United States
Environ mental Protection
Agency
Stormwater Structures
& Mosquitoes
EPA 833-F-05-003
Office of Water
August 2005
WHAT is
STORMWATER?
Stormwater is the rain or
snowmelt that does not
initially infiltrate into the
ground and runs off of
surfaces and is transported
into nearby waterways.
FACTS ABOUT MOSQUITOES
6 There are over 2500 mosquito
species worldwide, about 200 of
which are found in the United
States.
6 Only female mosquitoes transmit
diseases since they need the
protein from blood to breed.
6 The primary breeding habitat for
mosquitoes is stagnant or
shallow pools of water (generally
less than 3 feet in depth) that
exist for at least 7 days and/or
aquatic sites with dense floating
vegetation regardless of the
water depth.
6 Depending upon species, the
adult mosquitoes may live from 1
week up to 3 months maximum.
b Mosquito predators include
birds, fish, dragonflies, spiders,
and a wide variety of aquatic
insects.
What's the Issue?
Stormwater structures that temporarily or permanently retain runoff are
receiving increasing attention as potential mosquito breeding areas.
Mosquito-borne diseases such as West Nile virus, St. Louis encephalitis, and
eastern and western equine encephalitis are human health concerns. Measures
that lower mosquito production in stormwater structures are needed to protect
public health.
If designed properly, stormwater structures should not
promote mosquito breeding. Ensuring that these
structures are properly designed and maintained is the
key to limiting mosquito production.
How is STORMWATER MANAGED?
Historically, stormwater controls were designed to
quickly collect, store, and transport runoff away
from developed areas into nearby streams to prevent
flooding. However, it is now recognized that these
systems alone are often not the ideal solution because
they impact streams by increasing the volume and
velocity of water and amount of pollutants.
Today stormwater management promotes a variety
of practices and controls that help to infiltrate
runoff and minimize contact of runoff with pollutants. For example, infiltration practices
(which can be cheaper and easier to maintain than traditional stormwater practices)
involve using vegetated areas like swales and rain gardens (a.k.a. bioretention cells) to
slow the velocity of water and allow for percolation into the ground. When properly
designed and maintained, stormwater management practices are not conducive as habitat
for mosquito breeding.
WHAT SHOULD LOCAL AUTHORITIES Do?
Stormwater managers should incorporate design, construction, management, and
maintenance features into stormwater structures to minimize mosquito production (and
therefore decrease or eliminate the need for insecticides) without compromising water
quality functions.
Local authorities should properly inspect and maintain stormwater structures to ensure
their continued effectiveness, reduce the need for costly pesticide applications, and prevent
large outbreaks of mosquitoes.
However, it might still be necessary for state, county, or local governments to apply a
limited amount of insecticides to control mosquitoes. Mosquito control officials use EPA-
registered products that do not pose unreasonable risks to human health, wildlife, or the
environment. Monitoring efforts that involve field inspections by mosquito control
personnel determine when and where insecticide applications are needed. However, as
with all pesticide use, the use of insecticides in stormwater structures should be minimized.
Stormwater managers should work closely with mosquito control officials to help achieve
this goal.
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Basins
There are two main types of
basins used to manage
stormwater - dry detention
and wet retention basins. Dry
detention basins are designed
to hold water during storm
events and then release the
water within 3 days. Because these systems are designed to
hold water for only short periods of time, they are not
suitable habitat for mosquitoes. The aquatic stages of many
mosquito species require 7-10 days in calm, standing water.
Wet retention basins are designed to hold permanent pools
of water. These systems are usually between 3 and 8 feet in
depth. Most mosquitoes only breed in shallow standing
water (i.e. less than 3 feet) or deeply vegetated waters so
mosquito breeding should not occur.
There are several maintenance considerations associated with
basins to make these sites unsuitable as mosquito habitats.
Debris and sediment must be removed from inlets, outlets
and the bottom of the pond; eroded areas must be repaired;
bare ground must be seeded to prevent soil loss; and plants
must be harvested as needed.
Created Wetlands
Wetlands are vegetated areas
designed to contain shallow,
slow moving water. While these
two characteristics are typically
preferred by mosquitoes, healthy
wetlands can actually prevent
mosquito outbreaks. Mosquito breeding can be minimized
through site design and management considerations that
include mosquito predators such as fish and several types of
aquatic insects.
Wetlands must be inspected for invasive plants, which must be
removed; signs of erosion should be recognized and repaired;
and inlets and outlets should be checked and accumulated
debris or sediment should be removed.
Fountains
Fountains typically found near
large buildings retain and
slowly release stormwater.
These structures range in
depth. Aerators can
be added to these fountains to agitate the water thereby
deterring mosquitoes since they prefer standing water.
Aerators should be checked regularly to ensure that they are
working properly.
CONTACTS/RESOURCES
Stormmater
EPA's NPDES Stormwater Program http://www.epa.gov/npdes/stormwater
Stormwater Manager's Resource Center http: / /www.stormwatercenter.net
Mosquitoes/Pesticides/Mosquito-borne Diseases
EPA's Website on Pesticides http://www.epa.gov/pesticides/
The American Mosquito Control Association http: / /www.mosquito.org
Centers for Disease Control and Prevention http: / /www.cdc.gov
Storm Sewer Systems
Storm sewer systems include
structures like catch basins.
By design, catch basins, which
are sumps located directly
under storm drains, hold
standing water. These
structures require maintenance to ensure that debris does not
accumulate in the storm drain grate or the storage area allowing
mosquito breeding. Sometimes the only practical means of
mosquito control involves the use of insecticides to kill the
larvae.
Catch basins must be cleaned throughout the year to remove
accumulated sediment. Screens and other devices used to
remove debris must be checked regularly to ensure that they
are working properly.
Rain Gardens
Rain gardens, also known as
bioretention cells, are vegetated
areas designed to retain and
infiltrate stormwater. These
areas are designed to not have
standing water for more than a
I day or so except during very
large storm events. Therefore when properly designed and
maintained, rain gardens should not sustain mosquito
populations.
These areas have some maintenance requirements to ensure
their continued effectiveness. Accumulated litter and debris
must be removed regularly; areas must be remulched as
necessary; grassed areas must be mowed; areas showing signs
of soil erosion must be repaired; and dead and diseased
vegetation must be removed and replaced with healthy
vegetation.
Rain Barrels/Cisterns
Rain barrels and cisterns allow homeowners
to disconnect downspouts and divert runoff
into a storage tank. These barrels decrease
the volume of runoff and allow the owner to
reuse the water for irrigation. Several
precautions should be followed to prevent
mosquito breeding, such as keeping barrels
tightly closed, using debris screens to filter the water entering
the barrel, and using the collected water within several days.
COMMONLY OVERLOOKED BREEDING AREAS:
RESIDENTIAL BACKYARDS
Homeowners should check their property to eliminate mosquito
breeding. Water can collect in unused flower pots, buckets, cups, old
tires, etc. and may provide the perfect habitat for mosquitoes. What
can homeowners do to deter mosquito breeding?
6 Pick up trash, such as paper cups, which may have collected
in the yard.
& Clear clogged rain gutters.
& Cover containers, tires, wading pools, and all other items
which can hold standing water for extended periods of time.
Change the water in bird baths and pet dishes regularly.
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