Location and site design. Stormwater management structures should be located and designed to minimize mosquito-breeding potential. First, sites should be designed to preserve natural drainage and natural treatment systems using low impact development (LID) principles to reduce the need for manmade stormwater control structures.
Second, permanent pool ponds should be designed to minimize shallow depths and increase circulation to support continuous water flow to prevent stagnation and vegetative growth. Third, all site designs should take into account the natural conditions of the land: depth to rock, depth of seasonal high groundwater table, and Karst. Don’t force a specific management practice where it is impractical for proper drainage, but rather use the appropriate practice to reduce occurrences of poor drainage and stagnant water.
Maintenance and construction. Mosquito populations may be boosted by a lack of maintenance and improper construction of management controls. Vegetative overgrowth including floating algae, dead grass, emergent aquatic grasses and weeds, and cattails, along with trash, provide nesting and hiding areas for mosquitoes. If draining is inadequate in constructed wetlands, small puddles remain which support mosquito populations. To reduce mosquito populations, you should:
- Maintain and clean-out temporary erosion and sediment control traps and basins.
- Maintain stormwater ditches to ensure positive drainage.
- Conduct annual vegetative management, such as removing weeds and restricting growth of aquatic vegetation to the periphery of wet ponds.
- Remove grass cuttings, trash and other debris, especially at outlet structures.
- Avoid producing ruts when mowing.
Remember that dry ponds and underground structures usually detain water for less than 30 hours, so if it appears that they are retraining water for more than 5 days, they are poorly maintained. Infiltration trenches and sand filters should not hold water for longer than 2 days. The U.S. Environmental Protection Agency (EPA) suggests that stormwater treatment practices dewater within 72 hours to reduce the number of mosquitoes that mature to adults, as the aquatic stage of many mosquito species is 7 to 10 days. Some states, such as Massachusetts, have established a 72-hour dewatering rule in their stormwater management standards.
Larvicides. Larvicides may be applied as necessary to catch basins, oil/grit separators, wet basins, swales, detention basins, infiltration basins, and construction stormwater wetlands to reduce mosquito populations. A pesticide discharge management plan should be put in place that describes the time and method of application. However, larviciding should not be considered a long-term solution to preventing mosquito production; preconstruction planning and maintenance should be of high priority to avoid the unnecessary application of pesticides. In addition, the application of certain mosquito control larvicides now require a National Pollution Discharge Elimination System (NPDES) permit, increasing workload and technical requirements on site and facility owners and operators.
Amanda Czepiel, J.D., is a Legal Editor for BLR’s environmental law publications. Ms. Czepiel has over 6 years of experience as an attorney and writer in the field of environmental compliance resources and has published numerous articles on a variety of environmental law topics, including wastewater and NPDES permitting, brownfields and contaminated sites remediation, oil spill prevention, wetlands, and corporate sustainability. Before starting her career in publishing, Ms. Czepiel worked in hospitality consulting and for various non-profit organizations and government agencies in the environmental field. Ms. Czepiel received her law degree from the University of Connecticut School of Law.