than 75%). The grids set out in
199(1 wcrc placed oui the ends and ncarshoie edges of the beds as watermilfoil will be
more likely to spread laterally and into shallow water. The grids did not extend into deep
water as wateriiiilfoil abundance is probably limited on the deep edge of beds by light
availability. The grids were swum on 15 June, 13 July and 24 August dui ing the 1992
gro ing season.
In 1991, new giid was established in Lake Memphiemagog (Newport, Vt) in a bed of
ateimilfoil Just urnith of the Whipple Bay boat access. Lake Memphremagog is
approximately 3 miles northeast of Brownington Pond and is in the same watershed. We
qualitatively sampled this grid t ice in 1992.
Invem tebi ate Samples
Super Samples and Minisainpies. To describe the watermilfoil invei-tebrate
assemblage quantitative samples of watermilfoil and the associated invertebrates were
taken in the South and \Vest Beds. In aclclitioti, saiiiples of two abundant native
iiiaci ophytes ( Potamogetoui ampi ifol ius and Hctci anthera dubia ) were taken to compare
their invertebrate assemblages with those of wateimilfoil. Samples were collected using
two sizes of the Mobile Invertebrate Sampler (MIS) developed by Smith and Sheldon
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(unpublished inanuscripO. The larger sampler (the Super Sampler), used for both
watcrmilfoil and the two native macrophytcs, samples an area of 0. 1% rn 2 : the smaller
version (the Minisampler) was designed for sampling a single stern of watermilfoil. Both
samplers were employed by a SCUBA diver. An area or a plant to be sampled was
chosen IlaI)hazardly. The sampler tube was then slid over the plant(s) as the diver
descended. Plants weie cut near the sediment surface, the opening of the sampler was
then covered with a 50() urn mesh sieve and then the sample was returned to the sw face.
All samples were placed in scalable, plastic bags. Super samples were preserved in 70c/
ETOH: minisamples were picked soon after sampling while the animals were still alive.
Invertebrates were identilied to the lowest taxonomic level. Dry weights were recorded
for the pldnis after the invertebi-ates were removed. Super samples were taken on 8 June.
on 29 June, on 20 July and on 10 August. Mini-samples were taken weekly from 9 June
to 25 August for a total of 12 sample dates. In 1992, due to there beiiig many small
plants, we sampled long (>50 cm. n=3) and short (n=3) plants each date with the
in in isamp I cr.
Stern ‘I’rauisecls. In 199() we discovered that weevils lay their eggs on the apical
meristems of watci milfoil and that the early instar larvae burrow into the mci istem upon
hatching (Creed and Sheldon 199 Ia). We initiated “meristern transects’ across both
watermilfoil beds in 1990 to determine the density of eggs and early instar larvae in the
beds. In 1991 we continued taking these stern transects but we sampled larger pieces of
stem (approximatley 50 cm long) in order to collect late instar weevil larvae and pupae.
In 1992, 16 stems (on avet age), stems with intact apical meristems and stems without
apicdl meristems. were collected per transect. While it is possible to find all life stages
on both stein types (especially as weevils also lay their eggs on lateral ineristems), we
believed tlldt stei1 s with intdct mcristeins had a greater probability of containing eggs and
first instar lat-vac. We believed that stems without apical nieristcms were more likely to
contain late inslur larvae and pupae. These two stem types weie collected in pairs
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haphazardly by snorkelers swimming across the bed. Three such transects were sampled
for each bed on each sample date. Samples were collected weekly for a total of 12
sample dates plus one collection in September. Stems were examined under dissecting
micioscopes and all lifestuges of weevils were recorded for each stem.
Fish Samples
Only five species of fish have been collected from Brownington Pond (Unpubl. State
Fisheries Survey l9 0). These include yellow perch ( Perca flavescens) , smallmouth bass
( Microptetus dolotmeui) , chain pickerel ( Esox niger) , white sucker ( Catostomus
commcrsonii ) and brown bullhead ( Ictalurus nebulosus) . The state survey data indicated
that the yellow peich is by far the most abundant species numeiicully in this pond.
Because of the abundance of yellow perch and the fact that it is the species most likely to
consume macrophyte associated invertebrates, our fish survey focuses on this species.
Gill nets for large perch (>150 mm) were deployed in Brownington Pond on three
datcs in 1992: 25 June, 2 July and 10 July. These samples wete taken concurrently with
the fish exclusion experiment (see below) which was located in an area with scattered
clumps of watermilfoil 20 m west of the area which had previously supported the south
wutetinilfoil bed. A single net was deployed, perpendicular to shore, approximately 15
iii to the west of the western most row of cages in the fish exclusion experiment for all
surveys. A net with a 6.4 cm (2.5”) stretch mesh was used. The net was deployed for
approxuuiiately one hour at dawn on all three (lutes. Captured fish were measured (total
length) and weighed. The stomachs were then removed and pieserved in 70% ETOH.
Stomach contents were examined under a dissecting microscope and identified to the
lowest possible taxononluc level.
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I xperiinents
The Effect of Acentria and Euhrvchiopsis larvae on Watermilfoil Growth
Thus experiment was designed to determine the combined effect of these two
herbivorous insect larvae on watermilfoil. Several small waterinilfoil plants were
collected horn Brown ington Pond. Plants were first checked for herbivore damage.
Damaged plants (e.g.. with missing meristems, meristem dunidge, or significant stem
damage) were iejected. We selected twenty four of the intact plants which were the most
similar in size. All obvious invertebrates and weevil eggs were removed from these
plants. These twenty four plants were then weighed (blotted wet weight). We tied a
marker auouncl the stem at the base of the plant and the length of the stem from the
markei to the tip of the apical meristem was determined. We also counted the number of
whorls on each stem above the marker. The initial lengths of the watermilfoil plants
ranged fioin 206-23() mm; initial weights ranged from O.23-O. 8 g. Much of the
variation in weight was attributable to differences in root biomass and not above ground
bioniass (Ciced, peis. obs.)
After processing. each watermilfoil plant was planted in a numbered chamber. The
chambers consisted of clear plastic tubes (42 mm inside diameter) set in a PVC pipe
base. We first placed aquarium gravel in the bases to weight them down. We then filled
thc remainder of each base with strained pond sediments taken from one of the
vaterinilfoil beds in Brownmngton Pond. A tight-fitting cap covered with 500 micron,
Nitex mesh was themi plaLeci on the top of the tube. These ure the same type of chambers
desimihed in Creed and Sheldon (1991a&b). Plants were planted in the sediment up to
the tag on the stem. The chamiibers were then placed in a large wading pool set out of
doors in an unshaded area. The chambers were aerated with a slow trickle of air bubbles
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to plevent stagnation. Plants were allowed to acclimate to the chambers for one d y
bcfoie the Acentria and Euhrvthiopsis larvae were added.
The experimental design was a randomized complete block design with four
treatilielus per row and six replicates per treatment. The plants were randomly assigned
to rows in the wading pool. The determination of treatment within rows was also
detcriiiined using a random number table. The treatments were as follows: control (no
larvae), weevil (I Euhryehiopsis la va per tube). Acentria (I Acentria larva per tube) and
the combination tieatmcin(1 lai-va of each species in a Lube). Larvue for both species
were collected from the west bed and were paired by size for each row. Water
temperature in the pool was monitored using a max/mm thermometer during the
expei imelit. Water temperatures ranged fiom 12. -26. 10 C during the experiment (mean
minimum temperature was 16.60 C; mean maximum temperature was 22.70 C).
The experiment lasted for 12 days. Plains and larvae were then removed from each
chamber. AfLer removing the larvae, the watermilfoil plants were measured (length from
tag to tip of rooted stem) and weighed (blotted wet weight). Any plant material not
attached to the rooted stem was not included in the final plant weight. We also counted
the number of whorls of leaves remaining on each stem. Treatment effects were
compamed using an ANOVA with planned, orthogonal contrasts (Sokal and Rolhf 19 1).
The recovered laR’ae were preserved in 70% ETOH.
The Effect of Larval Weevil Damage on Stem Fragment Viability
Weevil hcrbivory, particularly larval burrowing, weakens the waternulfoil stem. This
can result in stem fragmentation. The production of fragments by other watermilfoil
contiol methods has beeii a concern as fragmentation can promote the spread of
wutcimilfoil. The following experiments were designed to determine if the viability of
stem fiagmcnts damaged by weevils was reduced compared to undamaged fragments.
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Undamaged fragments are similar to those produced by mcchanical control methods
(e.g.. mechanical harvcsting).
Experiment I. In this experiment, damaged and undamaged stem fragments were
collected from the pond on 15 July 1992. Undamaged fragments were checked for
weevil eggs: eggs were removed if discovered. Weevil larvae and all other invertebrates
were removed as well. The fragmetits were then cut to a standardized length of four cm.
The amount of larval weevil burrowing was not standardized for the damaged fragments
but all fra iiients displayed some degree of larval damage. The two types of stem
fragments were then assigned to four groups each containing five fragments. The
fragments ere then planted in eight 3 I aquaria, four aquaria per treatment. Each
1 f’ ‘ —
aquarium contained nell water and straiiicd pond sediment taken from the west
•
watermilfoil bed. The assignment of stems to aquaria and aquaria to treatments was r
randomized. All aquaria were covered with a tight-fitting translucent lid to prevent
herbivore colonization. The lids contained a panel of 501) micron mesh to allow for air
exchange and also aid in temperature regulation of the water. All aquaria were aerated.
Tcmperaturcs were recorded in four of the aquaria twice a day (9:00 am and 6:00 pm).
Tcmpci aturcs during the expei-mnient ranged from 140 to 300 C in the control treatment
(mean ( I S.E.) morning temperature 17.90 (±0.32), mean evening temperature 24.20
(± 0.51)) and from 130 to 300 in the damaged stein treatment (mean morning
tcnmpeialure I 7 50 a 0.33). mean evening temperature 24.30 a 0.47)).
The expci iment was terminated 2 clays later (12 August). The stems were gently
removed from the sediment. Herbivore damage was seen on stems in three of the four
control aquaria. These damaged control stems were removed from the analysis; thus n=5
for one control aquarium, n=4 for two of the control aquaria and n=2 for the remaining
aquarium. The pei enlage of stems with roots was determined for each aquarium. Then
the roots were removed, blotted dry and weighed. The production of stem tissue was
also detemmnuned. As stem tissue could be produced either by elongation of the original
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stern or by the production of lateral sterns fiiial length of both original and lateral sterns
was determined. Treatment effects were examined using an ANOVA. Due to the
varying number of stems in the controls the ANOVA was performed on the means for
each variable from each replicate. Root weight data, and original and lateral stem length
data ere log iransfot mcd due to substantial differences between the two treatments for
these variables.
Expciiment 2. In the pi-evious experiment, the growth of stems was evaluated in small
aquaria containing clear water. These stems were subjected to light conditions typical of
vei-y shallow water. Watermilfoil is most abundant in water 2.0-3.0 rn deep in
Brow niiigton Pond and many stenl fragments may settle in water with reduced light
iuitensities This is particularly true of weevil-damaged fragments which have reduced
buoyancy (Creed et al. 1992) and probably settle close to the source plants. To see if
iedueed light intensities had an impact on growth of stem fragments we coiiducted a
second experiment where light intensity was manipulated. Using a portable light meter
(Lutron LX— 101 Lux meter) we had determined that the light intensity at 2.0 In (at noon
on an overcast day) as approximately half that at the surface. To simulate these light
levels we made shrouds of window screen for half of the aquaria that reduced the
incoming light by half.
The collection and processing of stein fragmeiits and the set up of aquaria in this
expem iment was similar to the first experiment. Theie were four treatments:
I )uiidamagecl stems (control), normal light; 2) undamaged stems (control), shaded: 3)
wecvil-daniagecl stems, normal light; and 4) weevil-damaged stems, shaded. Each
treatment had ihice replicates. There weie five stem fragments per aquarium.
lempematures weie recorded using max/mill thermometers suspended in four of the
aqLRiiia (two shaded and two unshdded) which were i-cad once a week. Temperatures
during the experiment ranged from 10.30 to 33.1° C in the unshaded treatment; mean a
I S.E.) max temperature 29.50 a 1.20), mean mm temperature L3.0 0 a 1.50).
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Temperatures ranged from 11.10 t 31.1CC in the shaded treatment; mean (± I S.E.)
max temperature 27.50 a 1.30), mean mm temperature I 3.5° a I .70).
The experiment was started on 19 August and terminated on 17 September. Stern
fragments were processed in the same fashion as in the previous experiment. One control
stern in a shaded aquarium was damaged so n=4 for that replicate: otherwise n=5 for all
other replicates. The data were analyzed using an ANOVA with orthogonal contrasts.
The first contrast compared the control stems with the damaged stems. The second two
contrasts compared the effect of shading on the control sterns and the damaged stems.
The A NOVA was performed on the means for each variable from each replicate. Root
weights were log transformed.
Fish Exclusion Experiment
Predation by insectivorous fishes may influence either the establishment of a
wutermilfoil herbivore population in a lake or their distribution and abundance in the
/
system. The effect of fish may be through direct predation. Alternatively, fish may
indiicctly influence the distribution and abundance of herbivores through their influence
on herbivore predators and/or competitors.
In a previous fish exclusion experiment conducted in the summer of 1991. we found
no effect of fish on the abundance of E. Iecontei or A. nivea . The treatments for this
expei mment ere in place for about two months (mid-June to mid-August). At the time
the experiment was sampled, many yellow perch were feeding primarily on open water
Cladocera and to a lesser extent on littoral tiiveitebrates. Since yellow perch feed
primlial ily on Imitomal invcrtebiatcs in the early part of the summer we believed that a
strong effect of fish on watciii ilfoil herbivores might be observed at this time. Thus we
repeated the fish exclusion experiment, changing only the duration of the study.
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The experimental design for this version of the experiment was the same as the
previous study and included three treatments; a complete exclusion cage, a cage control
and an uncagcd control. Complete cages and cage controls were constructed by making
cylinders of 0.6 cm mesh that were open on one end. Cylinder ends were held open by
wire i ings. Four cork floats were attached to the top of each cage to suspend them in the
ater. Cage controls differed from cages only in that large slits were cut in the sides of
the mesh cylindci to permit access to fish. Open controls were simply areas of the
waterinilloil bed demaicated by a single buoy. Placement of cages and cage controls
invol ccl sliding the cylinder over the watermilfoil. Cages were held in place by both
pinning the lower ring into the sediment and placing bricks on top of the ring (four pins
and bricks per cage). The position of treatments within a row was randomized.
Six rows containing each of the three treatments were set out on 24 June 1992 in an
area with scattered clumps of watermilfoil 20 iii west of the area which had previously
supported the south waterinilfoil bed. The cages were sampled on 6 July. All three
treatments were sampled using the large MIS sampler. For cages and cage controls this
entailed skiii divci.s removing the top of the cage. Immediately upon iemoval of the cage
top a SCUBA diver descended to the bottom pulling the MIS sampler thiough the cage.
Upon i-caching the bottom all plants were clipped at the sediment surface, the sieve was
placed over the bottom of the sampler and the sampler was returned to the surface.
Samples were removed from the sampler and placed into labeled, sealable plastic bags.
Samples were placed in a sieve stack (3 sieves with 8 mm, 2 mm and 0.425 mm openings
for the top, middle and bottom sieves, respectively) and spiayed with a Jet of water to
separate the invertebrates from the Iargci plant pieces. Each sample fraction was
preserved in 7 0’/c ETOH. Invertebmates were separated from macrophytes in the
laboi-atom y and identified to the lowest feasible taxonomic level. The macrophytes were
then dried and weighed. Invertebrate abundance was standardized by watermilfoil
biomass for statistical analysis. The data were analyzed using an ANOVA with planned,
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oithogonul contrasts which compared 1) the full cage to the cage control and no cage to
deteimine a fish effect and 2) the cage control and no cage treatments to determine if
there wa a cage effect.
Hci bivore Enclosure Experiment
The enclosures used in this experiment were three meter tall plexiglass tubes (20 cm
O.D.) which were composed of two parts. The bottom section (I m tall) was driven into
the seclimeiu. The upper portion of the chamber (2 rn tall) was then bolted to the bottom
section. Along the sides of the upper poi-tion were four pairs of ports covered with 202
urn Nitex mesh which allowed for water exchange between the enclosures and the water
column. A lid also cove, ed with 2()2 urn Nitex mesh was bolted on the top of each tube.
There was a centimeter scale on the outside of the upper portion of each tube.
The bottom sectionS of ten enclosures were placed in the pond on the nearshore side
of the South Bed by a SCUBA diver on 17 June 1992. Due to the depth of the water, the
lops of the enclo urc bases were not flush with the sediment surface. Extra sediment
which was free of other pldnts was added to each base. The sediment c’ime from the
middle of the South Bed. We collected a number of small (approximately 40 cm long
shoots) watermilfoil plants fi-orn the West Bed on 17 June. The plants were cleaned of
obvious inacroinvertebrates and any weevil eggs. The plants were then sorted into 13
groups of six and weighed (blotted wet weight) in order to standardize initial biomass.
Ten of the groups of plants were iandomly assigned to the enclosures; the remaining 3
groups were di ied at 00 C for an initial estimate of dry weight. Six plants per enclosure
is equivalent to I I plants/ni 2 . This value is well within the range of densities
detetinined by ui-veys of watei-milfoil in the two beds during 1990 (Creed and Sheldon
199 Ia). The initial mean wet weight a I S.E.) of plants placed in the tubes was 5.61 ±
0.16g.
19
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On l June the plants were planted in the tube bottoms by a SCUBA diver. Plants
were gently pushed down into the sediments until the roots were buried. The upper
portion of the tube was then bolted to the bottom. The lids were then bolted onto each
cn losurc top. Four days (22 Julie) after the piants had been placed into the enclosures
the maximum height ot each plant in each tube was recorded by a SCUBA diver. The
height of the plants was measured weekly until the end of the experiment.
The original plan had been to allow the aiermilfoil plants inside the enclosures to
grow for three weeks before adding the adult weevils. However, duiing the first three
weeks of the experiment larval weevil damage was observed on a single steni in four of
the enclosures. These four enclosures were designated as the weevil treatment. As the
plants had been randomly assigned to tubes we assumed that the distribution of this
treatment across cnclosures was also random. On 9 July we added four adult weevils (2
males and 2 females) to these foui enclosures. Another three enclosures had beeii
contaminated by single Acentria larvae (Lepidoptera, Pyralidae) so we added an
additional, / \centria control treatment. These larvae appear to have entered the
enclosures after the watermilfoil had been planted. We assumed that contamination of
the ihicc enclosures by Acentria were also random events. The remaining three
enclosuies were considered uncontaminated controls. At the time the adult weevils were
added, the larval weevils and Acentria had not had a significant effect on mean plant
height in these enclosures when they were compared with the uncontaminated controls.
During the experiment the enclosures weie periodically cleaned of exteinal periphyton.
The enclosurcs were sampled on 2() August. First, ilie upper portion of the enclosure
was icino cd from the base. The plants ere then clipped at sediment level. The shoots
either floated or erc gently pushed into the upper portion of the enclosure which was
then scaled with a screen-covered bottom. The upper portion of the enclosure was then
returned to a boat. The tube was lifted out of the water and all of the plant material was
collected on the bottom screen. The plants were removed from the tubes and placed iii
20
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sealable plastic bugs. The roots were gently reiiioved from the sediments, gently shaken
to clean off ally adhering sediment and then bagged. In the laboratory, shoots were
sepatuted into the six original stems (i.e.. the plant tissue produced prior to the adult
weevil introduction) and the newer lateral stems. Roots were cleaned of any organic
debris. Shoots and ioots were dried to a constant weight at 00 C. Weevil larvae were
not found in OflC of the weevil enclosures so this enclosure was not included in the
auiuly is. Thus. N 3 for all treatments. Treatment effects were analyzed using an
ANOVA and treatment ii eun wete iompared using Tukey’s HSD test (Sokal and Rohlf
l9 l).
Results
Surveys
Pond Survey
The watermilfoil population in the pond declined substantially over the winter of
1991-1992 (Figure I A&B). in June of 1992 there were no areas of the pond where dense
watcrmilfoil beds reached the surface. The decline was most dramatic in the South Bed:
the bottoi’n of the pond in the area which once supported tile South Bed was devoid of
any wuiermilfoil growth. Scaneted plants wei-e present in tile West Bed. Some of these
were hillel shoots (uproximately I .5m high) which were probably survivors from the
pre iou’ season: most were shorter shoots (<0.5 iii) that appeared to have Just begun to
grow. B)’ the cud of the summer. four areas of moderately dense waicrimlfoil growth
were present (Figui-e I B). These included tile southern poition of the West Bed and three
scaitcieci. small patches located along the southern shore of the pond. Watermilfoil only
approached the surface in tile West Bed; the tops of these plants were still almost a meter
21
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•1
below the surface. Only scattered, small plants were pleseilt in the vicinity of the former
South Bed by the end of the summer.
