EPA/600/A-92/122
Clark, Jaaea 8. 1991, Hosqaito Control Pesticides: idrerse Iipacii to
Jrsshsater Aquatic and Hariae Orgaaisss. Ia: losqoito Control Pssticidas:
?cological Iipacta aid laiageieat Alteraatms. Thoiaa C. fuel aid Joha C.
Tucker, Editors, Scisitifie Pailishers, Saiiesfille, FL. Pp. 32-33. (SHL.G3
T4Tt).
Mosquito Control Pesticides:
Adverse Impacts to Freshwater Aquatic and Marine Organisms
James R. Clark
US. Environment! Protection Agencj
Environmental Research Laboratorj
Sabine Island
Gulf Breeze. Florida 32561
Abstract. Most toxicity information available for evaluating potential effecs of mosquito ecoctsl chemicals on non-
target aquatic biota comes from acute lethality tests of V- a 96-hr dunnon. Tiese studies generally scow that ia.«xac.des are
more toxic to aquatic inrverebrates than Gshes. Qusacssns. is particular, se extremely sessiate a mosquito control
insecticides. perhaps a result of their close phylogenetic reiaocmhips with insecx Effects of longcr-tfsn exposures cm survival
and growth or studies thai quantify other sublethal effecs are available only for selected. standard labotaary test species for some
chemicals. Held studies conducted by our laboratory following operational insecticide applications hare shown that exposures
can be shorter deration awl of lesser concentration dm inse used for worst-case scenarios in snecaing-ievei eavimmneatal risk
assessments. However, long-cess effects of repeated applications of the same chemical or cumulative effects of multiple-
chrmical txeatmesis have not been adequately assisted in die field.
Approaches to Estabttshmg Potential Hazards of
. Pesticides
. Most of the toxicity information available to
evaluate potential effects of mosquito control chemicals
on non-target aquatic biota comes from acme lethality
tests of 24- to 96-hr duration. ' Effects of Ionger-«nn
exposures on the survival, growth and reproduction of
aquatic biota are assessed less frequently using
standardized chronic toxicity testing approaches
(ASTM 1988 a.b,c) and shortened, chronic effect
estimator tests (USEPA 1987,1989). these toxicity
data form the base of a tiered approach generally
applied to environmental ride assessment (Dickson etaL
1979), which has been developed into & itanjurhzH
approach for pesticide registration (Urban and Cook
1986). Contemporary environmental risk assessments
and use restrictions for pesticides ase laboratory data on
the acute and chronic toxicity of a pesticide to establish
exposure-response relationships that are then compared
to expected or measured concentrations under actual
use conditions.
-Information on acute and chronic toxicity is
limited to those species that can be readily tested under
laboratory holding conditions. In order to ensure that
regulatory controls , and ose restrictions will be
protective of the numerous species which can not be
cultured or tested in the laboratory, environmental risk
assessments" employ a series of worst-case exposure
scenarios using the toxicity test results for the most
sensitive species tested: Other factors, such as potential
bioaccumuianon by aquatic biota and chemical late and
persistence in aquatic habitats, also are considered in
environmental risk assessment of mosquito control
chemicals (Urban and Cook 1986).
Because laboratory tests are conducted under
conditions that optimize the survival of test species
except for exposure to test chemicals, there are
questions as to the applicability of the exposure-
response relationships for jm'mak in the Geld that might
be stressed by fluctuations in temperature, salinity,
dissolved oxygen availability, food availability, etc. Chi
the other hand, laboratory- test conditions provide for
constant exposure concentrations far the duration of the
test (e.g., 24 tar, 96 hr, 14- days) and deny test animals
protective behavioral responses such as avoidance of
contaminated areas. Thus the degree of environmental
protection offered by routine environmental risk
assessment procedures can be questioned as being
insufficient or over-protective when the process is
scrutinized in detail. Attempts to verify the
applicability of environmental risk assessment
processes by conducting exposure-response stsdies
under Seld conditions have provided some insight into
these questions, but considerable work still needs to be
done.
This paper will discuss common mosquito
control chemicals and present selected examples of
acute and chronic toxicity data ton standard laboratory
tests using a variety of freshwater and marine species.
The comparative sensitivities of these non-target
species is discussed. Also, these data will be examined
as to the degree to which the exposure-response data
represent conditions that reflect anal-use applications
of insecticides for mosquito costol or resemble the
'actual ecological reality of environmental
contamination. Examples of exposure-response
relationships from field stsdies of application of
mosquito control chemicals in Florida will provide

