Because stormwater management usually deals with the transmission, storage and treatment of water, there is much concern about the proliferation of mosquito breeding habitat associated with Best Management Practices (BMPs). This is a well-founded concern because mosquitoes may colonize any source of standing water provided there is a source of organic material to provide sustenance to larvae (Messer, 2003). Although this basic fact often means that BMPs will result in more mosquitoes, there are many design and management measures that can be followed to minimize this increase in mosquito population.
The primary threat to Minnesotans from mosquitoes, besides the nuisance, is the transmission of serious disease. West Nile Virus (WNV) and various forms of encephalitis are the major concerns. In spite of this threat, the U.S. Department of Health and Human Services Centers for Disease Control and Prevention (CDC) and Minnesota Department of Health both point out that a very small percentage of mosquitoes are vectors for disease and many of those bitten by carriers will not experience major health consequences, although minor difficulties could develop. Both organizations advise avoidance of outside activity, use of repellents and good integrated pest management programs to avoid disease problems related to mosquitoes.
Mosquitoes in Minnesota
Minnesota is fortunate to have a major mosquito research and management agency, the Metropolitan Mosquito Control District (MMCD), in the Twin Cities metropolitan area, as well as research in other parts of the state by the University of Minnesota and the Minnesota Department of Health. They have been able to characterize the occurrence of mosquitoes and the problems they cause in the state.
Information provided by Nancy Read of the MMCD via education material (ex. Minnesota Erosion Control Association Annual Conference, 2004) included the following basic facts.
- There are about 50 varieties of mosquito in the state, but only a few are efficient transmitters of diseases such as WNV.
- All mosquitoes need water for the larval and pupal stages of development. The larval stage lasts anywhere from 5 to 7 days, so holding water for less than 5 days will prohibit the progression of life past the larval stage. Standing water for over 2 weeks can easily breed mosquitoes if not treated.
- Aedes vexans is the most common Minnesota mosquito. It is a “floodwater” mosquito that lays its eggs on moist surfaces near water and relies on periodic submersion for eggs to hatch into larvae. Eggs can remain viable on moist surfaces for years before hatching. It is a vector (or carrier) of heartworm disease and may have a small role in WNV transmission.
- Ochlerotatus triseriatus is a “treehole” variety floodwater mosquito that lays eggs in containers that periodically fill with water, such as tires, bird baths, or holes in a tree. This variety is a vector for LaCrosse encephalitis, which affects primarily children.
- Culex tarsalis is a standing-water species that is principally responsible for the spread of WNV in the western US. It lays eggs in “rafts” in standing water. The ideal habitat for Culex species are areas that will remain wet for about two weeks, contain vegetation for shelter and nourishment, and have few predatory fish.
- Culex pipiens and restuans are species often found in stormwater catch basins, rip-rapped areas and ponds with vegetative debris. MMCD treats 50,000 water-holding catch basins in the Twin Cities metropolitan area to control these species.
- The larvae of the cattail mosquito, Coquillettidia perturbans, attach themselves to cattails and breathe through the inner air tube. Eggs are laid in late summer, with larvae able to over-winter under the ice. These varieties emerge as adults in large quantities around mid-summer.
- MMCD uses an integrated pest management (IPM) approach to controlling mosquitoes that targets primarily the larval stage through the use of bacteria (Bti or Bacillus thuringiensis var israelensis) toxic to larvae and growth regulators (methoprene) that inhibit larval development. Some limited spraying with synthetic pyrethoids is done for adults. IPM also includes good site design for BMPs and encourages biological control agents like predators (especially fish).
Methods to limit mosquito breeding in stormwater facilities
The presence and behavior of water is the most important element to the continuing life cycle of the mosquito. Controlling standing and stagnant water, and adapting design and habitat conditions are the ways stormwater managers can avoid a proliferation of mosquito breeding in association with stormwater BMPs.
A number of technical publications, articles and fact sheets on mosquitoes ( Aichinger, 2004; Commonwealth of Virginia, 2003; Messer, 2003; Metzger, 2003; Nancy Read, MMCD, personal communications; Stanek, brochure with no date; USEPA, brochure with no date; Wass, 2003) were evaluated to come up with the following advisory material for homeowners (possible public information for SWPPPs) and stormwater managers.
- Eliminate standing and stagnant water around the home, such as in abandoned tires, boat covers, wheelbarrows, flower pots, or other containers. Change the water in wading pools, birdbaths, or dog dishes frequently.
