Surface Water Management Division
Stormwater Detention, Mosquitoes, and West Nile Virus
Should I be concerned about mosquitoes in detention facilities?
With the arrival of West Nile virus it is appropriate to learn about your relative risk and what precautions you can take. The Washington State Department of Health considers the current risk from West Nile virus to be “very low." Your best action is personal protection. Wear long sleeves and long pants at dusk. Screen your windows. Use insect repellants containing DEET.
Of the 28 mosquito species that live in Snohomish County, the one considered the primary carrier of West Nile virus breeds in the small pockets of standing water often found near homes and businesses. Clogged gutters, old tires, flowerpot trays, buckets, and birdbaths provide ideal breeding habitat with no predators. Removing or maintaining these pockets of water can stop mosquitoes at their source.
Properly maintained detention ponds attract natural predators of mosquitoes such as dragonflies, aquatic beetles, and amphibians, and are less likely to produce adult mosquitoes.
Should I attempt to control mosquitoes in my detention facility?
You are not required to control mosquitoes in your detention facility. However, after you have learned about your relative risk and what precautions are available, you may decide that some form of mosquito control is appropriate.
Mosquito populations can be safely controlled in detention facilities by a number of methods. Healthy wetland plants in your detention pond will help reduce mosquito populations and encourage mosquito predators.
As a last resort, larvicides (insecticides that kill insect larvae) can be applied to the water where eggs are hatching. By state and federal law, application of larvicide requires a permit and must be applied by a licensed pesticide applicator.
What should I NOT do?
You should NOT put pesticides or toxic chemicals into detention ponds, ditches, creeks, streams, or wetlands. Unless you are properly trained and licensed, such actions are illegal and violate county, state and federal laws.
Pesticides and other toxic chemicals can cause harm to people and the environment. They may even pose a greater health hazard than the mosquito-borne diseases they are intended to reduce. They can also kill natural predators of mosquitoes and may actually increase mosquito populations!
What is Snohomish County doing about stormwater detention facilities it owns?
Snohomish County is currently reviewing its detention facility maintenance practices to reduce potential mosquito habitat. Snohomish County is not currently applying larvicides, but may consider it if the Snohomish Health District indicates a significant public health problem.
What about mosquitoes in wetlands?
Mosquitoes are a natural part of our environment. They serve important roles as pollinators and as dinner for fish, amphibians, birds, and other wildlife.
Treating wetlands to kill mosquitoes can disturb the ecological balance of wetlands and reduce the food supply for beneficial insects and wildlife. Treating wetlands can also harm important predators of mosquitos such as dragonflies, aquatic beetles, frogs, and salamanders.
Download the Stormwater Detention, Mosquitoes, and West Nile Virus Factsheet (PDF)
Contact
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Detention facility maintenance:
Nat Washington, Surface Water Management, 425-388-3464 ext. 4608
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