Frequently Asked Questions (FAQ)

Are mosquitoes a problem with stormwater BMPs, can mosquitoes be eliminated from BMPs, and what preventive devices work best?

Certain BMP designs have the potential to create more mosquito habitat than others. Those designs that hold permanent water, like retention ponds, wet basins, or underground chambers, have a much greater potential to breed mosquitoes than those designed to drain or infiltrate rapidly. Mosquitoes can even utilize underground sources of standing water to breed. According to the California Dept. of Health Services, mosquitoes can "smell" standing water and can navigate in darkness with no problem. Adults may pass through spaces as narrow as 1/8 of an inch to access breeding sources. Consequently, those actions and designs that eliminate standing water, seal all access cracks, and cover openings with netting have been shown to be effective in above- and below-ground BMPs installed by Caltrans in its retrofit pilot program in Los Angeles and San Diego Counties. The long-term effectiveness, though, of those preventative measures requires further study.

While it is nearly impossible to completely eliminate mosquitoes from BMPs because of the large number of factors (e.g. sediment build-up, rain fall, non-stormwater discharges, etc.) that influence these devices, properly designed, managed, and maintained, this BMP type should produce negligible numbers of mosquitoes. For example, it is important that underground BMPs that hold standing water be built with tightly fitting covers. Similarly, inlet and outlet pipes from the BMP sump should be submerged (or sealed during no-flow periods) to minimize mosquito access to water.

Solutions for minimizing the potential for mosquito breeding will vary from one location to another depending on local influencing factors (e.g. BMP located in a neighborhood vs. along an interstate; BMP in southern California vs. northern California). Local mosquito and vector control agencies can provide valuable suggestions on how to minimize mosquito breeding in a given area. Ideally, open BMPs should be designed with steep slopes to discourage invasive vegetation from filling the structure, or from becoming too thick or dense along the perimeter. Thick or dense vegetation is conducive to mosquito breeding and allows mosquito larvae to hide from natural predators. Mosquito management should also include the use of predatory fish in the pond, such as mosquito fish, that eat mosquito larvae. Fish populations should be checked periodically and restocked when necessary, especially after cold winters. Maintenance to prevent mosquito breeding should include regular vegetation management, such as thinning and removal, to keep emergent and perimeter vegetation under control.

According to the California Dept. of Health Services, some devices and preventive actions are helpful while others are not encouraged for mosquito control. For example, surface agitators and surface sprinklers have been shown to be effective in wastewater treatment pond applications, but regular daily maintenance is critical and they are dependent on outside energy sources.

Time-release pesticide dispensers, however, would kill a large number of, but not all, mosquitoes. The Department of Health Services claims that mosquitoes and other vectors develop resistance to pesticides rapidly, so pesticides should only be used as a last resort. Those mosquitoes that survive will breed and quickly produce offspring that will be immune to the pesticide. Oil application is not always effective because the oil must cover the entire water surface, and oils can impair the performance of the BMP in removing hydrocarbons. Bug zappers for controlling flying insects are usually ineffective against mosquitoes and also require maintenance and an energy source. For more information, contact the California Department of Health Services, Vector-Borne Disease Section, at 909-937-3448.

  CA Department of Public Health

  Managing Mosqitoes in Stormwater Treatment Devices

  Caltrans BMP Retrofit Pilot Program--Final Report