Water Temperature
Sui face and bottom temperatures remained fairly constant for most of the summer
(Figuie 2). Mcaii maximum suifuce temperature from late June through mid-September
appi-oximutely 230 C; mean minimum surface temperature ranged from 170 to 190
C. Bottom temperatures were fairly similar.
Water Chemistry
Concentrations of orthophosphate, nitrite and nitrate varied little in 1992.
Concentration of orthophosphate ramely deviated from ().O 2 mgJl concentrations of
nitrite and nitrate were always 0.01 mg/I. These values were similar to those obtained in
the 1991 water chemistry samples (Cu-eed and Sheldon 1992).
Sediment Chemistry
Ammonitim was the only sediment nutrient which varied ignificantIy among sites
(Table 1). Interstitial water ammonium concentrations were significantly lower in the
South Bed than those for sediments fiom the native plant sediments or the West Bed
scdiiiiciiis. Exuhangable ummonium in the South Bed sedirnenis wa signifkanily lower
thami only the West Bed sediments.
22
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Plant Transects
Wateimilfoil biomass was low on both transects in mid-June (Figures 3 & 4). There
was a slight increase in watermilfoil biomass in the West Bed over the course of the
summer (Figure 3 A-C). i.e., some recovery of the bed occurred. There was little change
in watermilfoil bioinass in the South Bed (Figure 4 A-C). These data, combined with the
permanent grid data (sec below) and snorkeling observations demonstrate that the South
Bed did not uecovei immediately from the decline.
Comparing the plant transect data for the last (late for each of the three field seasons
illustrates the 1991-1992 decline (Figures 5&6). There was a 4-6 fold reduction in
watemmilfoil biomass in the center of the West Bed between 1991 and 1992 (Figure 5).
There was a 15-30 fold reduction in watermilfoil biomass in the center of the South Bed
between 1991 and 1992 (Figure 6).
Pci manent Grids
The percent cover of waterinilfoil on all four grids was very low on 15 Juiie
supporting the evidence from the plant transects that a decline had occurred (Figures 7-
10). Large sections of all four grids had no watermilfoil present at all (i.e., 0 percent
cover). Only two small sections of the South Cud (West Bed) and one on the West Grid
(South Bed) had peicent cover readings greater thLln 2Yh. There was little change in
percent cover readings by mid-July for three of the four grids. Only the South Grid
(West Bed) showed a substantial increase in watermulfoil cover. By late August there
still was little Lhauuge in watermilfoil cover except on the South Grid (West Bed).
When the last re iduiig of the grids for each of the three field seasons are compared the
extent of the 199 1-1992 decline is more apparent (Figures 1 l&12). The four grids
displayed varying degrees of watermilfoil cover at the end of 1991: heavy wateimilfoil
23
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co cr (>SO’ 7 c) on the grids ranged from 40% (North Grid, West Bed) to almost 100% of
the cover on the East Grid, South Bed. At the end of 1992, three of the four grids had
cover vulue that rarely exceeded 25% (a few small patches of cover >25% were present
on the West Grid of the South Bed). In the case of two grids (the east grid on the South
Bed and the north grid oii the West Bed), anywhere from one half to three quarters of the
grid area had () watermilfoil cover. The decline was most striking on the east grid of
the South Bed (Figure 12). This grid had had essentially 100% watermilfoil cover over
the entime grid t the cud of 1991. Little wutermilfoil cover was present on this grid in
1992. Only the south gu id fiom the West Bed had substantial watermilfoil cover by the
end of the summer of 1992: approximately 30% of the watermilfoil cover on this grid
exceeded 50%
Peucent ateruiülfoil cover on the Memphremagog grid did not exceed 20%. Most of
the quadrants in this grid had 0% cover at the end of the summer. One clump of
waicmmilfoul was present in the southwest corner of the grid (near marker E4) and this
accounted for the 20 cover icading in this quadrant. While percent cover was not
deteimuned for this grid in 1991, the grid had been placed in a sparse waterniilfoil bed
(i.e.. there was watermilfoil in every quadrant of the grid). Thus there appears to have
been a decline in watermilloil abundance at this site.
Invertebrate Samples
Super Samples. Tables 2 and 3 list the dominant taxa iii the super samples. The
mote abundant taxa include the amphipod Hvallela , Hydracam iiia (water mites),
Chiuonomiclae (midges). the mayfly Caeuiis (early in the season) and the siiails Amnicola
and Physa . These were also the dominauittuxa in 1990 and 1991. Most taxa had similar
abuiickiuiccs iii both the South and West Beds. Chironomidae, Caenis , and Ceraclea
(cacklisfly) were significantly more abundant in the South Bed. Oligochaets, Leptocercus
24
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(caddisily). Plaiiariudae and Phvsa were significantly more abundant in the West Bed.
Significant changes iii abundance over the summer were observed in some tuxa. Caenis
and Enallagma (ddmselfly) declined in abundance in both beds: Amnicola and Physa
increased in both beds. Taxa which increased in abundance in only one bed were the
Oligochacts and Oxycthira (SB) and 1-Ivullela (WB): taxa which declined included
Cci aclea (SB), and Acentria, Oecetis, Lept ercus and Planoibidae (WB).
Euhr chiopsis was found on both native plant species but the numbers were extremely
low coiiipaicd to the number collected in samples of M. spicatum (Table 4). All three
weevils found cii P. amplifolius were adults. There was watermilfoil in the 1992 P.
amplifolius sample and in one of the 1991 samples which contained weevils. Thus, only
one weevil adult appears to have been on . amplifolius . This weevil may have been
resting on the pondwecd while searching for more watermilfoil. There was watermilfoil
iii the one 1992 fl. dubia sample which contained weevils. Two weevil larvae were
found in an U. dubia sample in 1991. This was the first . duhia sample taken and may
havc been the first native plant sample taken on that date. As we always sampled M.
picatuin first the piesence of these larvae in the H. dubia sample may be the result of
contauiiination. Acentria were most abundant on watermilfoil. However, they were also
piesent. albeit at lower numbers, on the P. amplifolius . No Acentria were collected in
these saii ples of H. dubia . However, some were found feeding on this macrophyte iii
1992 (S. Sheldon, pers. obs.). Parapoynx was found on all three macrophytes but was
consistently moie abundant on . amplifohu . It was the least common of the three
hci bivores on wuiermilfoil.
Minisainples. The sunie laxa which were abundant iii the super samples were the
domninaiit taxa on long watcm milfoil stems sampled with the minisampler (Tables 5&6).
The only diffemences were in the abundance of Oligochaets and Hydra . These two taxa
appc mr to be much inure abundant in the minisamples. This is probably a result of
differences in sample processing. The minisumples are picked by hand while the animals
25
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are alive. The super samples contain much more plant biomass. The animals are
sepaiated from the plains using ujetof water which probably fragments the fragile
Oligot.haets and Hydra . The pattern of Euhrvchiopsis abundance over time differed
between the super samples and the minisumples. In the minisamples, weevils were more
abundant early iii the summer. The ie erse was observed in the super samples. We are
not sure why this is the ease. The stem transect data (see below) show a pattern similar
to the minisample data.
Both EuhrvLhiopsis and Acenti ia were consistently more abundant on longer
watermilfoil plains (Tables 7& ). Euhrychiopsis abundance was only greater on short
stcii s on one date iii the South Bed. Acentria abundance was greater on short stems on
only’ two d ites in the South Bed.
Figures I 3& 14 are plots of the abundance of weevils (based on minisample ) and
watcimilfoil biomass for 1990-1992. Weevil abundances were fairly low in both
watermilfoul beds during 1990. In general, weevil abundance increased through early
1992 and then begun to decrease. When waterniilfoil abundance is plotted for the same
period it is appdrcnt that the increase in weevil abundance coincides with the pronounced
decrease in watermilfoul abundance. The peak in weevil abundance occurs
appioximately one year after the peak in watermilfoil abundance.
Stem Transects. In general. the stem transect data show an increase in abundance in
weevil life stages early in the summer (up to 3 July) followed by a decrease in abundance
(Figures 15-20). This pattern was obseivcd in both beds. The number of eggs per stem
peaked on 26 June in both beds (Figures l5&l ). The increase in eggs in the South Bed
almost UI)PCJi to be exponential tip until 26 June. The mean number of eggs per stem in
the South Bed on 26 Juuie was 6.0, the highest we have ever observed. The numbers of
larvde were similar to those observed in 1991 samples. Larval abundance peaked on 3
July’ in both beds (Figures 16, 17, 19, 20). Eggs amid merislem larvae appeared to be
mole abundant on watein iilfoil stems with intact meristems; stem larvae appeared to be
26
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more abundant on watermilfoul stems with damaged meristems. No . lecontei pupae
were found in the 1992 steni transects (nor were any found in ally of the minisafllpleS or
supersamples). In 1991. 50 pupae were collected in stem transects.
Fish S inplcs
Eleven pcich were collected on 25 June, thirteen were collected on 2 July and
seventeen eic collected on l() July. On 25 June, all of the fkh contained prey: the
number of fish containing prey on 2 July and 10 July were 10 and IS, respectively. The
mean a I S.D) total lengths and weights of lush collected on each date were as follows:
25 J tine - 23 1.7 (±21.3) miii and 164.7 a 47.0) g; 2 July - 232.5 a I .5) mm and 161.1
(±36.6) g. l() July -237.9 (±23. ) mm and 175.1 a 55.4) g.
The dominant prey (detei mined as having a frequency of occurrence >20’ic for one or
mouc of the thuce dates) found in the peuch guts were the amphipod Flyallela , Cladocera,
chironoinids (larvae and pupae), Ceratopogonidae, Chaoborus (larvae and pupae), the
maylly Cacuis , the di-agonfly Tetragoneuria , the (laniselfly Enallagma , the snail Physa
and perch fry (Table 9). Piey which occurred less frequently in the perch guts included
‘ater mites (Hydracarina), Bactid mayfly nymphs, larvae of the beetle Gyrinus , various
Truchopicra lauvue ( Leptocercus, Occetis and Polvcentropus) , Neuroptera larvae, small
crayfish, and small planorbid snails. No Acentria larvae or Euhrychiopsis (adults or
larvae) were found in the perch stomachs.
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Fxperiments
The Effect of Acentria and Euhrychiopsis larvae on Watermilfoil Growth
l)oth Acentria and Euhiychiopsis larvae had negative effects on all three measures of
plant gro th (Figure 21 A-C). Watermilfoil plants with just one weevil larva were
shoi icr. had fewer whorls and weighed less than control plants. However, plants with
Acciitui larv ie , either alone or in conibin tion with a weevil larva, exhibited even more
damage than plants with just a week ii larva. All measures were negative for plants with
ALenu ia larvae. The damage to plants with both Acentria and weevil larvae was slightly
lcss tliziui that exhibited by plants that had a single Acentria whkh suggests that the
presence of the weevil laiva might have h d a slight, inhibitory effect on Acentria
feeding.
The Effect of Larval Weevil Damage on Stein Fragment Viability
Expcriuiicnt I. Slightly fewer damaged tenm produced roots compared to the
tundain igcd. control stems but the difference was not significant (Table 10). The bioniass
of the rooN pioduced by the undamaged. control fragments was 7X greater than that
produced by the damaged fragments (p<()A)( 0X). Ovciall. there was no significant
dificience in the amount of stem tissue pioduceci by the two fragment types. However.
undamaged stems produced significantly more stem tissue by elongation of the original
stein (p<() 01)05) while weevil-damaged stems produced more stem tissue by producing
latcial stems (p
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Experiment 2. Both weevil damage and shading had a negative impact on root and
stem production in stern fragrnemits (Figures 22&23). All of the undamaged sterns
produced loots regardless of the shade treatment (Figure 22A). A higher percentage of
the damaged stenis iii unshaded aquaria produced roots compared to sterns in the shaded
aqualma. Undamaged stems (both shaded aiid unshaded) produced significantly more root
biounass than the damaged stems (Figure 22B). Shading reduced root biomass for both
damaged and undamaged stems; the difference was significant only for undamaged,
conti-ol stems Production of total stem tissue was siguiificantly greater foi undamaged,
control stems (Figure 23A). Most of the stern productioii in the undamaged stems was
clue to elongation of the original stem (Figuie 23 B). There was some production of
lateral stems. All of the stern production in the damaged stems was due to the production
of lateral stems (Figure 23C). The difference between the undamaged control stems and
the damaged stems was highly significant for all three measures of stein production.
Shading appeamed to have a positive effect on stem elongation in the undamaged stems;
on average, the shaded control sterns had original stems that were 27 mm longer than the
uiishaclecl ones. l-lowever, the shaded control sterns produced less lateral stem tissue with
the result that the two treatments were almost identical in total stein tissue produced.
Shading inhibited the production of lateral stein tissue by damaged sterns. On average,
damaged, shaded stems produced 27 mm less lateral stem tissue than unshaded, damaged
stems.
Fish Exclusion Expem imcnt
Significant fish effects were found for four ot the nineteen taxa evaluated (Table II).
These were the weevil Euhiychiopsis , darnselfly larvae of the genus Emuilla nia , the
cacldisfly Oxvethira and Oligochacts. Marginally significant (p
-------
The Hydracarina and Euhrvchiopsis had similar abundances in the cages and cage
controls. The highest densities for Physa and EnaIIa ma were in the cages with
intermcdiatc dcnsities in the cage controls. The oligochaets and Oxvethira were more
abundant in the open watcrmilfoil bed.
Significant cage effects were observed for Oligochaets, Euhrvchiopsis and Amnicola .
Maiginally significant cage effects were observed for HvalIela , Hydracarina. Acentria,
Oxyethura. Hyal Ida . Hydracarinu and Euhrychiopsis were more abundant inside the
agcs and cage oiitrols. The cage effects for some of these taxa appear to be fish effects
which suggests that perch may not have foraged as extensively on the watermilfoil in the
cage contiols as in exposed watermilfoil. The abundances of Oligochaets, Acentria,
Oxvethira and Aninicola all appear to have been depressed by the presence of cages, i.e.,
the)’ weic more abundant in the presence of fish.
1-Ici bivore Enclosure Experiment
Weevils had a significant effect on watermilfoil biomass and plant height in the
enclosure experiment. Total biomass was significantly greater in the control and the
Acentria tteatments compared to the weevil treatment (Figure 24). The differences in
total bioinass cre attributable to differences in root weight and lateral stem weight:
there was no significant difference in the weight of the original stems (Figure 24). The
weevil-damaged, original stems in the weevil treatment tended to collapse during the
experiment. While the mean height of these weevil-damaged original stems in the water
column was usually lowei than that of the original stems in the control treatments, the
diffeicuce as hot significant until the last three weeks of the cxpei imcnt (Table 12).
The difference in the mean height of original stems bet een the weevil treatment and the
control tieaunents for this period ranged from I () - 25 cm (Table 12). The weevil-
damaged stems were often suppoi ted by the enclosures. In the abseiice of the enclosures,
30
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the diffeience in the height of weevil-damaged veisus undamaged plants may have been
greater.
l)iscussion
The weevil and watennilfoil survey data support the hypothesis that weevils were
invoked in the Biownington Pond waieu-iiiilfoil decline. In 1990, weevil abundance was
ai us lowest while that of watermilfoul was high. The summer of 1990 was the growing
season subsequent to the first observed watermnulfoil decline (see Creed and Sheldon
1991 b for maps illustrating the first observed Browningtomi Pond decline). With the
marked decline of their major food resource by 1989 (weevils do not appear to feed on
any other aquatic macrophyte present in Browningion Pond) it is not surprising that
weevil numbers were quite low in 1990. From 1989 through 1991 the areal extent of the
watermilfoil beds increased (see Creed and Sheldon 199 lb and Figure 1). This
expansioui as also reflected in the permanent grid data. However, the weevil population
also began to increase iii abundance in 1991 and wateimilfoil biomass did not continue to
increase over the 1991 glowing season as it had in 1990, i.e., peak wateimilfoil biomass
was in mid-summer of 1991 and not late summer as was the case in 1990. Overall, the
numither of eevils per stem increased through 1991 and were high at the onset of the
1992 growing seasoui. Watermilfoil abundance had declined dramatically by this point in
time. Subsequeiit to the watermilfoul decline, weevil abundance began to decline by mid-
summer of 1992. Thus, the peak abundances of watcrmilfoil and . Iecontei appear to be
out ol phase ith one another. These patteruls of abundance are similar to that displayed
by simple predator-prey or host-parasitoid models (e.g., Begon and Mortimer 1981.
Krehs 1985) and suggest that a similar interaction is occurring between Eutasian
waternitifoul and E. Iecontei . Additional collections over the next 6-9 years are needed to
verify this cyclic pattern of weevil and watermilfoil abundance.
31
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The lack of any weevil pupae and observations on watermilfoil size in Brownington
Pond in 1992 suggest a reason for the observed decline in the weevil population. Weevil
eggs and larvae weie very abundant early in 1992. However, very few long (>150 cm)
watermilfoil stems were present iii the pond early in the summer: all of these long sterns
were located in the southern portion of the West Bed. Few plants (and practically no
long plants) were present in the South Bed. It is possible that there were few stems large
enough in diameter in which weevil larvae could construct pupal chambers. Therefore,
many Lam vac may have died in early July. This would explain the lack of pupae in our
samples which in turn would explain the lack of other weevil life stages for the
remainder of the summer. Furthermore, the high densities of weevils early in the
summer would have severely damaged most of the wutermilfoil plants in the pond.
Many watci mifoil plains would have been prevented from growing longer with the result
that stems suitable for pupation would be in short supply. This hypothetical scenario
suggests that the lack of suitable stems for pupation is a major factor driving the
population oscillations of weevils and watermilfoil. High densities of weevils such as
those observed eaily in 1992 could prevent a watermilfoil population which has already
declined froiii producing laige plants. What few long plants that might be present might
have high densities of weevils (weevils were clearly more abundant on long plants in
both beds in the minisamples). Weevil numbers would subsequently crash. With reduced
weevil feeding the remaining, small watermilfoil plants could recover and time
watermilfoil could spread again. This hypothesis remains to be tested.
The results from both the eiiclosure experiment and [ he two lab experiments
deinonstiate that the weevil . le ontei can have a significant, negative effect on
Eurasian wateimnilfoil. lii both the pond experiment and the wading pool experiment, the
pi imary effect of weevik appears to have been a suppression of watermilfoil growth. In
the pond experiment, weevils suppressed production of new stems by damaging lateral
shoot meristems. The meristem damage observed in this experiment was due to both
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larval and adult feeding. Weevil attacks on the shoots appear to have had a negative
impact on loot production. Weevil damage may influence root production as the
removal of stcm vascular tissue by weevil larvae may interupt much of the flow of gases
ind photosyiithate to the root system. Weevil damage to the stem also caused the plants
to sink out of the water column. This result with rooted plants confirmed thc results of
earlier experimental studies (Creed et al. 1992) which demonstrated that weevils could
affect the buoyancy of floating watermilfoil fragments. Weevil damage also had a
negati e impau on the viability of watermilfoil stem fragineins. Like the rooted plants
in the cnclo uie experiment, the stem fragments had greatly reduced root production.
The ucduccd viability of these stem fragments suggests that the spread of watermilfoil
beds clurmn periods of mnteuise weevil herbLvory would not be as great as that observed
with oihei- methods of watermilfoul coutm-ol which produce fragments (e.g.. harvesting,
rotovating). The results of these experiments suggest that weevils have three effects on
Eura’ ian watermilfoil: I ) wcevils damage existing stems, possibly stressing the plants
physiologically as a iesult of disruption of gas balance and loss of vascular tissue, 2)
weevils inhibit the production of new stem tissue by destroying meristems and 3) weevils
inhibit the spread of watermilfoil beds by reducing stem fragment viability. These data
support the hypothesis that weevils played an important role in the Brownington Pond
watermilfoil declines.
Changes in water and sediment Lhemistry do not appear to have been the primary
cduscs of the Biowningion Pond decline. ConLcnti Itions of the measured nutrients in the
water column displayed essentially no chdnge between 1991 and 1992 or within the 1992
giowung season. It us possible that a change in some unmeasured waterborn
micu onutm icnt coLild have caused the decline. However, observations from Brownington
Pond suggest that this was not the case. First, watermilfoil did not disappear throughout
the pond which is fairly small and appears to have a well-mixed epilimnion (e.g.,
teinpelatures are nearly uniform around the epilimnion of the pond). Second, the
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Bi-ownington Pond enclosure experiment was conducted adjacent to the site of the former
South Bed where the ueduction in watermilfoil abundance was greatest between 1991 and
1992. The watermilfoil inside the enclosures readily grew at this site while little
wateiiiiulfoil guowth was observed immediately surrounding the enclosures. The latter
observation suggests that some other factor was preventing the reestablishment of
watermulfoil in this aica.
