Because stormwater management usually deals with the transmission, storage and treatment of water, there is much concern about the proliferationof mosquito breeding habitat associated with 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.
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 DiseaseControl 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 (see next section) to avoid disease problems related to mosquitoes.
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 5-7 days, so holding water for less than five days will prohibit the progression of life past the larval stage. Standing water for over two 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 once inundated. 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).
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.
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 concerned. 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 three-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 three 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 one-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, as outlined in Chapter 12. Although a recommendation above suggests that “shallow” water less than one-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 1m in width, whereas Chapter 12 recommends bench width at 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 one-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 in Chapter 10 and trying to get the extended detention pool to recede within three 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 five years and could hatch with the resulting larvae inhabiting the pool whenever water levels rise. Standing water mosquito varieties 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.