- Protect family members from mosquito contact via such measures as house screening, avoidance during hours of maximum exposure, repellents, and clothing coverage.
- Chlorinate, clean and cover swimming pools, and prevent water from collecting on cover.
- Unclog roof drains and downspouts.
- Aerate water gardens or use fish to prevent larval mosquito development.
- Screen rain barrels to keep adult mosquitoes from laying eggs.
Stormwater manager actions
- Use BSD/LID development techniques to reduce the amount of stormwater that needs to be conveyed and managed.
- Do not allow water to collect in “temporary” facilities for longer than five days, preferably less than three.
- Adhere to Minnesota Construction General Permit requirement to drain infiltration/filtration BMPs within 48 hours.
- Avoid allowing standing water to collect in inlets and outlets and in conveyance pipes; avoid corrugated pipe without constant flow and sumps in catch basins.
- Maintain and clean-out sediment traps/basins and all drainage structures, inlets, outlets and orifices (use only openings >3 inches in diameter to prevent clogging) to keep positive water drainage.
- Screen inlet and outlet pipes or place under water if no other control available (prevents fly-in).
- Eliminate standing stagnant water as part of any BMP appurtenance, including forebays, sediment traps, sump areas and pumps.
- Avoid the use of rip-rap that can catch and hold organic debris in a wet area.
- Design de-watering capability into every BMP for routine dry-down and maintenance.
- Minimize installation of BMPs that will collect stormwater for only brief periods then stagnate until the next event; this could include a water budget analysis to make sure some baseflow will occur through the BMP.
- Minimize shallow depths (less than 1 foot) as part of ponds and wetlands; if this cannot be done, make sure flow continually occurs over the shallow area.
- Design facilities to minimize vegetation overgrowth floating organic debris, algae, trash, sediment dead grass/clippings, and cattails.
- Avoid the use of mulch that will wash into any BMP (use geotechnical material or secured mats instead).
- Avoid vegetation cutting operations that leave debris, blow into standing water, or leave ruts for water accumulation.
- Keep dense emergent vegetation limited to narrow (<1 meter) bands around areas with standing water and prevent the development of cattail stands.
- Keep permanent pool embankments steep to prevent emergent vegetation, especially cattails, from growing; carefully plan plant species for aquatic/access benches to avoid cattail intrusion.
- Fall draw-down on cattail marshes can be a very effective control for cattail mosquitoes, which overwinter as larvae in the water.
- Design healthy natural systems that encourage mosquito predators to thrive and have access to mosquito larvae; this includes open water (over 4 feet deep) as part of wetland design (preferably oriented perpendicular to flow-through), minimization of stagnant, non-flowing water, creation of diverse vegetation along periphery of ponds.
- For stormwater wetlands, maintain a constant water table just below the ground surface (or above ground <5 days) to minimize mosquito production.
- Require a written inspection and maintenance plan that addresses stagnant water, water quality, and vegetation and debris management.
- Consider including mosquito control as a potential annual maintenance cost in some situations.
- Work with vector control agencies on integrated pest management approach to larval control.
- Always design access for vector control staff to reach entire BMP, not just the inlet or outlet.
- Properly design and maintain all stormwater BMPs.
Information: The recommendations listed with a* above could be designs that appear to conflict with common BMP use
Compatibility with Common BMP Design
A cursory consideration of the list of commonly used Minnesota BMPs relative to the above list would seem to indicate that some BMPs might be more desirable than others when mosquitoes are considered. The practices that would seem to be the best for preventing mosquitoes would be permanent pools with steep slopes below the water line, infiltration devices that drain effectively in 48 hours, bioretention that infiltrates or filters water then dries at the surface, dry ponds, ponds with a water quality volume that is fully treated and discharged within three days, and healthy pond/wetland systems (those with diverse vegetation, open water areas over 3 feet in depth, fairly steady water levels and low nutrient loads).
Practices that would seem to cause mosquito breeding to proliferate would include water basins or holding areas that hold water in a stagnant condition for longer than 3 days, sub-grade treatment systems that include sumps and are not properly sealed, poorly maintained water holding areas that contain substantial amounts of vegetative debris, wet meadows with less than 1 foot of standing water, and storage areas that bounce up and down repeatedly. Not all of these systems need to be dropped from the list of suitable BMPs, but their use should be supplemented with IPM techniques (ex. biological larvicides), physical sealing, or adequate maintenance.
Although some of the recommendations for addressing mosquito concerns appear to conflict with common BMP design, careful consideration can alleviate those concerns. Considerations include the following.