Changes in sediment chemistry ilso do not appear to have been important in producing
the decline. Only one sediment variable, the concentration of .immonium, was found to
vary significantly among sites. Ammonium concentrations in both the sediment and the
interstitial poie water were lowest in the sediments of the former South Bed. These
results were the opposite of those of Painter and McCabe (198K) who found that
ainmonuuni concenhiations were lowest in areas of high watermilfoil abundance. We are
not sure why amnionium abundance was lower at the South Bed site. As ammoniurn is
produced by the decomposition of organic matter by heterotrophic bacteria (Wetzel
19K3). we would have expected higher sediment concentrations at the South Bed site as
thete was a Ltyer of decomposing watermilfoil on the sediment surface for much of the
summer. Alternatively, the waterniulfoil bed that had previously been present at this site
may have sevemely depleted sediment ammoniuin concentrations with the result that
waterinulfoil w s unable to grow here. However, we used sediment from the South Bed
in the enclosure experiment. As the experimental waternulfoil grew on this sediment we
do not believe that Lhdnge in sediment quality was a primary factor in the Brownington
Pond decline. The results from Norton Brook Pond (see section on introductions) also
suppoit the hypothesis that herbivory arid miot changes in sediment quality was primarily
responsible fom this decline. The sediment was not disturbed in any way in the Norton
Brook Pond expeu-umcnt. The only cliffeicnce between treatments was the presence of
weevils. While there may be an interaction between nutrient availability and the effect of
the weevil on Eurasian wateimilfoil (e.g., reduced root production in the presence of
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weevil heibivory results in reduced sediment nutrient uptake), we do not believe that
changes in nutrient availability alone could have produced the Brownington Pond
decline. Admittedly. our assertions regarding the effects of sediment nutrients are based
on a limited number of samples from a single date. However, our results confirm those
of Painter and McCabe (1988) who could find no relationship between sediment quality
and the waterinilfoul declines observed at the Kawartha Lakes.
The fact that much of the Brownington Pond watermilfoil disappeared during the
winter suggests thdt wcevil hei-bivory stresses the plants in some maiiiier that makes it
difficult for watermilfoil to overwinter. For example, weevil damage to stem vascular
tissue could pievent movement of nutrients and/or gases from stems to roots (or vice
versa) which could physiologically stuess the plants (Wetzel 1983). Alternatively, the
weevil-damaged plants may be much more susceptible to decomposers than healthy
plants. At present. the reason that the wateu-milfoil in Browiiington Pond declined during
the winter remains unknown. However, this observation suggests that winter may be the
season when the greatest reduction in watermilfoil biomass occurs. Further reseaich is
needed to tinderstand thus potential I)’ important effect.
Yellow perch do 1101 appear to be a source of mortality for Euhrychiopsis or Acetitria .
O er the last three summers we have examined the gut contents of 175 laige perch plus
25 YOY. We have not found any Euhrychiopsis (adult or larva) or Acentria larvae in
any of the guts examined. While we do not have adequate samples of I + and 2+ perch it
is hard to believe that the contents of their stomachs would be dramatically different
from those of the 3+ and 4+ perch we have collected. In the 1991 fish exclusion
experiment. . leconteu showed little response to the treatments. Weevil numbers were
actually higher in (lie watci milfoil bed (controls) and the cage controls (see Creed and
Sheldon 1992). At the time that this expei iment was sampled (late August). the yellow
l)Cich weic not feeding as heavily on littoial prey as they had been earlier in the summer
(see Creed and Sheldon 1992. Figures 18 and 22). In the 1992 experiment, the opposite
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response was observed: weevils were more abundant iii the cages and the difference was
significant. These results suggest that many weevils were avoiding areas where they
might be exposed to fish. This result was observed during a time when yellow perch
were feeding extcnsi ely on littoral invertebrates (Table 9). Thus, during the early part
of the summer when yellow perch feed heavily on littoral invertebrates, weevils may
aggregate in potential refugia (e.g.. areas with a high density of watermilfoil) even
though they ate not being consumed by perch. If weevils are iinroduced into a body of
watel ejily in the season then it may be best to introduce them into a region of dense
watermilfoil giowth. Yellow perch may have had a positive iiichirect effect on
watermilfoil heibivores by consuming potential predators (dragonfly and damselfly
larvae). This potential indirect effect needs to be investigated.
Student Reseaich Projects
Two student research piojects were carried out in 1992 at Brownington Pond. One
project examined the ability of weevils to colonize and damage individual watermilfoil
plants at three different distances (10, 30 and 60 in) from a watermilfoil bed. Weevil
damage was assessed by detei mining the amount of stem burrowed by larvae and the
number of adult stem bites per stem. Larval damage decreased with increasing plant
clist iiicc horn the bed. There was no significant difference among locations in the
number of adult stein bites. A second study was designed to determine what cues female
weevils might be using to choose plants on which to lay their eggs. We had noticed that
there were more weevils on longer plants. The study evaluated the effect of stem length
and depth of the ineristems. Weevils laid significantly more eggs on longer stems.
There was no significant difference in the number of eggs oii shallow and deep stems of
the sanie length although theie tended to be more eggs on shallow meristems. These
results suggest that female weevils may be actively selecting longer stems.
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RESEARCH AT LAKE BUMOSEEN AND MII)DLEBURY
Research at Lake Bemoseen
liii rod uci ion
Lake Bomoscen is the laigest lake (I I2 hectures) contained entirely within the
bounclai ics of the state of Vermont. Eurasian watermilfoil was First reported in Lake
Bonioscen in l9 2. The lake currently has a serious infestation of this species. Attempts
at contiolluiig the wateri’nilfoil have included harvesting and an overwinter drawdown.
Hydroi iking and bottom barricis have also been used by camp owners on an individual
basis. One of the primary objectives of this project is to determine if the herbivores
under study could be employed to control the M. spicatum infestation in Lake Bomoseen.
The piiniary goal of the 1992 field season was to collect data on the effects of
watermilfoil harvesting on heibivoue abundance, primarily the weevil . lecontei .
Study Sites
Twelve sites were designated in 1992 to be avoided by the harvesters (Figure 25).
Thcsc sitcs were mui-ked with peimancnt buoys. These sites included I) the eastern shore
of the north end, 2) the west side of Eckley Point, 3) the east side of Ecklcy Point at the
southern end (= E. Eckley South). 4) the east side of Eckley Point at the uioi them end
(=E. ELkicy Noith), 5) the cast side of Ne hobe Island, 6) Green Bay, 7) the top of the
Channel (across from Indian Bay). X) Avalon Point Beach, 9) W. Castleton Bay (south of
State Paik beach, 10) the NW corner of W. Castleton Bay, I l)north of the slate quarry
and 12) eastern Rabbit Island.
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Three sites, E. Eckley South, the area south of State Park beach, and the east side of
Neshobe 1., were previously designated as “no harvest” sites in 1991. All other sites were
newly established in May 1992.
The distance each of the 110 harvest sites extended from shore varied depending on the
wutei depth off shore. At all sites, the buoys were placed close to shore in water no
deeper than 3 in. For all of the sites, the area designated to be left unharvested was
between the line of buoys and the shore.
The shoi chine dkiance of CULI1 of the no harvest sites ako varied depending on the
available lake shore puoperty and sire characteristics. All sites were approximately 50 in
along the houc.
Materials and Methods
Stein Tiansccts
Watcrinilfoil stems were collected weekly from three of the sites (E. Eckley South, E.
Eckley North and Neshobe I.). Transects were set-up parallel to the edge of the
harvested area (perpendicular to shore). On eaLh side of the line of harvest the first
transects were within 1-3 in of the line, and two more transects were placed progressively
farther from the harvest line, resulting in six parallel transects, with three tian ect
located in both the harvested and unharvested areas. Along a transect line snorklers
removed the 0.3 in uppermost portion of a plant auid placed it in a ziplock bag. For each
ti ailseu. five plauh with intact apical mei istems and five with damaged apical meristems
erc colle ted. resuhiiig in the collection of 60 stem tops per site per day. To determine
the distribution of weevils within sites, we separated the plants we collected in each
transect into two bags, a shallow bag and a deep bug. In each transect, the four plants
collected in the two stops closest to shore were called shallow plants. The other six
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plants, collected at the other three stops, were called deep water plants. Separation of
stems into deep and shallow bags began on 22 June and continued through the summer.
Samples were collected from 26 May through 2 September 1992.
On returning to the I b, plants were examined under a dissecting microscope. From
the tell plants within a tr insect, every weevil was removed, preserved and the iiumber
and life stage recorded. Differences between dates, sites and harvest versus unharvested
areas were compared using an A NOVA (the data were square root transformed for the
analyses). To analyze for depth effects, all sites weic combined and we compared the
number of cevils per stem in all the shallow regions versus the number of weevils per
stem in all the dcci ) regions for the entire summer.
Some of the ‘hai vested’ areas near the designated no harvest sites were often not
harvested hampering comparisons of harvested and unharvested areas. The “harvested”
area near E. Eckley South was not harvested throughout 1992. The “harvested t ’ area near
E. Ecklcy Noith was harvested once, in early July 1992. The “harvested” area near
Neshobe I. was hai vested oii a regular basis starting mid-July 1992.
Super Samples
Three sites (E. Eckley North and South, and Neshobe I.) were designated as no harvest
sites in 1992. Unfortunately, regular harvesting of adjacent watermilfoil was only
performed at Neshobe I.; watermilfoil was harvested only once adjacent to the E. Eckley
North no harvest aiea and no harvesting occurred adjacent to E. Eckley South.
Them efoic. only the d mta fi-om Neshobe I. will be discussed. Once a month (June through
/\ugust) six super samples were collected at Neshobe 1. (For a description of this
sampling method see the Brownington Pond section, page 9). Three samples were taken
fiom the hai vested bed and tlimee from the udjaLent unharvested amea. The sampler was
placed h ipIiazardly, although care was taken to distribute sample locations over an area
39
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within 10 m of the line of harvest. Samples were preserved in 70% ethanol. In the lab,
all of the invei tebrates were removed from the plants. identified and enumerated. The
plants wcrc sorted to specics. dried at jO C aiid weighed. Differences between dates
and harvested and uiiharvested areas for watermilfoil biomass and the abundance of
major invertebiate taxa were compared using an ANOVA.
Other Sampling
Three sites (the eastern shore site in the north end (site I ), the western side of Eckley
Point (site 2). and the Avaloii Point beach site(site )) were quantitatively sampled once
during the summer to determine the presence of weevils. All of these sites had been
harvested the previous summer. Weevil transects were conducted at these sites using the
same protocol described above.
Weevil Augincntatioii
On 3 August approximately 100 weevils (both adults and larvae) were caiefully placed
on tall M. spicatum plants close to shore at E. Eckley South. The introduced weevils
were in all life stdges. The weevils were collected in Glen Lake.
Results
Stem Tldilsccts
Of the three sites, E. Eckley South had the highest mean a I S.E.) number of weevils
per mci Istem with U.04 a (Ii) 10) for the entire summer (Table 13). The other two sites
had lower but not significantly cliffetent (p=0. 04) weevil densities throughout the
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summer. Neshobe I. had a mean of (1.042 (±0.012) weevils per ilieristem and E. Eckley
North had 0.039 a 0.007) weevils per meristem.
When the data from all of the sites were combined, the mean a I S.E.) number of
eevils per meristeni in harvested areas (0.026 ± 0.005) was significantly lower
(p 0.0Uń) than in the uiiharvestcd areas (0.05 + 0.010) (Table 14). The sites did not
cliftci significantly. Theie was little difference in the number of weevils in harvested and
unh ii vested areas early in the summer (Figure 26); many more weevils were present in
samples from unharvested areas later ii i the summer (Figure 26). Examination of the
data for the individual sites indicated that both Neshobe I. and E. Eckley North had
significantly more weevils in the unliarvested areas (Table 14). More weevils were
collected in the unharvested area at E. Eckley North early in the summer: the reverse was
true at Neshobe I. (Figure 27). E. Eckley South was never harvested in 1992. Weevil
densities in the sites designated as “harvest” and “no harvest” did not differ significantly
(Table 14. Figure 27).
All the weevil collection data from all of the sites were combined to test for
diffeiciiccs in weevil distribution with respect to water depth. The number of weevils per
incristcm on watenuilfoil in shallow water was significantly higher than the number of
weevils on deep water atcrmilfoil (p
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A plot of the abuiidauice of the different weevil life stages over the summer (data from
all sites combined) illustrates the decliiic in numbers of the different life stages between
1991 and 1992 (Figure 29). Three peaks in egg abundance (15 June, 13 July and 24
August) were apparent in 1992; the position of the two later peaks coincides with the 2
peaks in egg abundance ohscrved in 1991 . Larval abundance in 1992 was high in mid-
June following the first peak in egg abundance but did nor display a clear pattern of
abundance afterwards. Pupae were not abundant until late in the summer; adults were
never abundant in these samples. Data from both years indicate that weevil cgg
abundance decieases dramatically at the end of August. No eggs were found in either
ycai after the first week in September and larval numbers dropped to zero by the last
week of September.
Super Samples
There was a significant effect of harvesting on watermilfoil biomass; no significant
date effect for watermilfoil biomass was observed in 1992 (Table 17). Significant
rliffeiences between dares were observed for tell of the fourteen most abundant
macroinveitebrute taxa. Five of the ten taxa having a significant date effect (Oligochaets,
Isopodla. Chiionomidae. Caenis , and Zygoptera) were most abundant in June. Three of
the ten taxa ( Oxycthira, Orthotrichia and Amnicola ) were most abundant in July. Only
one taxon, Euhrvchiopsis , was significantly more abundant in August.
Only three macroin crtebrate taxa (Isopocla, Euhrvchiopsis and Caems ) showed a
signifiLant i-espouse to haivesring in 1992 (Table 17). Both the lsopocla and Cacuis were
inoic abundaiit in the harvested areas: Euhrvchiopsis was more abundant in the
unhai vested aicas.
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Other Sampling
On 3 August, one weevil larva and six empty pupal chambers were found in a
collection of thirty merisiems at the eastern shore site in the north end (site 1). No
ecvils were collected at the other two sites. However, during sampling snorkelers
reported signs of weevil damage on wutermilfoil at the west side of Eckley Point (Site 2).
A local property owner also reported that there appeared to be a large amount of
damaged wateruiiilfoil at thc sest side of Eckley Point and he attributed this damage to
weevils.
I)iscussion
Two of the sites (Neshobe and E. Eckley South) were sampled intensively in both
1991 and 1992 for the months of July, August and September. Although these two sites
differed with respect to weevil densities in 1991, our data indicated no difference
between the two sites in 1992 because the number of weevils (all life stages) at Neshobe
had decliiicd. The mean a I S.E.) number of weevils per stein at Neshobe iii 1991 was
0.204 (±0.026); in 1992 the mean was 0.072 (±0.016) per stem. This difference was
highly significant (p
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harvested within the last year. When only the Neshobe data are examined (the only site
whet-c regular hat vesting occurred in both years) we once again found a dramatic
difference between harvested and unharvested areas. These results support our findings
from 1991 iegaidung the effect of harvesters on weevil abundaiice, i.e., that constant
harvesting will prevent the establishment of large weevil populations.
By separating CaLh of the weevil transects into shallow and deep regions in 1992, we
were able to test the 1991 hypothesis that weevil densities were higher on watermilfoil in
shallow water. Overall, we found less than half as many weevils per meristem on the
waterunilfoil in deep atcr as oil the shallow water watermilfoil. The diffeience in
weevil density between shallow and deep habitats was especially pronounced at the
ELkIcy Bay sites At those sites, we found one third to one fifth as many weevils per
uiieristcm in deep water as compared to the shallow areas. One main difference in the
distribution of watermilfoil between the Eckley Bay sites and the Neshobe I. site was the
density of watenuilfoul in deep water. At Neshobe I., watermilfoil density was similar
throughout the areas sanipled. At both the Eckley Bay sites, on the other hand, the
satermulfoiI was fairly dense iiear shore but tended to be in small, dense clumps in
deeper water. These data suggest that the weevils are not responding to water depth per
se but waicrmilfoil abundance.
The 1992 supel. sample results from Neshobe I. are very similar to the results we
obtained in 1991. Most taxa displayed significant date effects in 1992 which was also
the asc in 1991 (Creed and Shcldon 1992, see Table 14). Only four taxa (Amphipoda,
Isopocla. Planorbidae and Physa ) displayed diffetent responses to date in 1992 compaied
to 1991. Aiuphipoda and Physa were significantly affected by date in 1991 but not in
1992. The ueversc was true for Isopoda and Planorbidue. Of tile three taxa which
exhibited a signifucaiit har esting effect in 1992, two of the three (Isopoda and
Euhiyclnopsus ) displayed suinilai responses in 1991. The 1992 results confirm our
pu-evious results that showed that harvesting had a negative impact on Eubrychiopsis
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abundance in Lake Bomoseen. The Isopoda were again more abundant in the harvested
areas and we still can not explain this result. The increased abundance of Caenis in
hai vested areas in 1992 may have beeii a response to an increase in the abundance of
pcrupliyton on watermilfoil stems which could have occurred due to the removal of the
dcime watermilfoul canopy. It is not clear why this mayfly taxon displayed different
rcsponses to harvesting in the two years. Two snail taxa (Planorbidae and Physa) . which
were significantly affected by harvestuiig in 1991, did not show a significant response to
harvesting in 1992.
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Research at Middlebury
Culture and Life History oft. leconici
Materials and Methods
Culture ol E. lccontei
E Icconici cultures wcre first established in June 1991. Batch cultures of wcevil eggs,
laivae. pupae and adults were maintaifle(I in the Middlebury “light loom”. The room was
illuminated with both standard fluoiescent and GroLux lamps on a 16h-on, 8h-off
pliotopcuiod Watci tempciuturcs ranged from 21.5- 24°C. Aquaria (approx 100 liters)
were filled with aerated tap water. All aquaria were continuously aerated.
fri. spicatuni plants were collected from Glen Lake or Lake Bomoseen. In some cases,
plants were held upiight by sliding their roots into weighted down, plastic mesh.
Oihci wke, plant roots wcie planted into 100cc cups filled with autoclaved lake sediment.
Alter being planted. all watet inilfoil plains regained an upright position. Weevil adults,
larvae, and M. spicatum with weevil eggs were added to the aquaria. When M. spicatum
plants beLaine heavily damaged (usually larval damage) weevils were moved into new
aquaria itli undamaged fr i. spicatum .
Life I listory of . lccontei on M. spicutum and . sibiricum
The lenglli of wccvil life stages were quantified in the culture room under the
Londitions described foi- weevil cultures. M. spicutum was collected from Lake
Boinoscen and Glen Lake. and M. sibiricum was collected from Beebe Pond.
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Myriophvllum spp. sterns were planted into cups of autoclaved lake sediment, were
enclosed in clear t.ylinclrkal polycarbonate tubes (30cm long, 6cm inside diameter),
covcied with a lid of 200um Nitex, and set in aquaria containing aerated tap water. Each
tube was also individually aerated. Weevil larvae and adults, collected from Glen Lake
or reared in the lab were placed in the tubes, one weevil per tube. Plants and weevils
wcic examined daily. often under a microscope at 7-15X magnification, and mortality or
metamoiphosis noted. In many cases, especially for pupae, this repeated handling
we..ikciied the plants, and in some cases the pupae were damaged.
For quantification of the lifetime egg production by a female, unmated females were
collected. then mated. and the number of eggs produced recorded. To get an unmated
weevil, individual pupae (which reside inside plant stems) were isolated, and after adult
emergence. the sex of the weevil was determined. A single, adult virgin female and two
males were placed in a tube as described above. Three to six M. spicatuni stems with
intact merislenis were planted into the autoclaved lake sediment. The meristems were
iemovcd and examined at 7-l5X magnification every 3 or 4 days, and the number of
eggs and larvae counted. New meristerns weie planted in the tube, and the female and
two males were returned to that tube. If a male died, he was replaced by another imile
weevil so that there were always two males in eaLh tube with a single female. Weevils
were also giown on M. sibiricum , however these cultures were difficult to establish.
Plants were collected froiii Beebe Pond and weevils housed and tieated as described
abovc.
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Resulls and Discussion
Culture ofE. leconiei
• lecoutet seenicci to do vcll under these conditions. For the 19 months, eggs, larvae
and adults have been continuously produced. The weevils generated were not quantified,
so the rate of weevil produuion under these conditions is not. kiiown.