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*
34 Clark; Adverse Impacts
some perspective on the utility of various approaches
used for pesticide risk assessments. This paper will
limit considerations to malathion, fenthion, dibrom, or
temephos because they are generally used by mosquito
control operations in Rorida for wide-scale applications
that could potentially affect permanent water bodies.
Acute Toxicity to Aquatic Biota
Johnson and Finley (1980) and Mayer and
Ellersiek (1986) present acute toxicity test results for a
variety of freshwater invertebrates and fishes tested
with a large number of chemicals, including malathion,
fenthion, dibrom, and temephos. Toxicity of these
mosquito control chemicals to various marine species is
reported in Mayer (1987), Borthwick et al. (1985), and
Parrish et al. (1977). Mulla et al. (1979) reviewed
laboratory and field toxicity of mosquito control
insecticides. Selected data for the subject mosquito
control chemicals are presented in Table 1. These
selected and limited examples generally show that
malathion, fenthion, dibrom and temephos are more
toxic to aquatic invertebrates than fishes. Crustaceans,
in particular, are extremely sensitive to
organophosphorus chemicals, perhaps a result of their
close phylogenetic relationships with insects. Because
there can be up to seven orders-of-magnitude difference
in LCSO concentrations among the most sensitive
species and other fishes and invertebrates tested (Mayer
and Ellersieck 1986), pesticide use restrictions and
controls designed to protect the most sensitive species
should provide protection for most other aquatic, non-
target biota.
Chronic Toxicity to Aquatic Biota
Effects of longer-term exposures on survival and
growth or studies that quantify other sublethal effects
are available only for selected, standard laboratory test
species for some chemicals. - Malathion has been
studied extensively due to its widespread use.
Threshold effects for chronic toxicity of malathion have
been reported at 200 to S80 ug/1 for fathead minnows
(Mount and Stephan 1967), 4 to 7 ug/l for bluegills
(Eaton 1970), and 4 to 9 ug/1 for sheepshead minnows
(Panish et al. 1977). These chronic effects data were
obtained from tests where adult fish were tested for op
to 140 days and tests with their progeny that lasted 28
days in laboratory systems. McKenney (1986) reported
that mysid growth was reduced after 4 days'exposure to
fenthion maintained at concentrations >0.17 ug/1 and
that exposure concentrations >0.08 ug/1 maintained for
14 days significantly 'inhibited mysid growth.
Reproduction also was affected by 14 days of
continuous exposure to fenthion >0.08 ug/1. No other
published information concerning chronic exposure
toxicity tests with invertebrates or fishes and malathion,
fenthion, dibrom, or temephos were revealed in a search
of the literature.
Environmental Persistence
All of the mosquito control chemicals of interest
degrade readily in aquatic environments. Malathion
degrades rapidly, with a half-life of 4 to 8 days, when
released into aquatic habitats with pH and temperature
ranges of 6.5 to 8.5, and 15 to 30 C, respectively
(Bourquin 1977, Wolfe et al. 1977). The aquatic half-
life of fenthion is 4 to 7 days, with enhanced
degradation in habitats with plant components and
sediment- associated bacteria (Cripe etaL 1989, O'Neill
et al. 1989). Dibrom breaks down within hours by
chemical hydrolysis once it enters aquatic
environments, leading to rapid decreases in exposure
concentrations for aquatic organisms (Chen 1984).
Temephos concentrations also diminished in the course
of 2 to 3 days in marine habitats (Lores et al. 1987,
Pierce et al. 1989, 1990). Because these insecticides
degrade or are mixed and diluted to very low or non-
detectable concentrations within days of entering
aquatic habitats, it seems unlikely that exposures
beyond those used for acute toxicity assessments are
needed to accurately characterize potential risks to
aquatic biota under most mosquito control use
conditions. However, research needs for assessing sub-
lethal and long-term effects of repeated, pulse
exposures are discussed in a subsequent section.
Field Studies or Pesticide Effects
Several mosquito control districts and state
laboratories in Florida have participated in field studies
of operational applications of malathion. fenthion. and
temephos to determine effects on son-target, aquatic
species.	-
Malathion was quantified in environmental
samples at concentrations <10% of the values predicted
by worst-case exposure scenarios for truck or aerial
ultra-low-volume (ULV) application (Tucker et al.
1986, 1987). These concentrations decreased to non-
detectable levels within 48 hours and no acute toxicity
among fish or invertebrates held in cages at the field
site was observed. Ground ULV and thermal fog
applications of malathion in a saltmarsh had no acute
toxicity to fish and invertebrates (Tagatz et al. 1974).-
Concentrations of malathion measured in water samples
from the treated area ranged from 0.49 to 5.2 ug/L
• Fenthion has been studied in the field by our
laboratory (Clark et al. 1987) and by staff at the Harbor
Branch Research Laboratory (Tucker etal. 1986, 1987,
Wang et al. 1987). These studies have shown that
exposures of estuarine biota to fenthion are of shorter
duration and of lesser concentrations than those used
for worst-case scenarios in screening-level