- Avoiding excessive vegetative growth does not mean minimizing vegetation; rather it means keeping a healthy mix that thrives and does not overwhelm the BMP or an (upland) area adjacent to a BMP. The same applies for emergent vegetation that is planted as part of an overall planting scheme.
- Shallow vegetated benches are part of the recommended access design for ponds. Although a recommendation above suggests that “shallow” water less than 1 foot be avoided in standing water situations, it might be necessary, depending upon access needs, to construct such a bench. In addition, a recommendation above suggests that dense periphery vegetation be limited to about 1 meter in width, whereas recommendations for pond bench width is 10 feet. Designers are advised to use their judgment on the mix of recommendations for edge-of-pond depth, depending upon priorities for access relative to mosquito control. Care should be taken in plant selection, particularly if bench depths less than 1 foot are anticipated.
- Rip-rap or similar structural armor for bank stabilization are options that are sometimes needed in erosive situations. The tendency for these materials to capture vegetative debris and to create small pools of water make them ideal mosquito breeding sites. If mosquito breeding is a concern at these installations, smoothing with a grout material or size grading can be used to minimize edges and pools that promote mosquito habitat, or alternative materials can be used.
- The required wet basin design in the MPCA CGP contains a water quality volume that is temporarily detained above the permanent pool. Although there are no CGP requirements for the amount of time this should be held, a minimum of 12 hours is recommended and trying to get the extended detention pool to recede within 3 days is a good goal to minimize possible mosquito breeding. Floodwater mosquito egg-laying on the moist side slopes above the permanent pool is almost impossible to control in this situation because the eggs remain viable for up to 5 years and could hatch with the resulting larvae inhabiting the pool whenever water levels rise. Mosquito varieties that require standing water can be minimized with a management plan that allows these areas to fully dry out between events. If conditions cannot be improved to minimize breeding habitat, biologic larvicides should be used.
- Forebays, sediment traps and treatment sumps could all be part of a well designed treatment train. The recommendation above to keep these from becoming stagnant is consistent with good design principles and should not preclude their use. The essential elements in keeping them “fresh” are to either drain them fully after use or keep baseflow moving through them. MMCD began a monitoring program in underground structures in 2005 and has found evidence of mosquito breeding in half of the structures tested through mid-summer of 2005. Studies in California outline more details of which structures are most likely to provide habitat for mosquitoes (Metzger, et al., 2002).
- In summary, there are many ways in which stormwater BMPs can become mosquito breeding grounds if caution is not followed in their design, operation and maintenance. The means exist to install BMPs that minimize the creation of mosquito habitat and/or to biologically attack the larvae that result even under the best designs.
- Aichinger, C., 2004. Understanding the West Nile Virus. Woodbury Bulletin (newspaper opinion page), August 11, 2004. Contact Ramsey-Washington Metro Watershed District, North St. Paul, MN.
- Commonwealth of Virginia, 2003. Vector Control: Mosquitoes and Storm Water Management. Stormwater Management Technical Bulletin No 8.
- Messer, D.F., 2003. Mosquitoes in Structural Stormwater BMPs: A Case Study. In Proceedings of the StormCon Conference of 2003, San Antonio, Texas. Published by Forester Communications, Santa Barbara, CA.
- Metzger, M.E., 2003. Mosquito Control Challenges Presented by Stormwater Treatment Devices in the United States. In Proceedings of the StormCon Conference of 2003, San Antonio, Texas. Published by Forester Communications, Santa Barbara, CA.
- Metzger, M.E., D.F. Messner, C.L. Beitia, C.M. Meyers, and V.L. Kramer, 2002. The Dark Side of Stormwater Runoff Management: Disease Vectors Associated with Structural BMPs. Stormwater. 3(2):24-39.
- Minnesota Department of Transportation, 2005. The Cost and Effectiveness of Stormwater Management Practices. Report 2005-23, St. Paul, MN.
- Stanek, S. (no date). West Nile Virus and Stormwater Management. Brochure prepared for the Minnehaha Creek Watershed District, Deephaven, MN.
- U.S. EPA (no date). Wetlands and West Nile Virus brochure.
- Wass, R.D., 2003. Mosquito Management Do’s and Don’ts in an Engineered Arizona Treatment Wetland System. In Proceedings of the StormCon Conference of 2003, San Antonio, Texas. Published by Forester Communications, Santa Barbara, CA.
Links to information on mosquito control