Life History of. . lecoutci on M. spicalum and fr i.. sibiricum
Under these lab conditions the duration of the egg phase averaged 3.90 days
(±0.20 SE. n=4 ). Larval duration ranged from 4 to 22 days. averaging l2.9S a 1.75
SE, n=9) days. Pupal duration ranged from 7 to 17 days and averaged 13.() (±1.52 SE,
n=5) days. The sum of these averages suggests that the Lime between egg deposition and
emergence as an adult is 29.9 days.
On average, females laid I .91) (± 0.44 SE, n=7) eggs per day. Eggs appeared to be
prcfeientually laid on the apical meristeni. If eggs were already present on the apical
meristein, eggs were often laid on the uppermost lateral meristems and if these also had
eggs. eggs were deposited on leaves near the plant apex. In general, hatching rate of
eggs was 87 .3’/c. While eggs were usually widely distributed, when weevils were
cn loscd with few pLints we found as many as 29 eggs on a single plant in the lab. Eggs
cic elliptical. 0.52 mm long and 0.39mm wide, dud appeared uyOlk) I i.e..they were
VCi yello uiid viscous.
Two fcmdlcs wcie both very long lived dnd fecund. These feinales as adults lived
I 61) and 162 days and IdId 562 and 469 eggs respectively.
First instar larvae bunow into the merustem, usually destroying the meristem. Later
unstar larvae spiral clown the outside of the stem, then burrowed in. Larvae spend most
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of theit time insidc the stein, burrowing through stein tissue, hollowing out the stern.
Sometimes, particularly when they reached the end of an internode, they will burrow out,
spiral up or down the stem to a new location, and burrow into the stem again. Larvae
were usually found in the top third of the plant. Puparia were formed inside the stem and
tended to be found fui ther down in the thicker portions of the stem.
Of the rates quantified in the lab, we are least confident of pupal duration. Repeated
handling of M. spicatum plants resulted in plant breakage and pupal mortality. Also, it
appears that successful metamorphosis is a function of stern diameter and health of plant.
Pupae appear to bLiild their chambers in thick (>2 mm diameter) stems of actively
growing M. spicatum . It is difficult to find . spicatum plants that have thick stems,
have roots, and arc less than 3() cm long.
We have found that we can put laige lar ae on thick stems, and within 2 days, larvae
will construct a pupal chamber. In the future, we will put large larvae on sufficiently
thkk M. spicatum stems in the field, enclose the stein in a longer tube, and quantify
pupal duration.
The dui-at ion of life history stage data collected in the lab are consistent with
observations of.E. lecontei phenology in the field. There appear to be 3 generations of
weevils on M. spicaturn each suniincr in the 2 lakes we have studied in Vermont. In the
field, eggs arc found primai ily on meristems near the surface, larvae are found in the top
meter of the plant, and pupae are typically found at >0.5 iii or more down the stem. The
first ccvils found in the spring are adults, thereafter eggs and then larvae aie found In
Scptcmber, weevil densities decline. C. O’Brien (pers. comm.) predicted that
Euhr ’chiop is leLontei may overwiiner as adults in leaf litter near lake margins. This
picdiuion is based on obscrvation’ of other aquatic weevils. This is consistent with the
single eevil e found in leaf and soil samples collected on shore in late October, 5 rn
from the margin of Lake Bomoseen.
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Weevils were also grown on M. subiricum . These cultures wcre cliflicult to establish.
Seventy percent of the eggs on M. sibiricum hatched. Mean a I SE) hatching time for
eggs was 4.7 ± ().4 ) days (n=7). For each of the three females placed in tubes on M.
sibiricum , no eggs wete found on the plants. This is in contrast to the mean a I SE) of
12.75 (±2.26) eggs for weevils (range 2-23 eggs) under these same conditions on M.
spicatum for thc same period of time. A singlc pupai ium was formed. We feel that the
stems of the M. sibiricum plants chosen for these cultures were too narrow and this may
expl iii the lack of pupation.
Student Research Projects
A series of student projects were curried Out during the summer of 1992 at
Middlebury. Project topics included: I) the effects of Phytobius leucogaster (0, 2, or 4
adults) on a spicatumn and M. sibii icum , 2) the effects of both Phvtobius and .
le oiitei on M. spkalum (treatments consisted of either 4 Phytobius alone, 4 . lecontei
alone or 2 Phytobius and 2 . lecomitel together, 3) determining the number of weevils
assouuted ith flouting watcrniilfoil rafts in Like Bomoscen, 4) evaluating weevil
behaviot in the presence of native mucrophytcs, 5) examining the tendency for weevil
larvae to move between plants (both bet een a spicalum plains and between M.
spmc itum .mnd natives). 6) examining the growth rate of different types of watermilfoil
fragineiits (uutofragnicnts. wcevi l—geiiematccl fr 1 igiiicnts and harvestei -generated
fragments). 7) couipai ing the buoyancy of iootecl watermilfoil fragments (both
undamaged and weevil-damaged fragments), M) identifying weevil damage in a
laboratoiy setting and 9) determining the life history of Phytobius on M. spicatum in Vt.
In the fimsi experiment, Phylobius did not have a significamit effect on the growth of
either M. sOicatum and a sibiricum . In the second experiment, there was no significant
diffeicnce among trealnicilts ( Phytobius alone. Euhrvchiopsis alone, and Phytobius and
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Euhrvchiopsis together) for either change in length or weight of M. spicatum . However,
thc behavioi of Euhi-vchiopsis appeared to be affected by the presence of Phytobius.
Eulwvchiopsis spent less time on flowers in the presence of Phytobius . lii the third study,
the number of weevils associated with floating watermilfoil mats was correlated with the
biounass of the mats. In the fourth study. adult weevils spent most of theii time
swimming ill (lie picscnce of most native macropliyte species. Substantial amounts of
time erc spcnt on R o inacrophyt taxa ( Ceratophvllurn and Chara ) which have
morphologies siinilai- to that of M. spicatuin . In the fifth study. weevil larvae were
observed to mo e between M. spicatum stems. Movement from M. spicatum to Elodea
was also observed in one instance and there did appear to be larval damage on the Elodea
stem. In the study (Study 6) examining the growth rate of diffeient types of watermilfoil
fragments (a utofragments. weevil-generated fragments and harvester-genei ated
fragmcnts) gro ’th rate of the autofragments and harvester-generated fragments was
greatci than wee il-gcucrated fragments but the differences were not significant. In the
study (7) which compared the buoyancy of rooted waterirtilfoil fragments (both
undamaged and ecvi I-damaged fragments), found that the height of weevil-damaged
frugiiicnts in the wutci column was significaiitly lower than that of undamaged
fragiucuits. The results of this study confium the results of the buoyancy experiment
conducted at Biowningtoui Pond in 1991 (Creed et al. 1992) and the enclosure
expcm milient conducted in Bro nington Pond in 1992 (see Bro nungton Pond section of
this ieport). lii the study ) hich examined weevil damage on watermilfoil in the lab,
similar damage (adult stem bites, leaflets iemoved by adults. larval burrowing etc) to that
obsci vcd in the field and previous lab expei iment was seen. In the last study (9), the life
histoiy of Plivtobius appeaied similar to that reported elsewhere for this weevil. Eggs
wei-c laid on and inside of flowers, larvae wcre observed feeding on flowers and pupae
erc tound on the stein just below the floral spikes. The duration of each of the life
histoiy stages was not determined.
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The occurrence of Mycoleptodiscus terrestris on watermilfoil iii Vermont lakes
Samples of Eurasian vatermilfoil from thuce Vermont lakes (Brownington Pond, Lake
Bouuosecii and Metcalf Lake) were collected in August 1992 and sent to Judy Shearer at
the U.S. Army Corps of Engineers Waterways Experiment Station to determine the
pueseiice of the fungus Mycolepiodiscus terrestris on these plants. Mycoleptodiscus
Icutestu is as found on the watermilfoil from Brownington Pond and Luke Bomoseen but
not on the wdtcrmllfoil from Metcalf Lake.
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The Effect of j . lecunici Adults on Native Plants
Introduction
lf, lecontei is going to be used as a biological control agent it is necessary to
determine if it ill have any impact on native macrophytes. To quantify the potential
effect of weevul on native plant species a series of feeding experiments were curried out.
Plants used for these expeliments were some of the more common (frequency, biomass
and distribution) macrophytc species in Vermont and included Ceratophvllum demersurn,
Chara sp.. Elodea canjdcnsis, Hetcu anthera dubia. Megalodonta becki i, Myriophyllum
sibirkum. Najas flexilis, Pot 1 imogeton amplifolius, Utriculuria vulgaris , and Vallisneria
americana . The series of experiments was run from 3 July to 26 August.
Materials and Methods
Plants <30cm total length were collected with roots intact fiom Beebe Pond and Glen
Lake. except Utriculat ia vulgaris (a species which is not rooted in the sediment) which
was collected fiom Lake Bomoseen. In the lab, all plants were examined under a
dissecting microscope (7- 15X)ancl all invertebrates and eggs were removed. Many
l)luflts weic discarded clue to condition, difficulty in invertebi ate removal, plant breakage
while being handled or other such damage. Of the remaining plants, those that were the
most similar in length. weight. number of leaf whot Is and number of merustems were
selected iou - use in the experiments. Only plants with intact apical merustems were used.
Plants used iii the experiment were examined for the initial condition of the meristem(s),
lcav s and stem. Each leaf us examined, and damaged or missing leaves were recorded
by whorl or leaf as appropriate for the species. Each plant stem was marked with a tag at
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the point dividing the shoot from the roots. The length of the plant above and below the
tag was recorded. Blotted wet weights were also recorded.
Siiiglc plants were placed in chambers similar to those used in previous wading pool
experiments. Plant roots were inbcdded in a container of autoclaved lake sediments to
the mai k dividing the shoot from the roots. Plants were enclosed in clear polycarbonate
cylinders (3()cm tall, 6cm inside diameter), except E. amplifolius which was in larger
(27cm high. and 12.7cm inside clianieter) enclosures. Each chamber was sealed by
pusliiiig the polycau bonute cylinder mo the sediment and covering the top with a lid of
2UU urn Nitex. The chambers were placed in large wading poois (375 I) filled with
aerated tap water in a greenhouse under ambient light conditions. Each chamber was
individually aerated.
The design of each experiillent was a randomized block design with three treatments
(0, 2 or 4 eevils PCI chamber) per row and six replicates for each treatment. The
diainbeis containing the eighteen plants were arranged in six rows in a wading pool: the
01 ientaiioil of the rows as perpendicular to a north-south axis. The . amplifolius and
Utricul aria vul aris plants which remained after the initial processing was completed
v cre obviously not homogeneous with respect to length. Relatively short plants (3 for .
amplifolius . (i for Ut, icularia ) were scpaiatecl from the other plants and placed in the
southein most row(s). The plants in these subsets were randomly assigned to chambers
within and between i.ows. After the chambers were in place. the assigned number of
adult eevils (from Glen Lake) weie placed in the chambers and the chambers were
cal)pC(l. The trcatmcnts were assigiied at rancloni to chanibeis within a row. For each
native plant species feeding trial, three chambers ofM. spicatum with 4 adult weevils in
each weie also placed in the pooi for the duration of a trial to determine if weevil
moi taut)’ ‘as due to host plant oi environmental conditions in the wading pools.
All trials except three ran for 10 or II days. The Elodea experiment was ended after
days bet ause all of the weevils enclosed with Elodea were dead. The Chara experiment
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was terminated after 8 clays because under all conditions including controls (no weevils)
some of the plants were stalLing to fall apart. The Utricularia feeding experiment ran for
only 7 days because it was clear that the weevils were not affecting plants by feeding but
by knocking off the bladders. The importance of this effect could not be tcsied under the
expciimenlal design used for the feeding experiments.
At the end of each experiment, each tube was opened and the number of surviving
wecvil adults was recorded. Plants were removed from the sediment and retuined to the
lab. In son e cases, it was difficult to remove the plants from the cylinders without
breaking them. Length was difficult to measure accurately for broken plants. Plant
lcngth above and below sediment level was recorded and blotted wet weights were
dctcminincd. Plants wet-c examined under a dissecting ImcrosLope aiid the number of any
weevil eggs and larvae wet-c counted. Plants weic again examined leaf by leaf and flCW
(relative to initial) leaf and stein damage was recorded. Plants were dried > 4 days at
60°C, and dry weights for shoots and roots recorded.
Due to the breakage of plants. two analyses were pci-formed on the length data for all
species: I ) for all plants in a treatilleilt (11=6) and 2) for all intact plants in a treatment.
Unless othei usc statcd, avemage length data reported are for intact plants. The data were
analysed using ANOVA: differences among treatments were compamed using Tukey’s
HSD test.
Results
The numbcm of intact plants by treatment for eaLh species is presented in Table 18.
With the exception of M. sibimicum , there does not appear to be any pattein of broken
plants with i-espcct to weevil tmeatmemmt. Tile mean numbeis of adult weevils surviving on
native species andM. spicatum for each experiment are presented ill Table 19. The mean
number of adult eeviIs surviving on M. spicatum was always higher than that for native
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plain spe ics. The lowest weevil survivorship observed oii the M. spicatum controls was
447c ( Ccratophvllum trial); survivorship was usually 67% or greater for these controls.
On the oilier hand, weevil survivorship on the native species was usually less than 25%.
The exccption was weevil survivorship on M. sibiricum which ranged froiii 46% in the 4
weevil I leatilient to 5X% in the 2 weevil ti-eatment. No weevils survived in the Elodea
and Hctcranthcra trials. No eggs or larvae were found on any of the macrophyre species.
The responses of eaLh inaLrophytc species to weevils except Naias are presented
below. Najas dctcrioiated during the experiment so the results of this tiial will not be
discussed. In the icsults (liscuSsed below the numbers of intact plants used to determine
length changes was variable (see Table la); for change in wet weight n=6 in each case.
CeiatophvUum : There were no significaiit differences among treatments for either
change in length or weight (Figures 30&3 I). Average change in plant length ranged
from 1.710 3.2 ciii for the three treatments. Aveiage change in plain wet weight ranged
fiom 0.37 to 0.55 g. There were no significaiit differences in the average number of new
branches (3.7-4.5 per plain) pioduced by the plants.
Char 1 i . There were no significant differences among treatments for either change in
length or weight (Figures 30&3 I). Average change in plant length ranged from 0.05 to
0.32 cm for the three treatments. Average change iii plant wet weight ranged from 0.00
to 0.07 g. The average number of whorls added per l)luflt ranged from 0. 1 7 to 0.50
s horls for the three ticalinents.
Ekxlca There weie no significant differences uirioiig treatments for either change in
length or weight (Figures 30&3 I ). Average change in plant length ranged from 0.0 to
0.50 cm for the three tieatnments. Average change in plant wet weight ranged from
-tL0U toO. l4M g. All but two plants produced new branches. The average number of
iie branches per plant was significantly higher (p<0.023) in the 4-weevil treatment than
in the Lontrol: the dilieremiLe between the 2-weevil treatment and the control was
marginally significant (p<0i)53). There was no grazing damage seen on Elodea ,
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although leaves were missing from three plants in the 2-weevil treatment and one plant in
the 4—weevil ticatmnent.
1-leretanthera : There were no significant differences among treatments for either
chaimgc in length or weight (Figures 30&3 I). Average change in plant length rangcd
fuoiii 0i 17 to I .3 () cm for the three treatments. Average change in plant wet weight
munged horn ( ).34 to O. 4 30g. Theie was some length loss recorded in all treatments due
to loss of the longest leaf. However, these losses were similar among treatments. All
plants ackied leaves. l’he mean number of new leaves per treatment ranged from II to
143 leavcs.
Mcgalodoiita . There weie nO significant differences among treutinent for either
change in length or weight (Figuies 3U&3 I). Average change in plant length ranged
from I .56 to I .83 cm for the three treatments. Average change in plant wet weight
ranged from 0.083 to 0.957 g. Some plants lost weight because some rool whorls broke
off in the sediments. AU plants added leaf whorls during the experiment (mean number
of whorls 1 tckled - control: 4.5: 2-weevil treatment: 4.7: 4—weevil treatment: 4.5), and the
numbet of new whorls did not diffei unlong Ireatn ents.
M. sibmi icuin : M. sibiricum did not grow well under these conditions and many plants
weic broken (Table I 8). When broken plains ere included in the analysis, mean ( 1
SE.) cinmuige in plant length was +0.667 (± 1.376) cm for the control, -3.750 (±2.670)
cm for the 2-weevil treatment and —6 333 (+ I .470) cm for the 4-weevil treatment. There
eic no significauit differences amoiig tieatments, although the difference between the
control and the 4—weevil treatment was marginally significant (p<0.053). When broken
plamits wcic excluded from the analysis the mean changes in length for the three
tueatuncnts weuc as follows, control ÷2.0() a 1.67) cm. 2—weevil treatment +1.17
(± 0.38). and 4-weevil treatment —3.0() (± 3.(J0) cm (Figwe 30). The differences among
Imeatments for change in length of intact plants were not significant either. Average
change in plant cight ranged from ft095 g in the 2-weevil treatment (n=4) to 0.248 g in
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the U-weevil treatiiient the differences among treatments were not significant. Plants
without eevils added more leaves than plants with weevils. There was leaf loss at the
top of some plants, and damage to apical meristems in both weevil treatments. All
treatmcnts plus the M. spicatum controls were covered by variably thick cpiphytic algae
in this experiment. Plants with weevils were more likely to be covered with algae and
broken
B amphfolws : There were no significant differences among treatnients for either
change in length or weight (Figures 30&3 1). Aveiage change in plant length ranged
from 0.5 1) to 1.15 ciii foi the three treatments. Average change in wet weight ranged
froin0.6 4 to 1.17 g. There were no significant diffeiences between the row of short
plants and the othci five rows for any of the variables measuied. On average, plants
added 3 to 4 leaves. Nine of the plants added runners. There were no significant
differences for eithei the nuniber of leaves or runners added among the three treatments.
Un iculat ia: Un icularia had the highest growth rates under these conditions. There
weic no significant differences in increase in length among the treatments (Figure 30).
Average change in plant length ranged from 3.30 to 3.9 cm. There appeared to be an
effect of weevils on plant weight. l-Iowever, there was considerable variability within all
treatments so these differences were not statistically significant. Average changes in wet
weight for the thrcc tieatments were as follows: 0-weevil tmeatmcnt +0.33 g, 2-weevil
trcatiiient -0.59 g, 4-weevil tieatmcnt -0.39 g. Most of the weight loss in the weevil
ticati1 ents appeared to be clue to the loss of bladders. The bladders eie not counted at
the beginning of the experiment, so we could not quantify bladder loss. There was also
some loss of bladders iii the 0-weevil treatment: half of the plants in the 0-weevil
n eatmcut lust weight despite then increase in length. There were no significant
diffeicilLes bci ecii the two rows of short plants and the rest of the plains for any of the
variables quantified.
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Vallisneiia : There were no significaiit differences among treatments iii change in
plant length or weight (Figures 30&3 I). Average change in plant length ranged from
0 017 to 0.51 7 cm. Average change iii wet weight ranged from 1.04 to I .50 g. Most
plants in all treatuiieiits had new leaves, averaging 0. to 1.5 new leaves per plant. In one
0-weevil replicate there was a damaged leaf, likely due to handling. Two plants, one
from each of (lie il treatments, lost length. One of these plants had a scar and the
other one had no visible scar. A number of the plants in all treatnients showed signs of
chlorosis hicli may have been due to handling during the initial processing. The
Vallisiteria plants weuc covered with Amnicola eggs. To remove the eggs, plants were
cxainuiied under lights, and eggs removed with forceps. This process resulted in some
desiccated sections of the leaves, and some tears and scars. Later, the desiccated areas
becanie chlorotic. If chlorotic plants are removed from the analysis, mean change in
plant lcuigth raiigcd from 0.35 to I . 15 ciii; mean change in wet weight ranged from 1.40
to I .7S g Again. there were no significant differences among treatments.
l)iscussiun
Adult weevils did not have a significant effect on the rowtli of any of the muerophyte
species tested . The only noticeable effects of weevils were on M. siburicum length and
L’tricul iria weight. Mean M. sibirmeum length decreased with increasing weevil density.
l’hcre was, however, no significant impact of weevils on M. sibiricum weight. These
rcstilts arc similar to those obtained iii a study conducted at Browniugton Pond in which
. Iccoiitci adults (11(1 not have a igiiiiicaiit, negative effect on M. sibiricum length or
cmght (Creed and Sheldon 1992). Weevils did remove a significant number of leaves in
that experiment, though. The effect of weevils on Utricularia weight appeared to be due
to the loss of bladders on plants with weevils, however the differences were not
significant. Also. theme was no significant effect of weevils on Utricularia length. The
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most commonly found plant damage was due to effects of pre-experiment processing.