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Clark; Adverse Impacts 35
environmental risk assessments. Measured
environmental concentrations following ground ULV or
aerial thermal fog applications of fenthion were at least
an order of magnitude less than those predicted by risk
assessment assumptions of 100% deposition in an acre-
foot of water (Clark et al. 1987). Tidal mixing and
exchange diminished concentrations of fenlhion to non-
detectable levels in open, estuarine environments. In
poorly flushed areas with minimal mixing, the chemical
persisted for up to 4 days at concentrations that were
not acutely toxic to test species. Estuarine biota such as
mysids, penaeid shrimp, spotted seatrout and
sheepshead minnows maintained in cages at several test
sites were not more sensitive to fenthion than
expectations based on laboratory toxicity data.
Dibrom has not been studied extensively due lo
its lack of persistence in the environment and difficulty
in quantitative analyses. A study conducted over
freshwater habitats on a military base in Georgia
reported environmental concentrations <10.5 ug/1 30
minutes after application, with non-detectable residues
at 24 h post-application, the only other sample time
(Livingston et aL 1974). No threat to aquatic resources
was observed k this study. Test applications of dibrom
at operational mosquito control rates near estuarine
environments by aerial, truck mounted, or lawn
equipment resulted in no significant mortality among
test populations of shrimp, crabs, and fish (Bearden
1967). Environmental chemistry was not included in
the study. :	1
Mosquito larvicide applications of temephos
have been studied by Pierce et al. (1989, 1990) in
mangrove habitats of south Florida. Toxicity from
temephos exposure was not observed among five
species of non-target crustaceans and fish studied in the
field. Measured environmental concentrations were
close to predicted concentrations immediately after
larvicide applications directly over water, but tidal
dilution and mixing as well as biodegradation
diminished concentrations in open habitats within hours
after application. Similar environmental concentrations
and persistence in mangrove habitats have been
reported by Lores et al. (1987). Applications of
temephos to freshwater ponds at labeled mosquito
larvicide rates had no adverse impact on bluegill
survival with minimal impact on aquatic invertebrates
other than mosquito larvae (Sanders et al. 1981).
Additional Data Ne«ds
What was not tested in any of these field
experiments was whether the sublethal concentrations
of insecticides persisted for sufficient duration to result
in adverse effects on populations of sensitive
crustaceans. Extended exposures to non-lethal
concentrations of mosquito control insecticides could
result in diminished growth or impairment of
reproduction among non-target species such as mysids,
however long-term characterizations of exposure
concentrations have not been completed. The long-
terra effect of repeated exposures to short-term pulses
of pesticide throughout a mosquito season also have not
been thoroughly assessed. Sublethal effects on non-
target larvae are being addressed by the Mote Marine
Laboratory in Sarasota, FL, through field studies of
larvicide application of temephos (R. Pierce, Mote
Marine Lab, personal communication). The cumulative
effects of multiple-chemical exposures of aquatic biota
in an urban setting that include pesticide application
activities such as insect control by homeowners, lawn
services, golf courses in addition to mosquito control
have not been adequately assessed in the field.
This paper is not intended to provide an
extensive or comprehensive review of effects of
mosquito control chemicals on all non-target aquatic
species. Rather, the reader should review the
environmental issues and risk assessment approaches
discussed in the various papers presented at this
symposium, and use this paper as a source of issues for
considering the extent to which laboratory toxicity data
used in the environmental hazard assessment match the
exposures and responses observed in the field. It is
unlikely that field studies will cover all exposure-
response scenarios in all habitats, thus we must rely on
conceptual approaches to various exposure scenarios
and laboratory toxicity ' test data to provide a
comprehensive basis for decision making and
environmental risk assessment The accuracy and
applicability of those approaches and data should be
evaluated within the context of their intended use.
Table 1. Acute toxicity of malaihion, fenthion, dibrom and temephos to selected freshwater and marine organisms.
All LC50 values reported as ugfl from 96-h tests unless otherwise noted. Daphnia tests are 1C3Q results based on
immobilization of test animals.
Insecticide Species		2£ii	LC2Q	M
Malathion (Cythion)
Freshwater species	....
Water flea (Daphnia pulex)	1,8	(48-h)	1
Amphipod (Gammarus fasciaius)	0.76	1
Glass shrimp (Palaemonetes kadiakensis)	90	I

