Mosquito Control Practices Near Wetlands: Methods, Risks and Limits

We’ve mentioned before how Kohl Construction’s plans to build condos near wetlands would place a large number of residents in an area that’s buzzing with mosquitoes during the warmer months. It would be hard to blame those residents from wanting to apply pesticide and other mosquito control measures to prevent disease and use their outdoor spaces in comfort. Mass Audubon has these comments on mosquito control practices:

  • Source reduction involves removing areas of stagnant water where mosquitoes breed. For example, buckets, tires, and other artificial containers can be removed. Stormwater detention basins should be designed to drain within a few days after each storm or to hold permanent ponds of water that can support mosquito predators like fish. Catch basins should be cleaned regularly. Roadways and stormwater outfalls should be cleaned and maintained to minimize deposition of sediment into streams…
  • Adulticiding involves spraying chemical pesticides, such as malathion, resmethrin, or sumithrin (Anvil) to kill flying adult mosquitoes. This method has only short-term local effectiveness, as new mosquitoes soon fly into the area or emerge from breeding pools. Spraying of broad-spectrum pesticides also exposes people, pets, and wildlife to the chemicals. Butterflies, bees, aquatic invertebrates and fish are particularly sensitive to some of the commonly used adulticides. Routine spraying for nuisance mosquito control also may lead to pesticide-resistant mosquitoes, which would be more difficult to control in the event of a public health emergency. Pesticide spraying may present some level of human health risk as well, particularly for certain sensitive individuals such as asthmatics. For information on the health effects of pesticides, contact the Department of Public Health’s Bureau of Environmental Health Assessment at (617) 624-5757. Mass Audubon opposes adulticiding for nuisance control of mosquitoes.
  • Larviciding involves application of chemical or bacterial materials to mosquito breeding areas to kill mosquito larvae. Bacillus thuringensis israelensis (Bti) is a toxin producing bacterium that, unlike broad-spectrum chemical pesticides, narrowly targets mosquitoes, midges, and other closely related flies. However, Bti may disrupt the food web in vernal pools where amphibians breed.

    Another commonly used larvicide is methoprene. It is a growth regulator, which acts by interfering with the normal metamorphosis process thereby preventing mosquito (and various other insect) larvae from reaching the adult stage. Methoprene briquettes are often used in catch basins, because they provide a much longer duration of control than Bti…

The effectiveness of mosquito control in a rural or suburban landscape with large amounts of wetlands is questionable. There is little documentation of the effectiveness of mosquito control activities in Massachusetts, and most of the available information is on short-term effects within a few days after pesticide applications.

Pesticide applications and wetland ditching or draining can harm or kill beneficial species (including pollinators and mosquito predators), alter water chemistry; lower water levels, and degrade wetland habitats. The high reproductive rate and short life-cycle of mosquitoes may allow populations to evolve which are resistant to the pesticides, while local populations of mosquito predators (such as frogs, fish, and predatory insects) are less resilient.

The benefits of simply giving wetlands a wide berth–50 feet or more–are apparent. In addition, the article below suggests how difficult it can be to design stormwater mitigation systems that don’t promote mosquitoes yet remain effective, practical, attractive, and safe for residents. One solution is designing projects that eliminate or minimize the use of the systems described below, by reducing impervious surface and favoring detached houses instead of larger structures.

University of Sydney and Westmead Hospital, Australia: Freshwater Wetlands – Mosquito production & management
Natural freshwater wetlands (swamps and marshes) provide habitats for a diverse range of mosquito species, including some potential pests and vectors (carriers) of pathogens. They are often not productive of large numbers of mosquitoes, because predators and other factors exert some control, but they must be carefully assessed individually for control requirements.