For cxamplc, there was desiccation of some leaf margins on . amplifolius . None of this
damage was more common on the plants enclosed with weevils than controls.
At the end of an cxpciiment weevils were classified as either alive, dead, or missing.
Many weevils (148 out of a total of 324 weevils) were not found at the end of the
experiments. In this discussion, only the weevils found alive iii the tubes were used to
calculate survivoi ship. Thus the values represent the lowest possible survivorship.
assuming that all of the missing weevils were dead. The highest survivorship was on the
fri. spicatum controk. The highest survivorship on a native plant was on M. sibiricum .
Thcue was extcnsive weevil moitality in the experiments. No weevils survived at either
density on either Elodea or Heteranthera . Many more (lead weevils (128 of 324) were
found associated with the native plants. If weevils on M. sibiricum are not considered,
this becomes I 28 of 288. In contrast, of the 108 weevils placed in theM. spicaturn
controls (not including theE. amplifohius trial) only I was dead and 24 were missing.
Overall. adult wee il survivorship on non-target plants was 15%. 1CM. sihiiicum is
ex ludcd fi om this group. the overall survivorship on native plants was 11%. compared
to an avei age suivivorship of 75% on M. spicatum . Foity-five percent of the weevils
could not be found. There aie a number of possible explanations for their absence: I)
they weie not put in the chambers in the first place, 2) they found some way to escape, or
3) they died, fell into the sediment and decomposed. lii four sets of the fri spicatum
controls all of the weevils ere found alive. In the othei four trials, on average 4.25
weevils of the original 12 weie missing. As we have rarely found adult mortality at this
rate on . spicatuni . it seems likely that at least some of these weevils may have escaped
or been ovci looked.
lii condusion, the macmophyte in Vet mont which appears to be most vulnerable toE.
lccc’ntei is the native watermilfoil, M. sibiricum . We have found E. lccontei and weevil
damage on this plant in the field, but the weevils (ho not appeau to have a significant
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negatI .e effect oui the plant. At one location, Inrnaii Pond in Fair Haven, Vt, it appeared
that i. sibiricum responds to weevil damage by increasing the number of lateral
branches.
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WEEVIL INTROI)UCTIONS
Norton Brook Pond
Site Description
Norton Brook Pond is a small (< hectare) impoundment in Bristol township. Vt. M.
picatum as hi si identified in the lake in I 9 5, and currently is the dominant (percent
co eu, biotnass, pers. obs.. and H. Cuosson pers. comm.) macrophyte in the lake. No
otliem submerged plant species were seen. M. spicatum ringed the impoundment.
Materials and Methods
Befome weevils ci-e intuoduced, invertcbiates on transects were collected to determine
whether . lecoutci us already present iii ihe lake.
Cylindiicalenclosuues were used for weevil additLoj . The 30.5 cm diameter, 2.5m tall
enclosures wete ouistructcd from impeimeablc 4um polyethylene sheeting held open by
external rungs. The tops and bottoms of the enclosures were held open by approximately
cm tall PVC rings. The tops of the enclosures were covered with 200um Nitex mesh,
and ere held at the waler surface by floats. Six enclosures were placed in a line running
north to south on the 2. I iii depth contour. Thc enclosures were placed over cleiise M.
spkatum . and the bottom ting was pushed into the lake sediment enclosing 730 cm2 of
sedmineiit. Fifty adult . letontci weie placccl n every other enclosure. No weevils were
J(kled to controls. Enclosuies were examined weekly. and dissolved oxygen was
mcasuued at mid-day at the bottom of the enclosures thu-ee times over the otirse of the
experiment with i clissolvcd oxygen meter.
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After 36 da) s in situ, a diver went to the bottom of each enclosure, cut all plains at
scdiincnt Icvel, clainpcd a 200uni mesh sieve to the bottom of the enclosure, and all of
the mate, ial was brought to the surface. All plants and invertebrates were washed into
scalable bags and prcservcd in 70% ethanol. ‘ro quantify the cffects of enclosures, three
similai samples weic also taken under ambient conditions at haphazard locations between
enclosure sites on the 2. I depth contour on the day the enclosures wcre removed.
At the lab. plants wete examined. Met isten is were iemoved and examined under a
dissecting mictoscopc (7-l5X m agnificution) for weevil eggs and early inslar larvae. All
niacroiiivertebiates were removed, identified and enumerated. Plants were placed in a
drying oven for >4 clays at X0°C, and dry weights recorded. The data were analyzed
using A NOVA and the treatirlent means were compaiccl using Tukey’s HSD test.
Results and I)iscussion
There cie no . lecontei found in preliminary samples in Norton Biook Pond.
Weevil endosuics had significantly lower M. spicatum biounass compared to control
enclosuies (p=0.007) and to open water (p=0.043) (Figure 32). This appears to be a
weevil effect as therc were no significant differences in M. spicatuin dry biomass
between t.otitrols and open lake. There were also visual differences in the position of the
plants in the waler column between weevil enclosuics and controls when the endosure
tops were removed. In all control endosures M. spicatum fom med a canopy on the
surface. as it did in the open water. In the weevil enclosures there weme no plants at the
surta e in any of the weevil enclosures: the plants were approximately one meter below
the suriate. No plant species other than M. spI atum were found in any of the samples.
There weie fc ’ diffeiences in inveriebiate species and abundance in enclosumes
compared to open water. There were no significaiit differences in larger, more benthic
ina roin eriebiatcs such as mayflies. caclclisflies. mites and diironomids. There were
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significantly more mucrozooplunk ton (Cladocerans and Copopods) in enclosures
compared to open water (p <0.02). This is not surprizing as there were no planktivores
in the cndosures. There were no differences in overall taxa richness, or organism
abundance. excluding zooplaiikton, among the enclosures, and open water (Figui-e 33).
Dissolved oxygen did not differ between enclosures with and without weevils. Mean
(± I S.E.) dissolved oxygen concerinations at the bottom of the enclosures averaged 8.28
± 0 37 iiig/l. extept on day 6 when one control had a dissolved oxygen concentration of
1.18 ing/l iid on c l ii )’ 36 lien one weevil enclosure had a dissolved oxygen
concentiat ion of 1.46 mg/I. On day 36, dissolved oxygen at the bottom was somewhat
lo er thaii under ambient conditions, 7.1 ± 1.29 mg/i. in enclosures coinpated to 8.5%
mgIl outside tile enclosures. On day 36, water temperatures at sediment level and at the
surface weic 21.5 and 21.9. respectively.
There wcie very few weevil eggs. larvae and pupae found in the enclosures. There
were 6 eggs found iii one cnclosure (5 eggs on one meristem and I on another) and I
pupa found in another enclosure. No larv 1 ie were found. It is uncledr why there was
apparently so little weevil reproduction. Giveti the longevity of the weevils seen in the
ith imis possible that the adults found in the enclosures, which averaged 3() (±2.64)
iepresent survivorship of the original 50 adults placed in each enclosure. It is possible
that the apparent lack of successful reproduction was due to low dissolved oxygen. We
mcd uied dissolved oxygen during the day. at the bottom of tile enclosure. We
anticip ited that dissolved oxygen would be lowest at sediment level both because low
light intensities would minimize oxygen from photosynthesis, and because
decomposition of oigaiimc matter would (leLiease tile available oxygen. On the three
(fates that we quantified dissolved oxygen, the concentrat ions were usually high.
Fuithci mom-c. thcie was no significant cifect of the emiLlosures on othci niacioinvei tebrate
tixa associatcd s ith watcimilfoil.
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One adult • . lecontei was collected in an open waler sample. Given that there was
none of the charaueristic . lecoutci damage found on M. spicalum plants in any area in
the luke, it is likely that this was a result of sample contamination. When we took the
open watcr samples, we reused one of the enclosures we had just removed. Unlike the
Super Sunipler. which is used in the field for brief periods of time and dried between
uses. thc cmiclusurcs remained in the lake for 36 days. The enclosures were encrusted
with pci ipliyloii making the inside of the tube less smooth, therefore increasing the
likelihood of happing a weevil in the sampler.
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Van Vieck’s Pond
Site I)escriptiun
VanVlcek’s Pond is a small man-made pond (approximately 0.7 hectares) located in
Cornwall, Vt. It hits a cirLumfelence of approximately 240 m. Mean depth is 2.3 in. It
has a small sticam inflow and an outflow pipe. The pond was built with a drain hole in
the NE corner.
Materials and Methods
We divided VaiiVleck’s Pond into eight shoreline sections (30 m of shoreline each)
and one center seition using permanent stakes and removable ropes. Thiee corner
sections were selected for introduction sites. The NE corner whete the drain is located
was not used.
To detcimiiic if eevils were present prior to the introductions, the three introduction
sections weic syslcniatically sampled immediately prior to the weevil intioduction. In
each section. two (2 in apait) transects parallel to shore were swum by snorkelers. On
ea .h ransect, ten milfoil meristems (top 50 ciii) were collected. Of these, five meristems
had signs of in crtebrate daniuge and five were undamaged Three (3 in apart) transects
m uniting nomih to south weie also sampled in the middle of the pond. Overall, nine
tiahisCcts were sampled, with a total of 90 ilieristenis collected.
Wcevils were intl odticed on the same day as the pre—inuoduction sampling, 9 July
1992. Wecsils eie added to three sites using three different stiategies. At site A (the
SE coiiici . 5 (1 adult weevils ‘ere added cii the first day. Fifty mote adults were added
c ciy othci week until 19 August 1992. At site B (the SW corner), 50 adult weevils
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weic added on this date only. At site C (the NW corner), 50 adult weevils were added
with approximately 5() eggs and larvae on this first day only.
All introcluctioii sites were examined on 19 August 1992, Day 41 after the
iiitioductioii. Obsei-vations of weevil presence were recorded but no saniples were
cot ICL ted.
We conducted e Lensive saiiipling of the entire pond on 25 August 1992, Day 47 after
the Intl oduction. Collection transccts were made in all portions of the pond. Two (site
B) or ihiec ( ilcs A and C) transects were taken at each of the three introduction sites.
One Ii aiisccl was taken in cacti of the oilier five pond sections. Three transects were
taken in the pond center as described above. On each liansect, fifteen meristems (top 50
ciii) were collcctcd. Of these, five meristems had signs of invercebi ate damage, five were
utidainaged. and five were flowering meristems. (Flowering meristems erc not
amplcd in the pre-intioduciory sampling because they were not present. We added them
10 the sampling iegiiue to be cci lain we wcrc not overlooking any possible weevil
habitat.) We also collected five mci istems (top 50 Ciii) from some of the floating
watciiuilfoil fioni cacti of the shoreline sections. In total, sixteen transects of rooted
wateiinilfoil wcie sampled and 240 attached meristems collected. Forty meristems from
floating atermilloil in were collected.
Results
In pie—introduction sampling on Day 0 (9 July 1992), we collected one utiideuiifiable
weevil Lii ‘ia fioni one of the center transects. Because we collected 90 meristems and
one weevil. this suggests a pre-intioduction weevil density of 0.() II weevils per
hid istein.
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Obseivatioiiat data from Day4 1 after introduction indicated oiie weevil pupal chamber
and some larval damage at site A, possible larval damage at site B, and 3 adult E.
lecontei and a fair amount of larval damage at site C. No eggs were seen at any site.
In extensive siinpliiig on Day 47 (25 August 1992), e collected 27 weevils (seven
adults, one pupa, eighteen larvae, and one egg). Bused on 240 collected meristems. we
found a final weevil dciisity of 0. 113 weevils per meristem for the entire pond. The
highest iiuunbers of weevils were collected near sites C and A. By examining these two
sites indepeiidcnt of the rest of the pond. weevil densities in the weevil trai1 ects at these
two sues cre 0.29 (site C) and 0.20 (site A) weevils per meristem. No weevils were
collected horn site B.
A total of five wees ils (one adult and four larvae) were collected from six non-
intioduction sites Five wce ils collected on 12(1 meristems suggests a weevil density on
Day 47 of 0.042 weevils per meristem. This density is higher than that initially measured
in the pond. No weevils were collected from floating milfoil pieces.
I)iscussion
We collected twelve larvae at introduction site C. Weevil eggs and larvae under
luborutoi y conditions need approximately 30 days at 21 . 1°C to go from egg to adult.
Mean sui face tenlperatuie on Day 47 after the introductions was 27.4oC. At 30 cm fi-om
surface, water tempciatuie was 25.50C. This suggests that the larvae collected on Day
47 were not the same lai vae we placed in the pond.
Based on the laige total numbei of weevils (13) collected from site C (tile site with a
single intioduclion ci 100 eggs, laivae, and adults), this type of mixed life stage
intiocluction appears to be most productive. While we did collect nine weevils from site
A. this site had biweekly intioduuions of 50 adult weevils, or 20(1 weevils in the
summer.
-------
Oveiull. the density of weevils throughout the pond appears to be increasing.
Excluding the three introduitioii sites, our final sampling suggested a four fold increase
in the number of weevils per meristern in the pond.
69
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Betuuniey’s Poiid
Site Description
Betourney’s Pond is an oblong, man-iiuide pond located in Vest Salisbury. Vt. It is
small, meisui lug only 12 ni across at its widest point, and only 24 in long. It has a mean
depth of 1.5 in and a maximum depth of 2.1 in.
Materials and Methods
We divided Betourney’s Pond into cighi shoreline sections (9 in of shoreline each) and
one center section. The SE corncr section was selected as an introduction site. This
cornci was fairly shallow (mean depth of 0.75 in) and had M. spicatum growing to the
surface.
The entire pond was systematically saiiipled immediately prior to the weevil
lilt, oduction to detci mine pre-inti oduction weevil abundance. Snorkelers swam two
parallel transects along the long axes and two transects along the short axes of the pond.
Along the long axis, for each transect, ten milfoil ilieristems (top 50 ciii) were collected.
Along the short axis, eight inilfoil nieristems vcic collected pei transect. For each
tianscet, half of the total meristems collected had sigils of invertebrate damage, the other
half of the collection was undamaged nlcrislems. In total, foui transects were sampled,
with a total ot 36 mci Istems collec ted.
Vcevils wcie introduced on the same day as the pre-iinioduction sampling, 30 July
I 992. One hundied weevil larvae (with a small number of eggs and pupae) weme added
to the shallow, south end of the pond. Fifty weevils (both adults and lzii vae) were adtlcd
on 13 August 1992, fouitecn days after the initial introduction.
70
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The pond was examined on 19 August 1992, twenty days after the introduction. No
wccvils or weevil damage were observed.
Wc conducted extensive sainpluiig of the entire pond on 26 August 1992, 27 days after
the intiocluction. The same four transects were sampled. Three transects were also taken
parallel to the shore at the intioduction site. As with the pre-introductory sampling, ten
met istenis cre collected in each of the long axis tiansects, eight meristems were
collected iii the short axis transects. Ten meristems were also collected in each of the
neai-shoi-e ti-au ects taken at the introduction site.
Results
In pre-intioductioii sampling, no weevils were collected.
in extensive observations on Day 20 after the introduction, we found no weevils and
110 evidence of weevils.
In extensive sampling on Day 27 (26 August 1992), we collected no weevils. Within
oui collection, we found seven plants (Out of 66 collected plants) that had signs of adult
eevil damage to the leaves.
Water temperature on the day of introduction was 22.50C.
I)iscus iun
The fate of the weevils in Bctourney’s Pond remains a mystery. On the final collection
day. we noted that illost of the watermilfoil in the center of the pond was cut up and iio
longer rooted to the bottom. While we have been assured by the property owner that the
pond i emainecl untouched, it appeared that much of the watermilfoil had been
“haivcstecl” or cut at tile bottom by sonic means. Othei invertebrates appear to have
survived in the pond.
71
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COMMUNICATION OF RESEARCH RESULTS WITH
PUBLIC GROUPS, STA’I’E ANI) FEI)ERAL AGENCWS,
AND AT SCIENTIFIC MEETIN(;S
Public Guoups
S. Sheldon lndclc a presentation on our research to an audience comprised of state and
federal aqualuL plant managers and the general public in Minneapolis. Minn. (sponsored
by the University of Minnesota). R. Creed also presented a seminar on the research to a
wctlaiids cLology class in the Fisheries and Wildlife Dept. at the University of Vermont.
R. Creed m ude an informal presentat ion of research results to the boaid of directors of
the Lake Bomoseen Assoiiation and Castleton town officials. The purpose of this
meeting was to discuss the impending drawdown on Lake Bomoseen and its potential
mmpa ts on the weevil research. The slide show on the prospect of biological control of
Eurasian watemniilfoil was not updated this year. A woikshop for the general public was
held at Middlebury College during the summer. Information about the reseafth has also
been made available to the public in the Vt. Department of Environmental Conservations’
biannual newsletter. We have also responded to numerous queries from the public
regarding the research (pi imarily in the form of phone calls) and have sent out materials
clesci ibung the project when they aic requested.
State and Fedcmal Agencies
lii November. S. Sheldon and R. Ci-eecl attended the annual Aquatic Plant Control
RescaiLli Puogiam (APCRP) nicetings sponsored by the Aimy Corps of Engineers in
Bcllevue, Washington. The results of rese irch conducted at Brownington Pond and at
Middlebuuy College weie presented by S. Sheldon. S. Sheldon gave an informal
72
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pi escuitat ion to i lie Vermont Eurasian Watermi I foil Study Committee which is charged
with foiiuulatiiig an aquatic herbicide use policy in Vermont.
R. Cuecd presented a paper on recent research results at the annual New England
Association of Envuionmental Biologists meetings in Meridun. Couiui. iii March 1993.
Scientific Meetiuigs
S. Sheldon attended a woikshop at the annual meetings of the Aquatic Plant
Managenictit Society iii Daytona, Florida. She also wrote a paper on the distribution of
exotic aqihutic plants iii New England for this woikshop. S. Sheldon also attended a
symposium omi biologiLal contiol in Minnesota and presented a paper on biological
couit;ol of aquatic niucrophytes (sponsoued by the University of Minnesota). S. Sheldon
ga c a talk at the annual meeting of the New England Botanical Society in Boston.
73
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SUMMARY IMSCUSSION
The iesearch undei taken during 1992 addressed five of the six pi imary objectives
proposed lot thus puopect (Table 20). Considerable progress was made at the
Buo ningtoii Pond (BP) field site wlieie we are examining the watermilfoil decline
(Objective I). We continued the plain, invertebrate and fish surveys begun in 1990. We
ako continued monitoring water chemistry and temperature and obtained usable sediment
samples. Laboia tory experiments documented strong ulegative effects of herbivores ( . .
lecontel and A. nivca ) on wateruiiilfoil. The herbivore exclusion expcriment
dcuuonstiuted that weevils can have strong effects on established watermilfoil plants.
The second fish exclusion experiment also demonstiated that yellow perch have little
direct cffcct on the abundance of weevils. Once again, we did not work with Parapoynx
as this species was still rare on watermilfoil in BP. While we have not yet demonstrated
the cause of the decline at Brownington Pond the results of the 1992 field season lend
further suppout to the hypothesis that Euhrychiopsis , auid possibly other herbivores such
as the Aceiitu Ia . played an iuipoi tant role in both watermilfoil declines.
Rescauch conducted at both Biownington Pond and at Middlebury (M) examined the
effect of hcrbuvoies oil watermilfoil and native macrophytes (Objective 2). Strong
effects of Acentria and Euhrvchiopsis on M. spicatum were observed in the BP
experiment. Weevil larvae did not have as strong art effect on M. spucatum as Acentria,
howcvcr,bw rowing by larval weevilsp1 y _ çpj o1bute _ the _ mQst t the icduciiou in
Eurasian wutcrmilfoil buoyancy. Weevils weie not very comnrnn on native mucrophytes
in BP in 1991 and 1992. Weevils had no significant effect on native macrophytes in
wacluuig pool cxpeiiments coiicluctcd at Middlebury.
Weevils veie successfully introduced into Norton Brook Pond and Van Vleck’s Pond
(Objective 3). We have had continued success at maintaining a small culture of . .
74
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Iceontet at Middlebury and documenting their life history. Successful culturing is
importailt if we are to undcitake controlled introductions iii the future.
Ouu- data from L. Bomoseen indic ite that this lake already supports a population of the
weevil . lecoutci which suggests that a natural eeviI introduction has already begun
(Objective 4). In 1991. the weevils had begun to reduce watermilfoil abundance in
certain paiLs of the Iukc (Objective 5). Weevil abundances were much lower in Lake
Bomo ccii in 1992. As mentioned pieviously (Creed and Sheldon 1992). extensive
h iResting could pievent this wecvil population from expanding and affecting
e ! ! throughout the lake. We have had considerable difficulty in influencing
waieiinilfoil management in Lake Bomoseen. The severity of the watermilfoil pioblern (
in the lake makcs lakeshore pioperty owners reluctant to suspend wateiinilfoil control
acrivitic bug enough for a weevil populatioii to become established.