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36 Clark: Adverse Impacts
InsecTOifc SgSCiSS
mL
.ICSO
Fathead minnow (Pimephales promelas)
Channel catfish (Icmlurm punctams}
Bluegill (Lepomis macrochirus)
Largemouth bass (Micropenis salmoides)
Saltwater species
Mysid (Mysidopsis bahia)
Pink shrimp (Pemeus ducranun)
Grass shrimp (Palaemonetes pugio)
SUverside (Menidia beryllina)
Sheepshead minnow (Cyprinodon variegams)
Spot (Leisotomus xanthurus)
Fenthion (Baytex)
Freshwater species
Water flea (Daphnia pulex)
Amphipod (Gammarus lacustris)
Glass shrimp (Palaemonetes kadiakensis)
Faihead minnow (Pimephales promelas)
Channel catfish (Ictalurus punctams)
Bluegill (Lepomis macrochirus)
Largemootta bass (Mkroptems salmoides)
Saltwater species
Mysid (Mysidopsis bahia)
Pink shrimp (Pemeus dmrarum)
Grass shrimp (Palaemonetes pugio)
S&vereide (Menidia beryl Una)
Sheepshead minnow (Cyprinodon variegam)
Spot (Leisotomus xanthurus)
Dibrom (Naled)
Freshwater species
8,650
8,970
103
285
5.0
20
30
51
0.S
8.4
10
2,440
1,600
1.380
1,540
0.15
0.11
4.7
2,150
1,890
U00
(48-h)
(48-h)
(48-h)
(48-h)
(48-h)
1
1
1
1
2
2
2
1
1
1
1
1
4
4
4
4
4
4
Water Oca (Daphnia pulex}
0.4
(48-h)
I
Amphipod (Gammarus fascials)
18

I
Glass shrimp (Palaemonetes kadiakensis)
92

t
Fathead minnow (Pimephales promelas)
3.300

1
Channel (Ictalurus punctaais)
710

1
Bluegill (Lepomis macrochirus)
2,200

1
Largemouth bass (Mkroptems salmoides)
1,900

I
Saltwater species



Mysid (Mysidopsis bahia)
13

5
Pink shrimp (Penaeus duorarum)
66

5
Grass shrimp (Palaemonetes pugio)
16

5
: Sflvaside (Menidia beryllina)
2,800
(48-h)
5
Sheepshead minnow (Cyprinodon variegam)
1.900
(48-h)
5
Spot (Leisotomus xanthurus)
240
(10.000

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Clark: Adverse Impacts 37
Insectici1,000 (48-h)	5
i; 3 - Pairish et al. (1977); 4 - Borthwick et al. (1984) 5
Most toxicity information available for evaluating potential effects of mosquito control chemicals on non-target
aquatic biota comes from acute lethality tests of 24- to 96-hr duration. These studies generally show that
insecticides are more toxic to aquatic invertebrates than fishes. Crustaceans, in particular, are extremely sensitive to
mosquito control insecticides, perhaps a result of their close phylogenetic relationships with insects. Effects of
longer-term exposures on survival and growth or studies that quantify other sublethal effects are available only for
selected, standard laboratory test species for some chemicals. Field studies conducted by our laboratory following
operational insecticide applications have shown that exposures can be of shorter duration and of lesser concentration
than those used for worst-case scenarios in screening-level environmental risk assessments. However, long-term
effects of repealed applications of the same chemical or cumulative effects of multiple-chemical treatments have not
been adequately assessed.
References
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87/028. 417 pp.

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Clark: Adverse Impacts 39
, T.C., R.A. Lenahan and J.W. Tucker, Jr.
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Environ. Sci. Technol. 11: 88-93.

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4. Ttru-ANLO S4JM1TL*.
TECHNICAL REPORT DATi
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