Constructed (artificial) wetlands, built to manage and treat urban storm and/or wastewater, come in various formats; they may start out as simple vegetated pools but can develop into dense swamps. They have the potential to be more productive of mosquitoes than their natural counterparts, and must be carefully assessed for mosquito productivity and management…

Although mosquito management principles are often incompatible with objectives and operations of constructed wetlands, the health issues are not insignificant. Engineers and other professionals associated with wetland construction should be aware of the various requirements and opportunities for mosquito management within the framework of their objectives for water control and treatment…

Constructed wetlands contain various zones. Gross pollutant traps and sedimentation zones must be managed to prevent blockages and buildup in pollution which will promote stagnant water breeding. Rock flume riffle zones dissipate energy and prevent scouring; they are also useful in maintaining the integrity of the edges and in oxygenating the water, and low flows can infiltrate the porous ramp and thus reduce pooling. A weir at the downstream end of the wetland can maintain habitat for macrophytes and a reservoir for larval predators.

Sedimentation areas intended to be ‘deep’, and vegetated treatment zones intended to be ‘shallow’, provide different opportunities for mosquitoes. Shallow vegetated water typically supports more mosquito breeding; deep pools with steep and deep edges, and no emergent or surface vegetation, provide less suitable habitat for mosquitoes.

Any wetland area, constructed as a shallow vegetated pond, will be a major concern for mosquito breeding. Ponds with simple shapes and a low edge to area ratio are likely to be less productive than those with a greater proportion of edge to area in more intricate designs.

Concrete vertical edges are ideal but these are usually not acceptable for aesthetic reasons. Steep earthen edges or grassy banks are often unacceptable for public safety reasons. The safety issue is becoming more important, but there has to be a recognition of the local mosquito concerns and a compromise considered.

With larger areas, profiling the bottom with the depth greatest at the inflow end is advisable. Higher numbers of mosquitoes can be associated with smaller rather than larger basins, but if the site is too large it may become a shallow marsh supporting large numbers of mosquitoes.

…incoming water that is high in chemicals may kill vegetation and predators, and possibly the current mosquito population, but mosquitoes are likely to reestablish more quickly than the predators.

…chemical usage should not be viewed as a long term strategy, and should be resorted to only when there are occasional episodes of heavy uncontrolled breeding. Prolonged use will lead to development of resistance in mosquito populations, thereby limiting overall management options. Relatively few chemicals can be recommended for use in wetlands, whether natural or constructed (which usually flow into natural water systems), because of environmental concerns…

Recall that a particular challenge on Kohl’s land is its high water table. Test pits (PDF, 15.3MB, pages 30-38, 98-121)  show that seasonal high groundwater is as close as 18 inches to the surface. Kohl’s plans call for detention and bioretention basins that might be forced to be shallow, favoring mosquito breeding.

See also:

Photos Show: Man-Made Lakes and Stormwater Retention Systems
Are No Substitute for Natural Wetlands

“A band of white pollutants and algae float at the top of a stormwater
retention pond at Belle Hall Plantation. Though created to handle
pollution and often full of mosquito larvae, this type of pond is
considered a functioning wetland under definitions by the U.S.
Department of the Interior.”

More Reasons Why Smart People Don’t Build Near Wetlands: Mosquitoes and Disease

EPA: Do Stormwater Retention Ponds Contribute to Mosquito Problems? [emphasis added]
…Reducing our reliance on stormwater ponds for runoff control is
another way to reduce potential mosquito breeding habitat.
More people
are turning to alternative non-structural techniques, such as rain
gardens, bioinfiltration, infiltration, and vegetative swales, that
slow down water and help it infiltrate without extended periods of
ponding. These techniques are successfully minimizing or eliminating
the need for stormwater ponds or significantly reducing the pond size
requirements. Care must be taken to ensure that these alternative
controls drain all standing water as designed over the years.

Similarly, efforts to reduce the amount of impervious surface
in communities can reduce the need for stormwater ponds.
streets, sidewalk-less communities, and elimination of cul-de-sacs are
just a few of the ways that communities are now reducing the need for
stormwater controls. That is not to imply that stormwater ponds can be
eliminated easily. Retention/detention ponds use less space than many
other types of stormwater controls and are often found to be the best
and cheapest way to control runoff–especially when flooding is a

Mosquito proliferation in stormwater ponds is a concern,
especially when so many wet and dry ponds are in place and continue to
be installed across the country. Many ponds are not properly
maintained, particularly in cases where they are installed in
subdivisions and other developments where the entity responsible for
long-term maintenance is not clearly defined once the construction is

Detention Pools, Children and Drowning