We have presented the results of our woik to public gu-oups and at scientific meetings.
We have also picpaied a slide show on watermilfoil control that is available to the public
ilitougli the Vermont DEC (Objective 6).
FindIly, a list of equipment purchased on thc grant from June 1992 to Api 111993 is
presented in Table 21.
75
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Acknowledgments We wish to thank Kristin HenMiaw, Diana Cheek and Gibe Cries
for their invaluable help at Brownington Pond in 1992. Chris Alessi, Sara August, Ruth
Kelly. Kit van Wagner, Joel Gerwin, Jim Rodda, Lad Racha, Susan Lardner and Rennie
Peddue helped with all of the Muddlebury research. We are also grateful to the relentless
bugpickeus without whose dedication much of these data could not be presented! This
work was funded by the EPA Clean Lakes Demonstration Program. the US Army Corps
of Enginceis. the Vermont Department of Environmental Conservation and Middlebury
College.
76
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LiI’ERATURE CIi’EI)
Aiken, S.G., P.R. Newroth and I. Wile. 1979. The biology of Canadian weeds. 34.
Myriophyllum spicarum L. Can. J. Plant Sci. 59:201-215.
Anonymous. 1990. Eurasian watermilfoil in northern latitudes: results of management
workshop. The Freshwater Foundation and Minnesota Department of Natural
Resources. Nuvarre, Minnesota.
B iylcy. S., Ft Rabin anclC.H. Southwick. 1968. Reccnt decline in the distribution and
abundance of Eurasian milfoil in Chesapeake Bay. Ches. Sci. 9:173-181.
Bcgon. M.. and M. Mortimer. 1981. Population ecology: a unified study of animals and
plants Sinauei Associates, Inc., Sundei lund. Mass.
Couch. R.. and E. Nelson. 1986. Mvriophvllum spicatum in North America. In The First
Inici national Symposium on Watermilfoil ( Mvriophvllum spicatum ) and Related
I-lu lorugaccac Species. pp. 8- 18.
Creed, R.P.. ii.. and S.P. Sheldon. 199 Ia. Thc potential for biological control of
Euiasian watermilfoil ( Mvriophvlluin spicatum) : Results of the Research
Programs initiated in 1990. Prepared for Region I, U.S. EPA, Boston, Mass.
Creed. R.P.. Jr., and S.P. Sheldon. 19) lb. The potential for biological control of
Eurasian watermilfoil ( Mvnophvllum spicalum) . Results of Brownington Pond.
Vennont, study and multi-state lake survey. In: The Proceedings of the 25th
Annual Meeting of the Aquatic Plant Contiol Research Program. Miscellaneous
Pupei i\-9l-3, Waterways Experiment Station, Vicksburg, Mississippi. pp. 183-
193.
Creed. R.P., Ji., and S.P. Sheldon. 1992. l’he potential foi biological control of
Eurasiaui watermilfoil ( Myriophyllum spicatum) : Results of the research
programs conducted in 1991. Prepared for Region I, U.S. EPA, Boston, Mass.
(unpubl.) 197 pp.
Ciced. R.P., Jr.. S.P. Sheldon and D.M. Cheek. 1992. The effect of herbivore feeding
on the buoyancy of Eurasian wateiniilfoil. J. Aquat. Plant Manage. 30 75-76.
Krebs. Ci. 1985. Ecology. the experimental analysis of distribution and abundance.
3rd edition, Harper and Row. Publishers. New York. N.Y.
Nichols. S.A.. and Ci. Cottani. 1972. Harvesting as a control for aquatic plants. Water
Rcs. Bull. 8:1205-1210.
77
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Nichols. S.A.. and B.H. Shaw. l9 6. Ecological lifc histories of the three aquatic
nuisdnce plants, Mynophyllum spicatum. Potamogeton crispus and Elodea
can 1 idensis . Hydrobiol. 131:3-21.
Painter. D.S.. and K.J. McCabe. I9HH. Investigation iiito the disappearance of Eurasian
watermilfoul from the Kawartha Lakes, Canada. J. Aquat. Plant Manage. 26:3-12.
Reed. C.F. 1977. 1-listory and disuubution of Eurasian watermilfoil in United States and
Canada. Pliyiologia 36:417-436.
Sokal. R.R., and F.J. Rohlf. l9 l. Biomeiry. 2nd Ed. W.H. Freeman and Co., New
Youk. N.Y.
Spcntcr. N.R.. and M. Lekic. 1974. Prospects for biologicdl .oiitrol of Eurasian
w teim,lfoil. Weed SLI. 2:4W-404.
Wetzel. R.G. lY 3. Limnology, 2nd Edition. Saunders College Publishing. New York.
N Y
0
7
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TABLES
79
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Table I. Results of the analysis for sediments collected
from five sites in Brownington Pond. Values in the table
are means (± I S.E.) . The units for the sediment extraction
samples are mgIg; the units for the interstitial water
samples mg/i; the units for sediment density are g/ml.
Variable
Natives
Site
South
Bed
South
Shallow
West
Bed
West
Shallow
Sediment
Extractions
ab b ab a ab
Exch. NI-i 4 0.099 0.034 0.056 0.134 0.069
(0.014) (0.016) (0.017) (0.035) (0.011)
a a a a a
Exch. K 0.074 0.050 0.088 0.109 0.075
(0.027) (0.017) (0.032) (0.008) (0.017)
Avail ibIe a a a a a
0.147 0.131 0.145 0.173 O.J62
(0.013) (0.003) (0.032) (0.016) (0.020)
Inter st it i a 1
Water
a b a ab
N1-14-N 2.88 0.68 1.17 3.16 1.33
(0.38) (0.20) (0.22) (0.90) (0.20)
a a a a a
SRP 0.010 0.006 0.006 0.031 0.007
(0.003) (0.003) (0.004) (0.010) (0.001)
a a a a a
Fe 0.24 0.35 0.63 0.45 0.17
(0.13) (0.17) (0.13) (0.04) (0.02)
-------
Table Continued .
Sediment a a a a a
Density 0.057 0.059 0.070 0.073 0.069
(0.009) (0.003) (0.015) (0.004) (0.004)
%Organic a a a a a
Mcitter 48.28 41.26 39.46 35.46 41.63
(0.41) (1.47) (6.07) (1.05) (0.87)
-------
Table 2. Dominant macroinvertebrate taxa associated with .
siDicatum in the South Bed in Brownington Pond in 1992.
Samples were collected using the Super Sampler (MIS) . Data
in the table are mean number (± I S.E.) of individuals per
gram (dry weight) of M. soicatum . Five samples were taken
on each date. Statistical comparisons were made using an
ANOV k with Tukey s test on log transformed data. Means with
the same letLer are not significantly diffei-ent.
Taxon Date
8/6 29/6 20/7 l0,’8
Annelida
Oligochaeta 0.4 a 3.5 ab 5.4 ab 3.9 b
(0.3 (1.5) (2.7) (1.1)
Ar L h ropodd
Amph ipoda
Flyallela 12.1 a 13.5 a 18.0 a 20.8 a
(4.5) (4.5) (7.0) (3.4)
Cladoceta 5.1 a 1.4 a 0.2 a 12.2 a
(1.9) (1.4) (0.2) (10.3)
Hydtacarina 9.0 a 7.8 a 2.2 a 11.6
(.1.8) (2.7) (0.7) (4.5)
In i cc t a
Col cop t era
EuhLychioDsis I . 8 a I . 2 a 2 . 3 a 2 . I a
(1.4) (0.7) (0.7) (0.7)
Dip t era
Chironomidae 42.5 a 74.4 a 39.1 a 14.6 a
(12.5) (34.7) (6.7) (2.9)
Ephemeropt : era
Caenis 63.3 a 48.8 ab 13.4 b 2.7 c
(13.1) (12.4) (2.0) (1.2)
Lepidoptera
Acentija 2.4 a 0.4 a 0.6 a 0.4 a
(1.0) (0.4) (0.6) (0.4)
-------
0.4
(0.2)
a
0.6
(0.5)
a
0.3
(0.1)
a
0.2
(0.2)
a
6.6
(2.J)
a
3.0
(2.0)
ab
0.2
(0.2)
b
0.8
(0.5)
b
0.0
(0.0)
a
0.0
(0.0)
a
0.9
(0.6)
ab
2.8
(1.0)
b
3.2
(2.2)
a
1.9
(1.4)
a
<0.1
(<0.1)
a
0.0
(0.0)
a
1.1
(0.6)
a
0.4
(0.4)
a
0.2
(0.1)
a
0.6
(0.6)
a
0.2
(0.2)
a
0.0
(0.0)
a
0.1
(0.1)
a
0.0
(0.0)
a
6.5
(2.2)
a
3.0
(1.7)
ab
<0.1
(<0.1)
b
0.0
(0.0)
b
Table 2 Continued .
Cidonatci
Anisoptera
Zycjoptera
Enal 1a’iin t
Ti ichopt era
Oxy e t hi ra
Oecetis
Tri enodes
Leo t oc e r c us
C eraC lea
P latyhe lininthes
P lanariidae
Iviol lusca
GcLS ti opoda
Amn ic o 1 a
Ph ’ a
Pla norbiclae
0.6 a
0.1 a 0.3 a 1.5 a
(0.5)
(0.1)
(0.3)
(0.9)
37.1
(7.2)
ab
9.7
(6.1)
c
24.4
(10.1)
bc
155.3
(60.7)
0.1
(0.1)
a
0.8
(0.4)
ab
5.6
(2.6)
bc
11.5
(4.0)
4.3
(0.5)
a
1.4
(0.9)
a
5.4
(5.1)
a
0.6
(0.4)
a
C
a
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Table 3. Donunant macroinvertebrate taxa associated with .
s icatum in the West Bed in Brownington Pond in 1992.
Samples were collected using the Super Sampler (MIS) . Data
in the table are mean number ( 1 S.E.) of individuals per
gram (dry weight) of . soicEltum . Five samples were taken
on each date. Statistical comparisons were made using an
ANOVA with Tukey’s test on log transformed data. Means with
the same letter are not significantly different.
r1 LxQ!.I Date
8/6 29/6 20/7 10/8
Anrielida
Oligochaeta 6.0 ab 13.3 a 0.9 b 8.3 a
(1.8) (1.6) (0.2) (4.1)
Arthropoda
Arnph i. poda
Hvallela 13.5 al) 8.8 b 21.6 ab 36.2 a
(4.2) (2.3) (5.1) (4.3)
Cladocera 2.6 a 1.8 a 0.0 a 0.5 a
(1.2) (1.7) (0.0) (0.3)
Hydracarina 14.6 a 10.3 a 3.2 a 11.5 a
(3.7) (4.7) (1.4) (4.0)
Insec ta
Cc .leoptera
Euhrvchioosis 1.1 a 1.8 a 3.6 a 3.0 a
(0.2) (0.6) (0.8) (0.8)
Dip t era
Chirononudae 10.6 a 5.2 a 8.7 a 1.6 a
(3.5) (1.7) (3.1) (0.4)
Ephemeroptera
Caenis 8.0 a 3.4 a 0.2 b 0.2 b
(2.5) (0.8) (0.1) (0.2)
Lep i dopI era
AcentLia 5.2 a 2.5 ab 0.4 bc 0.0 c
(1.7) (1.6) (0.2) (0.0)
-------
Table 3 Continued .
Odonata
Anisoptera 1.3 a 0.0 a 0.3 a 0.9 a
(0.6) (0.0) (0.1) (0.3)
Zygoptera
Eillcvirn i 2.1 a 1.4 a 0.3 b 0.1 b
(0.5) (0.4) (0.1) (0.1)
Trichoptera
Oxvethira 0.2 a 0.5 a 0.2 a 2.2 a
(0.2) (0.5) (0.2) (1.5)
Oecetis 2.1 a 2.3 ab 0.1 b 0.3 ab
(0.5) (1.0) (0.1) (0.2)
Tria’ nodes 0.9 a 0.8 a 0.4 a 0.0 a
(0.3) (0.5) (0.3) (0.0)
LeDtocercus 2.0 a 1.2 ab 0.0 b 0.0 b
(0.6) (0.7) (0.0) (0.0)
Ceraclea 0.7 a 0.9 a 0.1 a 0.0 a
(0.3) (0.7) (0.1) (0.0)
Platyhelminehes
P lanariidae 1.9 a 11.0 b 0.6 a 2.7 ab
(1.0) (3.0) (0.3) (0.9)
Mo Ilus c a
Ga:, t rc poda
Ainnicolt 35.4 a 42.3 a 24.2 a 100.8 b
(3.7) (6.0) (5.1) (19.2)
Plivsa 0.6 a 15.9 b 7.9 ab 24.5 b
(0.2) (5.5) (3.1) (7.3)
P lanoubidae 3.2 a 2.7 a 1.2 ab 0.7 b
(1.1) (0.9) (0.4) (0.3)
-------
Table 4. The total number of • . lecontei , . nivea and .E.
badiusalis collected in the Super Samples (both N. sDicatum
and native plants) taken in 1991 and 1992. For N. soicatum
n=38 in 1991 and n 4O in 1992; for .E. amolifolius n=11 in
1991 and n=16 in 1992; for jj. dubia n=lJ. in 1991 and n=12 in
1992.
Euhrvchioosis lecontei
2. amrlifolius ff. dubia N. soicaturn
1991 2 296
1992 2 188
Acentria nivea
F. amolifolius H. dubia . soicatum
1991 20 0 352
1992 12 0 72
Paraoovnx badiusalis
2. alnrDllfolius li• dubia N. soicatum
1991 72 11 23
1992 85 16 3
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Table 5A. Dominant macroinvertebrate taxa associated with
long N. soicatum stems from the South Bed in Brownington
Pond (9 June - 14 July, 1992) . Samples were collected using
the smaller MIS sampler (minisampler) . Data in the table
are mean nuiuber of individuals per stem ( I S.E.) . Three
samples were taken on each date.
Taxon Date
9/6 16/6 23/6 30/6 7/7 14/7
Annelida
Oligochaeta 1.0 12.3 4.3 40.3 41.3 49.3
(0.6) (12.3) (0.3) (16.3) (11.9) (5.2)
Art hr opoda
Amph 1 pC)da
H\’ 1l1ela 4.0 - 1.0 3.3 4.0 1.3
(0.6) (0.6) (0.9) (2.0) (0.3)
C. ladocera 26.7 2.3 1.0
(0.9) (2.3) (0.6)
Hydiacariria 1.7 1.0 0.7 1.3 0.3 0.3
(0.3) (0.6) (0.3) (0.9) (0.3) (0.3)
tnsecta
L:oleop tet-a
EuhivchioDsis 3.7 1.3 1.7 3.3 2.7 1.7
(1.5) (1.3) (0.9) (0.7) (1.2) (0.9)
Gvrinus 0.3 0.7 - 0.3 0.3
(0.3) (0.3) (0.3) (0.3)
P . p t r a
Chironomidae 2.3 0.7 3.3 10.3 8.0 3.3
(1.5) (0.3) (1.7) (7.9) (2.5) (1.5)
Lepidoptera
Acentria 0.3 0.3 0.3 0.3 0.7 0.3
(0.3) (0.3) (0.3) (0.3) (0.7) (0.3)
Odona ta
Anisoptera
Zy go pt era
Ena llacima L.O 0.3 0.7
(1.0) (0.3) (0.3)
-------
Table . Continued .
Trichoptera
Oxvethira - 0.3 1.0
(0.3) (1.0)
Oecetis 1.3 0.3 0.3 0.7 0.3 0.3
(0.3) (0.3) (0.3) (0.7) (0.3) (0.3)
Coe1eiitera t
Hydra 31.0 78.3 4.3
(16.7) (19.8) (1.9)
Platyhelminthes
Planariidae 0.3 - 0.7 2.0 2.3 3.7
(0.3) (0.3) (0.6) (1.2) (1.5)
Mol lusca
Gastropoda
Arnnicola 6.7 2.0 1.3 1.7 5.0 6.0
(2.2) (1.0) (0.9) (0.9) 0.6) (1.0)
Phv a 0.3 0.3
(0.3) (0.3)
Pl.ariorbiclae 0.7 - 0.3 1.0 0.7 0.7
(0.3) (0.3) (0.6) (0.3) (0.7)
-------
Table 5B. Dominant macroinvertebrate taxa associated with
long N. soicatum stems from the South Bed in Brownington
Pond (21 July - 25 August, 1992) . Samples were collected
using the smaller MIS sampler. Data in the table are mean
number of individuals per stem ( .j I S.E.). Three samples
weie taken on each date.
Taxon Date
21/7 28/7 4/8 11/8 18/8 25/8
Annel ida
Oligochaeta 73.3 87.0 63.0 9.7 12.0 5.0
(17.2) (4.0) (10.4) (4.8) (5.5) (0.6)
Arthiopoda
Arnph ipoda
Hva lle l a 4.0 6.7 0.3 - - 1.3
(1.0) (3.7) (0.3) (1.3)
Cladocera - 0.7 0.3 — 17.3 0.3
(0.7) (0.3) (5.4) (0.3)
1-lydracarina 0.7 2.3 0.7 - 0.7
(0.3) (1.5) (0.3) (0.3)
Insecta
Coleoptera
Euhrvchioosis 1.0 3.3 2.0 - 0.7 0.7
(0.6) (0.9) (1.2) (0.7) (0.3)
Gvrinus - 0.7
(0.3)
Di PLO L
Chironomiclae 2.3 1.7 1.3 1.0 2.0 1.7
(0.7) (1.2) (0.3) (0.0) (0.6) (1.2)
L ep 1 d opt era
Acentria 0.7 0.7 - 0.7 0.3
(0.3) (0.7) (0.7) (0.3)
Odorw ta
Aflisoptera
Zygopt era
Enallacirna 0.3 0.3
(0.3) (0.3)
KY
-------
Thble 5 Continued .
Tn choptera
Oxvethira 4.0 2.7 0.7 2.7 0.7
(1.2) (1.5) (0.7) (1.2) (0.3)
Cecetis 0.3 0.3 - 0.3 0.7
(0.3) (0.3) (0.3) (0.7)
Coc’ lenterata
Hvdr i
F’ Ia tyhe 1 mint he s
P1 inariidae 5.7 15.0 7.3 1.0 2.0 6.0
(2.3) (8.1) (3.0) (0.6) (1.0) (1.5)
Mollusca
Gastropoda
Axnnico la 5.7 10.3 9.3 5.7 10.3 25.3
(0.9) (3.0) (3.8) (0.3) (5.3) (2.9)
Phv a 0.7 4.3 2.0 1.0 1.3 2.3
(0.3) (3.3) (1.0) (0.6) (0.7) (1.2)
Planorbidae 1.0 1.7 1.0 0.3 - 0.7
(0.6) (0.9) (0.6) (0.3) (0.3)
90
-------
Table 6A. Dominant macroinvertebrate taxa associated with
long i. soicatum stems from the West Bed in Brownington Pond
(9 June - 14 July, 1992) . Samples were collected using the
smaller MIS sampler. Data in the table are mean number of
individuals per stem (± I S.E.) . Three samples were taken
on each date.
Taxon Date
9/6 16/6 23/6 30/6 7/7 14/7
Anrielida
Oligochaeta 27.3 25.7 63.3 36.7 54.3 25.7
(6.0) (12.0) (7.9) (21.2) (22.9) (19.9)
At tJnopuda
Amphipoda
Hvalle la 1.0 0.7 0.3 0.3 0.7 0.7
(1.0) (0.7) (0.3) (0.3) (0.3) (0.7)
CladOCeLa 4.0 1.7 9.0 0.7 0.3 0.7
(2.3) (0.9) (6.7) (0.7) (0.3) (0.7)
Hydiacarina - 1.7 0.7 1.0 3.3
(0.9) (0.7) (1.0) (0.9)
IflSCCtcl
Coleoptera
Eulirychionsis 3.0 0.3 4.3 2.0 0.7 3.7
(1.2) (0.3) (0.7) (1.0) (0.3) (1.9)
Cyrinus 2.0 - 0.7 0.3
(2.0) (0.3) (0.3)
Diptera
Chironomidae 1.3 2.0 1.7 2.0 0.3 3.3
(0.7) (0.6) (0.9) (1.2) (0.3) (0.7)
Lep 1 dop t. e rEt
Acentria 1.3 2.0 - 1.0 1.0 1.0
(0.9) (1.5) (1.0) (0.0) (0.0)
Odonat a
Ani 3opt era
Zygopt era
En:t1la ima — 0.3
(0.3)
91
-------
Table . Continued .
Tr ichopt era
Oxvethira - 0.3 1.3
(0.3) (0.3)
Oecetis 2.0 0.3 0.3 0.7
(0.6) (0.3) (0.3) (0.3)
Coe It. e rat a
Hydr 130.7 24.3 0.3
(76.5) (6.3) (0.3)
} 1atyhelrninthes
P lanarildEle 8.0 4.3 15.7 5.3 6.0 39.0
(3.5) (1.7) (3.2) (2.0) (1.5) (8.9)
MolLusca
Gas tropoda
Amnicola 3.7 4.3 16.3 11.7 8.0 21.0
(0.9) (3.4) (3.5) (7.3) (2.5) (4.5)
_____ - 0.3 5.0 2.0
(0.3) (3.1) (2.0)
Planorbidac 2.3 0.3 - 0.7 1.0
(1.9) (0.3) (0.7) (0.6)
92
-------
Table 6B. Dominant macroinvertebrate taxa associated with
long . soicatum sterns from the West Bed in Brownington Pond
(21 July - 25 August, 1992) . Samples were collected using
the smaller MIS sampler. Data in the table are mean number
of individuals per stem (± I S.E.). Three samples were
taken on each date.
Taxoii Date
21/7 28/7 4/8 11/8 18/8 25/8
AIIIIeU.da
Oligochaela 62.0 62.7 99.0 15.0 23.0 24.0
(8.1) (39.7) (39.1) (4.0) (9.3) (11.5)
Ar thropoda
Amphi poda
llval lela 3.7 0.7 3.3 3.3 1.0 0.3
(0.7) (0.7) (1.5) (1.9) (1.0) (0.3)
Cladocera 0.3 0.3 - 1.7 0.7 1.7
(0.3) (0.3) (0.9) (0.7) (0.9)
Hydracarina 0.3 0.3 1.0 0.7 1.3
(0.3) (0.3) (0.6) (0.7) (0.7)
ln ;ecta
Coleopt era
Euhrvchioosis 3.3 3.0 1.7 1.3 1.3 0.3
(0.9) (1.7) (0.7) (0.9) (0.3) (0.3)
Gvriiius - 1.0
(1.0)
D i pt era
Chironomidae 2.0 1.0 0.7 - 1.0
(0.6) (1.0) (0.7) (0.6)
Le pi dope era
Acentria 0.3 1.3 0.3 0.3 — 0.3
(0.3) (0.3) (0.3) (0.3) (0.3)
0dOflcit t
Ani3optera - 0.3
(0.3)
Zyguptei a
Enallacirna 0.3 - 0.3
(0.3) (0.3)
93
-------
Table . Continued .
Trichoptera
Oxvethira 1.3 0.3 0.3 0.3 2.0 0.7
(0.3) (0.3Y (0.3) (0.3) (1.2) (0.3)
Oec.etis — 0.3 0.3 - 0.7
(0.3) (0.3) (0.7)
Coelenterata
Hvdr i
Platyhelmint hes
Planariidae 19.7 39.0 57.7 11.0 9.7 20.3
(5.8) (14.8) (35.0) (7.0) (9.0) (10.8)
Mo 11 usc a
Gast ropoda
Amnicola 14.7 11.0 30.7 19.7 20.3 37.0
(4.6) (2.0) (8.9) (0.3) (5.2) (11.7)
Phvs3 2.7 1.0 1.3 4.7 6.3 5.0
(1.2) (0.6) (0.9) (2.7) (5.8) (1.5)
P1anorbid ie - 0.7 - 1.3 1.0 0.3
(0.3) (0.9) (0.6) (0.3)
94
-------
Table 7. The abundance of . lecontei and . nivea on long
(>50 cm) and short watermilfoil stems in the West Bed. The
samples were taken with the small MIS sampler (Minisampler)
Values in the table are means ( 1 S.E.). N=3 for all
samples.
Date • .
Long
lecontei
. nivea
Short
Long Short
9 June 3.00 0.67 1.33 0.33
(1.15) (0.67) (0.88) (0.33)
16 June 0.33 0.00 2.00 0.00
(0.33) (0.00) (1.53) (0.00)
23 June 4.33 0.00 0.00 0.00
(0.67) (0.00) (0.00) (0.00)
30 June 2.00 1.67 1.00 0.33
(1.00) (1.67) (1.00) (0.33)
7 July 0.67 0.33 1.00 0.00
(0.33) (0.33) (0.00) (0.00)
14 Ju1y 3.67 0.00 1.00 0.00
(1.86) (0.00) (0.00) (0.00)
21 July 3.33 1.00 0.33 0.00
(0.88) (0.00) (0.33) (0.00)
28 July 3.00 0.67 1.33 0.00
(1.73) (0.67) (0.33) (0.00)
4 Augu:3L 1.67 1.33 0.33 0.00
(0.67) (1.33) (0.33) (0.00)
11 Auçju t 1.33 0.00 0.33 0.33
(0.88) (0.00) (0.33) (0.33)
18 August 1.33 0.00 0.00 0.00
(0.33) (0.00) (0.00) (0.00)
25 Aucju: t 0.33 0.00 0.33 0.00
(0.33) (0.00) (0.33) (0.00)
95
-------
Table 8. The abundance of . lecontei and . nivea on long
(>50 cia) and short watermilfoil sterns in the South Bed. The
samples were taken with the small MIS sampler (Miiiisampler)
Values in the table are means (± 1 S.E.) . N=3 for all
samples.
DaLe E.
Long
lecontei
. nivea
Short
Long Short
9 June 3.67 0.00 0.33 0.67
(1.45) (0.00) (0.33) (0.67)
16 June 1.33 0.00 0.33 0.00
(1.33) (0.00) (0.33) (0.00)
23 June 1.67 0.00 0.33 0.67
(0.88) (0.00) (0.33) (0.33)
30 June 3.33 0.00 0.33 0.00
(0.67) (0.00) (0.33) (0.00)
7 July 2.67 1.00 0.67 0.00
(1.20) (0.58) (0.67) (0.00)
1/1 July 1.67 0.67 0.33 0.33
(0.88) (0.33) (0.33) (0.33)
21 July 1.00 0.33 0.67 0.33
(0.58) (0.33) (0.33) (0.33)
28 JuLy 3.33 2.67 0.67 0.00
(0.88) (0.88) (0.67) (0.00)
4 Aucjusl 2.00 0.33 0.00 0.00
(1.15) (0.33) (0.00) (0.00)
11 August 0.00 0.67 0.00 0.00
(0.00) (0.33) (0.00) (0.00)
18 August 0.67 0.33 0.67 0.00
(0.67) (0.33) (0.67) (0.00)
25 Augu t 0.67 0.00 0.33 0.00
(0.33) (0.00) (0.33) (0.00)
96
-------
Table 9. Dominant prey found in the guts of yellow perch
collected in !3rownington Pond in June and July of 1992.
Values in the table are frequencies of occurrence.
Prey Taxon Date
25 June 2 July 10 July
CrusLacea
Arnph ipoda
Hvalle la 46 31 35
Cl doce .a 36 0 35
insect a
O . pt era
ChlLonomldae L . 73 31 71
Chiionumiclae P. 55 46 47
Ceratopogonidae 2 46 31 29
ChaoboLus L. 27 15 24
Chaoborus P. 27 0 0
EpherneLoptera
Caenis 73 31 59
Odona ta
T tracioneuria 46 23 48
Enallagma 46 54 29
Mol lusca
Phvsa 27 8 12
Chordat a
Perch fry 27 39 18
L. = Larvae, P. = Pupae
Ceiat.op’xjonidae = Heleidae (used in previous reports)
97
-------
Table 10. The effect of weevil damage on stem fragment
viability: results of Experiment 1. Values in the table are
means ( I S.E)
Variable
Treatment
F
Value
P
Value
Control
Damaged
Peicent of Stems
with Roots
100.0
(0.0)
85.0
(9.6)
2.80
—
Root. Weight (g)
0.257
0.036
37.90
0.0008
(0.018)
(0.011)
Change in Stem
Length (mm)
Total
114.8
(6.5)
90.7
(13.0)
3.22
-
Original.
111.7
5.1
45.52
0.0005
(5.6)
(5.6)
Lateral
3.1
(1.8)
85.6
(15.4)
33.26
0.0012
98
-------
Table 11. The response of the dominant macroinvertebrates
found on . soic tum in Brownington Pond to the exclusion of
fish. Values in the table are mean number of a taxon per
grain of watermilfoil (± .1 S.E.) . The treatments were
compared using an ANOVA with planned, orthogonal contrasts.
The Fish contrast in the table is the comparison of fish vs
no fish, i.e., the cage treatment vs the cage control and
the control treatments. The Cage contrast tests for a cage
effect and compares the cage control vs the control. The
ANOVA as performed on log(X+l) transformed data.
Significance levels are as follows: # p<0.l (marginally
significant), * p<0.05, ** p O.0l, p O.OOl. Ct=Control,
Cc=Cage Control, Ca=Cage.
Taxon Treatment
Ct Cc Ca
Contrast
Fish
Cage
In sect a
Coleoptera
EuhrvchioosisL 2.82 6.02 6.84 * **
(0.64) (1.03) (0.40)
Diptela
Chironc ’midae 8.85 9.34 5.02
(3.98) (2.51) (1.08)
CotatupogonlLlae 6.03 2.87 3.49
(2.17) (0.65) (1.11)
Ephemeroptera
Caenis 8.26 10.94 5.02
(2.90) (3.66) (0.65)
Lepidoptera
Acentria 0.06 0.23 0.05
(0.06) (0.12) (0.03)
Odonata
Anisopteia 0.27 0.46 0.29
(0.09) (0.19) (0.60)
Zygopt era
Enallaruna 0.42 0.87 1.58 **
(0.23) (0.31) (0.16)
Trichopterci
Decetis 0.57 0.25 0.33
(0.20) (0.12) (0.12)
99
-------
Table II. Continued .
Taxon Treatment
Ct Cc Ca
Cont.
rast
Fish
Cage
C’xye’:hira 0.99 0.28 0.03 *
(0.36) (0.15) (0.02)
Iriaenodes 0.09 0.29 0.34
(0.09) (0.07) (0.11)
Ceraclea 0.38 0.50 0.22
(0.20) (0.15) (0.06)
Crustacea
Aniph I pcda
L-Ivu lella 12.27 19.19 23.04
(4.95) (2.50) (2.86)
ilydiacarina 3.09 6.16 6.52
(1.11) (1.99) (0.67)
t ropoda
Amnicola 12.68 7.17 12.99 *
(3.53) (1.57) (2.40)
9.62 11.95 15.45
(1.56) (2.00) (1.48)
Immature
P lanorbidae 2 7.78 10.36 11.96
(2.00) (1.98) (1.66)
PJ.a tyhe luu. nt.lies
Planaria 0.43 0.32 0.18
(0.18) (0.19) (0.08)
Otigochaeta 13.32 1.92 2.12 * *
(5.24) (0.54) (0.86)
Hirudinea 1.84 1.46 0.87
(0.76) (0.39) (0.08)
I - . Iecontei adults and larvae combined.
2 - Immature Planorbidae consists of Gvraulus sp. and
ilelisonu sp.
I 0()
-------
Table 12. The effect of weevil and Acentria feeding on the
height of original sterns(cm) for the last three weeks of the
enclosure experiment conducted in Brownington Pond in 1992.
Values in the table are treatment means ( I S.E.).
Treatment means that are significantly different (p
-------
Table 13. The mean (.±. 1 S.E.) number of weevils (larvae,
pupae and adults) per meristem collected at three sites in
Lake Bomoseen in 1992 during stem transects. Data are from
all samples taken at a site, i.e., samples from harvested
and unharvested sites are combined.
Site
N
Number of
per mer
weevils
istem
Nioshube Island
19
0.042
(0.012)
East Eckley N.
17
0.039
(0.007)
East Eckley S. 1
19
0.048
(0.010)
All Sites
Combined
19
0.042
(0.006)
1- This site was not harvested in 1992.
I 2
-------
Table 14. The effect of mechanical harvesting on weevil
abundance (larvae, pupae and adults) in 1992. Data are from
the stein transects at three sites in Lake Bornoseen. Values
in the table are the mean (± 1 S.E.) numbers of weevils per
meristem. DiEferences between harvested and unharvested
areas compared using ANOVA; data were square root
transformed for the analysis.
Site
N
Harvested
Unharvested
p value
Neshobe 1.
19
0.011
(0.004)
0.074
(0.024)
0.012
East Eckley
N.
17
0.024
(0.006)
0.055
(0.012)
0.043
East Ecktey
S. 1
19
0.046
(0.011)
0.051
(0.013)
0.772
All Sites
Combined 2
L9
0.026
(0.005)
0.058
(0.010)
0.006
1- This site was not harvested in 1992.
2- There were no significant ditferences among sites
(p=O. 444 ) when harvest an no harvest data were pooled.
103
-------
Table 15. The distribution of weevils (larvae, pupae and
adults) with respect to water depth in 1992. Data are from
the stem transects at three sites in Lake Bomoseen. Values
in the table aie the mean (±1 S.E.) numbers of weevils per
meristem. Differences between shallow and deep areas
compared using ANOVA; data were square root transformed for
the analysis.
Site
N
Shallow
Deep
p value
Neshobe I.
15
0.056
(0.016)
0.050
(0.017)
0.896
EcI . t Eckley
N.
15
0.078
(0.016)
0.009
(0.004)
0.001
East Eckley
5•2
15
0.081
(0.016)
0.028
(0.010)
0.013
All Sites
Combined 3
15
0.071
(0.011)
0.029
(0.005)
0.001
1- Shallow-deep comparisons were started later in the summer
thus the reduced number of samples compared with Tables
13 and 14.
2- This site was not harvested in 1992.
3- There were no significant differences among sites
(p=0.578) when shallow and deep data were pooled.
104
-------
Table 16. The mean number (± I S.E.) of weevils (all life
stages) collected per date at each site in Lake Bomoseen for
1991 arid 1992. The data are from the stem transects and
samples from approximately the same dates for each year were
compared. For each comparison n=12.
Year
Site
Neshobe
Eckley- 1 -
Neshobe
Ec k 1 ey
÷
L991
12.08
(1.48)
2.08
(0.57)
14.17
(1.92)
L992
4.94
(0.88)
3.58
(0.77)
8.53
(1.06)
•T Statistic
3.966
1.501
2.395
p value
0.001
0.148
0.026
1- Eckley samples are just for E. Eckley South which was the
only area in Eckley Bay which was sampled both years.
105
-------
Table 17. Results of the ANOVA of the super sample data
from the licirvested and unharvested areas at Neshobe Island
in Lcike Bornoseen in 1992. Samples were collected on 30
June, 27 July 31 August. Three samples were taken in both
the harvested and unharvested areas on each date. The ANOVA
was performed on log transformed data. The table presents
the level of significance (i.e., based on p values) of
watermilfoil biomass and major taxa for the effects of date
and harvesting.
Taxon
Ef
fect
Date
Harvesting
Milfoil Dry Weight
-
**
. [ nvert ebiates
Oligochaeta
*
Ar thropoda
Crustacea
Amphipoda
-
-
Isopoda
*
*
l!ydracarina
#
#
Iris ec t. a
Euhrvchioosis
*
*
*
Chironomidae
Caenis
**
Zygoptera
*
-
Apraylea
-
Oxvethira
*
Orthotrichia
*
Mo 11 us ca
Gas t ropoda
Planorbidae
kk
Amnicol i
***
Phvsa
#
—
Significance levels: - not significant, # marginally
significant (p<0.l0), * p<0.05, ** p<0.0l, p
-------
Table 18. The number of intact plants tor each macrophyte
species in each of the three treatments for the native
macrophyte trials.
Macrophyte
Species
T
reatment
0
weevils
2
Weevils
4
Weevils
Ceratoohyllum
5
4
6
4
4
6
Chara
Eloclea
5
4
4
Heteranthera
6
5
4
6
6
3
6
6
2
Meoalodonta
. sibiricum
.E. amolifolius
6
6
6
6
5
6
6
6
6
Utricularia
Va llisnerip
107
-------
Table 19. The civerage number of weevils surviving per
chamber (2 and 4 weevil treatments for natives and the j .
soicatum controls (4 weevils/chamber)) in the native plant
trials. N=6 for the native plant treatments and n=3 for the
. soicatum controls.
Ikitive Species
Trial
Treatment
2
Weevils
4
Weevils
.
soicatum
Control
Cer atoohv llum
0.50
0.33
0.00
1.00
1.75
4.00
Chara
Elodea
0.00
0.00
2.00
Heterantheta
0.00
0.33
1.17
0.00
0.67
1.83
2.67
4.00
2.67
Meualodonta
M. sibiricum
Ł. amolifolius 1
UCricularia
Vallisneria
—
0.66 2
0.17
-
1.33 2
0.33
-
3.00
4.00
1- Weevil survivoiship not recorded for . amolifolius .
2- The IJtricularia experiment was terminated early (after
only seven days)
I OX
-------
Table 20. A list of the six primary objectives of this study
and the work conducted during the 1992 field season that
addresses these objectives. As the ideas in the objectives
overlap, some projects are listed under two or more
objectives.
Objective 1. Determine t:he probable cause(s) of the
Eurasian watermiltoil decline in Brownington Pond.
-aLl Brownington Pond (BP) research
UbiecLive 2. Examine the grazing/boring effects of all
major heLbivoL-es on Eurasian watermilfoil and native aquatic
plant species.
-Wading Pool Experiment (BP)
-Stem Fragment Viability Experiment (BP)
-Herbivore Enclosure Experiment (BP)
-Nat.Lve Plant Experiments (Middlebury (N))
Objective 3. Determine the feasibility of herbivore
introductions into other milfoil-infested lakes in Vermont.
-Weevil culture data (M)
-Weevil transect data (Lake Bomoseen (LB))
-Introductions at Norton Brook, Van Vieck’s and
Betourneys (N)
Objective 4. Determine if Lake Bomoseeri is a suitable site
for Iieibivore introductions/collect pie-introduction base-
line data.
-Weevil transect data (LE)
Objective 5. If determined to be feasible and appropriate
based on previous research (a high likelihood of success and
relatively free from causing negative impacts to non-target
species), use herbivorous insects to control Eurasian
w iLetnulfoil in Lake Bomoseen.
-augmentation o [ weevils at Eckley Bay (LB)
Objective 6. Develop a public education program to keep
Vermont’s citizens abreast of the results of the re . earch.
--pLesentcitlens given by Sheldon and Creed
-slide show on watermilfoil control prepared (M)
109
-------
Table 21. Equipment purchased on the EPA grant from June
1991 to April 1992.
Item Amount
Post Script Option for 329.00
iBM Printer
Statistix (Statistical
package for Zenith
Laptop computer) 201.00
Underwater Camera 252.86
I I()
-------
FIGURE LEGENDS
Figure I. The distribution of waterniilfoil in Browningion Pond in 1991 (A) and 1992
(B).
Figure 2 A and B. Water temperatures in Brownington Pond for 1992. Tempeiatures
wci e recoided iih maximum/minimum thermometers suspended 0.5 m below the
suifa c and 0.5 in above the bottom. Values in figures are means (+ I S.E.) for two pairs
of thermometcis located aiound the pond. A. Surface temperatures. B. Bottom
tcmperalui es.
Figure 3 A - C. Results of the plant transects for the West Bed in 1992. Bars represent
the mean (± I S.E.) biomass of watcrmilfoil or combined native macrophyte species
(=Otlicr).
Figiiic 4 A - C. Results of the plant tu-ansects for the South Bed in 1992. Bars u-epresent
the mean a I S.E.) bioiiiass of waterinilfoil or combined native macrophyte species
(=Olhcr).
FigureS A - C. Results of the plant transects for the West Bed: 1990-1992. The figures
for each ycar ieprcsent samples takeii at about the same time of year. Bars represent the
fflCdil (± I S.E.) biomass of watermilfoil or other maciophyte species.
FigLiic ô A - C. Results of the plant transects for the South Bed: 1990-1992. The figures
for each ycai iepiesent samples taken at about the same time of year. Bars represent the
mean a I S.E.) hioiriass of watermilfoil or other macrophyte species.
Figure 7. Maps of the percent cover of Eurasian watci milfoil in the South Grid. West
Bed loi- OUC (late Iii 1991 and three dates in 1992.
Figure . Maps of the percent Lover of Eurasian watermilfoil in the North Grid. West
Bed for one date in 1991 and three dates in 1992.
Figure 9. Maps of the percent cover of Eurasian watermilfoil in the West Grid. South
Bed foi one date iii 11)91 and three dales in 1992.
Figure U). Maps of the percent cover of Eurasian watermilfoil in the East Grid, South
Bed for one date in 1991 and three dales in 1992.
Figure II. Maps of the peicciit cover of Eurasian watermilfoil for the West Dccl: 1990-
1992. 1’lie figuies for each grid iepresent the last map for each year.
Figure 12. Maps of the percent Lover of Eurasian watermilloil for the South Bed: 1990-
1992. The liguics for each giid iepiesent the last map for each year.
III
-------
Figuic 13. Euuisian watermilfoil and weevil abundance in the West Bed from 1990-
1992. A. Watcimulfoil buounass (mean ± I S.E.). Data for 199() are from a series of
quaciral samples. The number of samples for a given date ianges from three to six. Data
for 1991 and 1992 are from the plant transects. All samples from the 2.0-3.5 in depth
intervals were used (n=9 for each date). B. Weevil abundance as mean (± I S.E.)
number of adults and Iar ae pCi stem. Samples were collected using the small MIS
samplet. N=5 for all dates in 199() and 1991. N=3 for all samples in 1992.
Figure 14. Eurasian waicumilfoil and weevil abundance in the South Bed from 1990-
1992. A. Watcrmmlfoul buomass (mean ± 1 S.E.). Data for l99() ui-c from a series of
quadrat samples. The number of samples for a given date ranges from three to six. Data
for 1991 and 1992 ale from the plant transects. All samples from the 2.0-3.5 in depth
inteuvals were used (ii=9 foi each date). B. Weevil abundance as mean a 1 S.E.)
nuimiher of adults and luu ac per stem. Samples were collected using the small MIS
sampler. N=5 l.oi all dates in 1991) and 1991. N=3 for all samples in 1992.
Figure 15 A and B. Results of the stem transects in the West Bed in Brownington Pond
in 1992. The data in the figure are the mean a I S.E.) number of eggs found associated
with A) waterniilfoil stems with intact apiLal meristems and B) watermilfoil stems
without intact apical mci istems.
Figure 16 A and Ii. Results of the stem transects in the West Bed in Brownington Pond
in 1992 The data iii the figuic we the mean a I S.E.) number of mneristem larvae found
associated with A) watcuuiiulfoil stems with Intact apical meristems and B) watermilfoil
stcms without intact apical ulicristems.
Figuic 17 A and B. Results of the stem ti-anse ts in the West Bed in Bmownington Pond
in 1992. The data in the figure we the mean a I S.E.) number of stem larvae found
associated with A) watermilfoil stems with intact apical meristems and B) watermilfoil
stems without intaLt apical meristems.
Figume l A and B. Results of the stem tiansccts in the South Bed in Brownington Pond
in 1992. The data in the figure are the mean a I S.E.) number of eggs found associated
with A) wateiinilfoil stems with intact apical meristems and B) watermilfoil stems
without intact apical meristems.
Figure 19 A and B. Results of the stem transects in the South Bed in Brownington Pond
in lY )2. The data in the fuguie are the mean a I S.E.) number of meristcm larvae found
associated with i\ ) aid milfoil stems with intact apical inerustems and B) watermulfoil
stems itliout iiltact apical mci istems.
Figuue 20 A and B. Results of the steull transects in the South Bed in Brownungton Pond
in 1992. The data iii the figuie are the mean a I S.E.) number of steni larvae found
associated with A) watci inilfoil stems with intact apical meristems and B) watermilfoil
stems without intact apical ineristems.
112
-------
Figure 21 A - C. The effect of feeding by Eulii ychiopsis and Acentria larvae on
waterunilloil plants. The bars iii the histogram represent the mean change in a response
variable a I S.E.) for each treatment. The lines with significance values above the
histograms show the results of ANOVA eomparison with orthogonal contrasts. In each
figure. the upper line repucsenls the comparison of the control vs the herbivore
trcatulieills: the middle line repiesents the comparison of the weevil neatment versus the
two tieatnlculIs coiitainiiig Aicntria larvae: The lowest line represents the comparison of
the Acentri alone ticatment versus the treatment with both the Acentria and the
Euhiyduiopsis laivae (combined). A. Chaiige in plant weight (in grains). B. Change in
plant length (in millimeters). C. Change in the number of whorls per plant.
Figure 22. A and B. The effect of weevil damage on the viability of watermulfoil stein
fragments. The biu s in the histogram represent the mean change in a response variable
a I S E.) for e i h treatment. The lines with significdllee vdlues above the histograms
show the results of A NOVA comparisons with orthogonal contrasts. In each figure, the
upper line repuesents the Lomparison of the undamaged control fragments (C) vs the
(Iain igCd fragments (D): the lower line oii the left represents the comparison of the
unshaded conti-ol treatment (CU) vs the shaded control treatment (CS): the lower line on
the u ight represents the comparison of thc unshaded damaged stem treatment (DU) vs the
shaded damaged steull treatment (DS) A. Percent of stems with roots. B Root weight
(in guams).
Figuue 23. A - C. The effect of weevil damage on the viability of watermilfoil stem
fragments. The bars iii the histogram represent the mean change in a response variable
(± I S.E.) for each ticatment. The lines with significance values above the histograms
show the iesults of ANOVA ompurisons with orthogonal contrasts. In each figurc. the
upper line iepiescnts the conipai uson of the undamaged control fragments (C) vs the
damaged fragments (D): the lower line on the left represents the comparison of the
unshaded tontrol treitincnt (CU) vs the shaded control treatment (CS): the lower line on
the right replesents the comparison of the unshaded damaged stem treatment (DU) vs the
shaded (Lunagedi stem treatment (DS) A. Total stem tissue produced. B. Stem tissue
produced by the oiiginal stem. C. Stem tissue produced by the lateral stems.
Figure 24. Results of the Brownington Pond enclosure experiment. The data shown
un lude the total watermilfoil biomass pei ticatnient (solid black bais) plus the
distribution of that biom 1 iss by its components (i.e., original stem bioinass, lateral stem
hiomass and root biomass). The bai s repiescut mean biomass a I S.E.). Treatments
conucued by the stime lcttci arc not significantly cliffeient.
Figuic 25. The location of study sites in Lake Bomoseeii, Vermont.
Figuie 26. The total number of all E. lccontei life stages (larvae, pupae and adults)
sampled in hai vested iiid unharvested areas for all three sites (Neshobe I., East Eckley
North and East Eckley South) conibineci. Data are from the stem transects.
113
-------
Figure 27. A comparison of the total number of all E. lecontei life stages (larvae, pupae
and adults) ainpIed in harvested and unhar estcd areas at the thiee sites. Data are from
the sicm transects. A. Nesliobe L B. East Eckley North. C. East Eckley South.
Figuic 2 . The mean number of weevils pei meristem on watermilfoil in shallow and
deep watcr at the thice sites (Neshobe I.. East Eckley North and East ELkIey South) in
Lake Boniosecii in 1992. Data are from the stem transects. The bars represent the niean
a I S.E.) abundance of weevils in each of these habitats (n=l 5 for each site).
Fieuic 29. The nuiiiber of caih life stage of E. lecontei , summed for all sites, for each
week in A) 1991 and B) 1992.
Figuic 3 . The effect of adult weevils on change in length (cm) for nine species of
native aquatic maLropllytes. Bars in the histograili represent the mean (±1 S E.) length of
iiltaLt plants for each of the species for each of the three weevil treatments.
Figure 31. The efieu of adult weevils oii change in wet weight (g) for eight species of
native aquatic macrophyles. Bars in the histogram replesent the mean (±1 S.E.) wet
weight of each of the species for each of the three weevil treatments. Utricularia is not
included in this figure in order to expand the scale for the remaining seven species.
Utricularia weight data are discussed in the text. N=6 for all species except for the 2-
weevil tieatinent forM. sibiricum where n=4.
Figuic 32. Mvriophyllum spicatum average dry weight a I S.E., n=3) in enclosures
with and without ee ils, and in open water in Norton Brook Pond.
Figure 33. Macioinvcitebiates in enclosures with and without weevils. and in open water
in Noitoii Brook Poiid. A. Average number a I S.E.) of macroinvertebrates excluding
zooplankton. B. Average taxa iichness a I S.E.. ii=3).
114
-------
FI(;LJRI S
115
-------
Figure 1.
A. 11991 milfoilI
BROWNINGTON POND VT
B. 11992m11f0hh 1
TN
1 00 m
-------
Figure 2.
Brownington Pond 1992
Surface Temperature
Date
Bottom Temperature
— a—— — Maximum
Minimum
—a--— Maximum
Minimum
A.
C.)
0
C)
-I
C)
C)
0.
E.
C)
I-
C.)
0
C)
I-
(0
C)
0.
E
C)
I-
= — = = = = = = x X X )C
30
20
10
0
30
20
10•
0’
B.
I ’ I • I
5: = — = = = = = = = x X X )(
Date
-------
Figure 3.
A
140
- 120
100•
B
01
>1
0
C
140
120
100
80
60
40
20
0
140
120
100
80
60
40
20
0
1992 Plant Transects
0.5 1 1.5
2 2.5
Depth (m)
Depth (m)
0.5 1 1.5 2
Depth
(m)
3
3.5
U M. sp catum
• other
U M. soicatum
U other
U M.spicatum
N other
10 June 1992
West Bed
- -
0.5 1 1.5 2 2.5 3 3.5
N
2.5 3 3.5
-------
Figure 4.
A
140
— 120
140
120 -
100
80-
60-
40-
20-
0-
140
120
100
80
60
40
20
0
1992 Plant Transects
-a---
0.5 1 1.5 2 2.5
Depth (m)
11 June 1992
South Bed
3 3.5
9 July 1992
South Bed
0.5 1 15225 -
Depth (m)
0.5 1 1.5 2 2.5 3 3.5
Depth (m)
• M.
• other
M.spicatum
• other
•
• other
N
-C
C
N
-C
>1
I-
0
-------
Figure 5.
240
200
N
160
120
C)
80
40
c, J
E
C)
C)
I . .
0
M. spicatum
other
M.spicatum
other
A
West Bed
15 August
1990
1-li
U
M. spicatum
other
0 .
0.5
.— ‘————--—. .———
1
.
1.5
2
2.5
3
I
3.5
Depth
(m)
July 1991
jail
B
240
- 200 31
N
. 160
C)
120
80
g b
240
__ 200
160
120
80
40
Q
C
-u
.
0.5 1
1.5
2
2.5
3
3.5
Depth
(m)
.
12 August1992
j
0.5 1 1.5 2 2.5
3 3.5
Depth (m)
-------
Figure 6.
3 3.5
I
M. spicatum
other
M. spicatum
other
M.spicatum
other
A
South Bed
a)
B
0.5 1 1.5 2 2.5 3 3.5
Depth (m)
c J
a)
>1
240
200
160
120
80
40
0
240
200
160
120
80
40
0
240
200
160
120
80
40
0
7 August 1991
i rII
I
L
0.5 1 1.5 2 2.5
Depth (m)
C
C J
>1
I-
a
13 August 1992
ii
0.5
1 ‘ 1.5 2 2.5
3 3.5
Depth (m)
-------
West Bed, South Grid
Figure 7.
August26, i i
June 15, 1992
2
3
::::
::::
:::
::::
:•::
:
:•:•:•:•
•:•:•:•:
0% Cover
1-25% Cover
4
25-50% Cover
50-75% Cover
>75% Cover
July 13, 1992
August 24, 1992
2 3 4
A
A
E
E
A
Al
:•:•:•:
E r.
E
-------
West Bed, North Grid
Figure 8.
August26, 1991
June 15, 1992
Al 2 3 4
E
y
0% Cover
1-25% Cover
25-50% Cover
50-75% Cover
>75% Cover
A
July 13, 1992
2 3
!
:*.
i—
4
A
E
August 24, 1992
2 3 4
f ’:
* :
::::
::::
):::
::::
:::
E
E
-------
South Bed, West Grid
Figure 9.
August 26, 1991
41
E
June 15, 1992
2 3
4
::::
!i.::::.
..... . p_
‘S tS5•
t7_
,
0% Cover
• 1- 25% Cover
25-50% Cover
50-75% Cover
>75% Cover
July 13, 1992
2
3
:::::::::::::::::::::‘
::,“
4
A
E
August 24, 1992
2
3
::::
:•:•:•:•
•:: :•:
:•: :•
•: :•::
:• •
•: :•:•:
::::::::
::::
::::
:•:•:•:•
::j”
Al
E
E
-------
South Bed, Eest Grid
Figure 10.
August 26, 1991
June 15, 1992
2 3 4
1- 25% Cover
25-50% Cover
50-75% Cover
>75% Cover
July 13, 1992
August 24, 1992
ç :
:1:1
(
(
:\
2 3 4
A
E
A
Ł
0% Cover
El
A
A
E
E
-------
Figure 11.
West Bed, North Grid
September 9, 1990
August 26, 1 99 1
August 24, 1992
2 3
4
: .
2 3 4
/::
: :
::::
::::
):::
::::
:::(
West Bed, South Grid
0% Cover
1-25% Cover
25-50% Cover
50-75% Cover
>75% Cover
September 9, 1 990
August 26, 1991
August 24, 1992
2 3 4
.
LUIILIIILLI”
Al 2 3 4 41
A
E
:
-
E
E
A
E
E
E
-------
South Bed, West Grid
Figure 12.
September 9, 1990
August 26, 1 99 1
August 24, 1992
2 3 4 4
South Bed, East Grid
E
2
3
::::::::
: : :•
•:•: :•:
•:•:•:•::•::•:•
•:•:. “
E
U
0% Cover
1- 25% Cover
25-50% Cover
50-75% Cover
>75% Cover
A
September 9, 1990
A
August 26, 1991
August 24, 1992
41
1
:• J
: :•:•:)
•1
:
4
E
E
A
E
E
E
-------
Figure 13.
1990 1991 1992
1990
WEST BED
1991
S
1992
ii
Date
A.
100
80
60
40
20
0
B.
6
5
4
3
2
1
0
E
C)
C l)
C)
C l )
>
C)
C)
I-
0
C)
.0
E
z
Date
-------
Figure 14.
1990 1991 1992
1990 1991 1992
SOUTH BED
Date
A.
100
80
60
40
20
0
B.
6
5-
4-
3.
2
1-
0
0)
C l)
Cl)
E
0
0
I-
E
E
w
C l)
I-
C)
0.
(I)
>
C)
C)
I-
0
C)
.0
E
z
II1
1 1
a’
Date
-------
Figure 15.
West Bed
A.
E Apical Meristem
I __
— — - - —. - — — — — - .— —
,- i- N w N C ’) ‘ N N
Date
B.
8 No Apical Meristem
::
i-i-N i-i-NC’) i-NN
Date
-------
Figure 16.
West Bed
Apical Meristem
No Apical Menstem
ji
I
I
Date
ii th
A.
1.50
1.25
1.00
0.75
0.50
0.25
0.00
- . . ‘•% - —. - - —.
‘- -C J ‘ i C JC!) JC%J
0
z
WE
0>
z
B.
1.50
1.25
1.00•
0.75
0.50
0.25
0.00
. NCV) -C JC J
Date
-------
Figure 17.
West Bed
Apical Neristem
jililIjIji a
A.
2.00
1.75
1.5O
1.25
1.00
0.75
0.50
0.25
0.00
B.
2.00-
1.75
1.50-
1.25 -
1.00.:
0.75
0.50
0.25
0.00•
C’)
0.
a-
a-&
z-J
0
C l)
‘-a-
0.
a-
C).
.0c5
a-a-
z-J
- . — - -.. - -.- - _%.
,- - N - ,- N CV) ‘-
Date
No Apical Menstem
n1 ILHiń
‘•% — - - - . .
- - N - -NC’) ‘-
Date
-------
Figure 18.
South Bed
A.
Apical Menstem
E 8
Cl)
6
In 5•
iJI$IIIIihE-
D(OCDWNNNNNCO COQ)
— - - -.- - - -
i- -C J w- INC’) JCsJ
Date
No Apical Meristem
E 8
Cl)
6
U)
LU 4
z
I- C C’) w-CSJCJ
Date
-------
Figure 19.
South Bed
A.
Apical Menstem
1.50
E 1.25
&-cl)
• 1.00
O 0.75
0>
0.50
O.25
0.00-
- - - - - - -.- , . -
- -C%J ‘-v-C JC’) —CsJOJ
Date
B.
No Apical Meristem
1.50
1.25
( f l .
U) i.oo-
, 0.
0.75
1.0
0.50-
Z 0.25
0.00 -
— - - — . - -
JO COc )ON i-
c J ‘- ,- C’4 CV) - C%J C J
Date
-------
Figure 20.
South Bed
A.
Apical Menstern
2.00
1.75
EE
1.50-
1.25-
1.00-
E 0.75
j E I LA 1 _
- - - - - - - - -.. - - -. .
‘- i-N i-i-N CV)-
Date
B.
No Apical Menstem
2.00
1.75
EE -
1.50
Cl)
1.25
1.00
E_ i 1 1nii
i—N i-i-N C )-
Date
-------
— pcO.0001
Figure 21A.
pcO.02
n.s.
Acentria Combined
Treatment
0,
a,
C)
C
0.
C
C)
0)
C
C
C -)
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
Control Weevil
-------
p
-------
— pcO.0001
Figure 21C.
— pcO.0001
Treatment
10
pcO.05
I-
C)
.0
E
z
I-
0
C)
0)
(U)
5
0
-5
-10
-15
Control Weevil Acentria Combined
-------
Figure 22.
pcO.003
n.s.
— p
-------
Treatment
p
-------
Figure 24.
Total
• Original
• Lateral
Roots
a
I
a
-
0
Cu
4
3-
2-
1-
0
•
I
Initial
Control Acentria Weevil
Treatment
-------
Figure 25.
Lake Bomoseen
Sampling Sites 1992
•1
4
3
IN
-------
Figure 26.
A. WEEVILS IN HARVESTED AND UNHARVESTED AREAS
15
U)
-J
>
LU
LU
10
U-
0
LU
z
-j
I-
0
I-
0
D Harvest, All sites
• No Harvest, All sites
dli.i 1
I
I
F:
-------
Figure 27.
WEEVILS IN HARVESTED AND UNHARVESTED AREAS
NESHOBE
nil nriilitI lifliL
E. ECKLEY NORTH
O Harvest
I No Harvest
c o - ‘
C. E. ECKLEY SOUTH
I
11
Ii
111111
N Ot’ N
— - N N
A.
C l)
-J
>
LU
LU
U.
0
LU
z
-J
I-
0
I -
14
12
10
8
6
2
0-
6
5
4
(I)
-J
LU
LU
LI.
0
LU
z
- I
F-
0
I-
3
2
1
0
C l)
-J
>
LU
LU
U.
0
LU
z
-J
I-
0
I-
6
5.
4.
3.
2
1•
0-
-------
Figure 28.
Mean number of weevils per meristem in shi llow
and deep areas of Lake Bomoseen 1992
Neshobe
E. Eckley North
E. Eckley South
I
I
0.10
0.08
0.06
0.04
0.02
0.00
.
0
Shallow Deep
Depth
-------
Figure 29.
Bomoseen weevil fransects 1991
July August SeptEmber
Bomoseen weevil transects 1992
Eggs
Larvae
0 Pupae
Adults
• Eggs
Larvae
0 Pupae
Adults
A.
30
10
0
No samples taken.
16
14
12
10
8
6
4
2
0
April May June July August September
-------
Figure 30.
CERATOPHYLLUM
CHARA
ELODEA
HETERANTHERA
EGALODONTA
M. SIBIRICUM
P. AMPLIFOLIUS
UTRICULARIA
VAWSNERIA
I
I
4.5
3.5
2.5
1.5
0.5
-0.5
-1.5
-2.5
-3.5
-4.5
-5.5
-6.5
U
0
U
0
U
0
0
U
0 2 4
NUMBER OF WEEVILS
-------
Figure 31.
CERATOPHYLLUM
CHARA
ELODEA
HETERANTHERA
GALODONTA
M. SIBIRICUM
P. AMPLIFOLIUS
VALLISNERIA
+1
2.0
1.5
1.0
0.5
0.0
.
0
0
0
0
U
U
0 2 4
NUMBER OF WEEVILS
-------
Figure 32.
p — 0 . 043
p — 0.007
-H
-r
Cage control
Treatment
P
p>0.0 5
a,
a,
w
0
E
1
L
Enclosure
Ambient
-------
Figure 33.
50 -
0-
A
300 -
‘U
I-
4
uJ
LU
z
0
4
150-
I L
0
100-
w WEEVILS w/o WEEVILS OPEN LAKE
B
C l,
C l ,
LI I
z
I
C.)
4
4
I-
UI
C,
4
LU
>
4
12
10
w WEEVILS w/o WEEVILS OPEN LAKE
------- |