Mosquito Control: Taking a Bite out of the Food Web
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| Culex quinquefasciatus is one of more than 35 mosquito species that can infect people with West Nile virus. (James Gathany/CDC) |
County and state governments are struggling to find the best way to limit mosquito populations and hopefully lower the incidence of West Nile virus in their jurisdictions. Nearly every method available to these agencies has its drawbacks, adding another element to the question of how to combat West Nile.
Some agencies favor adulticiding, or spraying insecticides in wetlands and other areas, which alarms conservationists. "I think the general consensus is that adulticiding is the least effective means of controlling mosquitoes, although that doesn't mean that it doesn't have its place in certain situations," says Patti Bright, vice president for pesticide programs at the Washington, D.C.-based American Bird Conservancy. "The key to controlling mosquitoes is larviciding early in the season [in spring] before the populations explode. It's taken a while for some mosquito control groups to get up to speed with that." Larvicides kill developing mosquito larvae, which are concentrated in smaller areas than those covered by adult mosquitoes later in the season.
The chemicals and bacteria that agencies use for mosquito control can damage other organisms, so they must be used carefully. Pyrethroid chemicals, while not directly toxic to birds and mammals, can kill a broad spectrum of aquatic invertebrates, fish, and amphibians along with the mosquitoes. Larviciding with a bacterium such as Bacillus thuringiensis israelensis, or BTI, is more effective in targeting just mosquitoes, but even it kills non-biting midges and other insects that are important food sources for wildlife. Methoprene, a substance used to keep mosquito larvae from reaching adult stage, can be toxic to amphibians and shellfish in concentrated doses.
Conservationists are calling for more targeted and informed methods. Mosquito species differ not only in appearance but also in habits and habitat, and different species are West Nile vectors in different parts of the country. "Spraying when you don't know which species you're going after is ineffective," says Bright, who adds that many local governments do just that. Why bother? "Counties may feel pressured to spray," says Bright, "because the public wants to see that something is being done. Spraying is more visible than larviciding."
The mosquito genus Culex by far represents the majority of West Nile vectors, but the day-flying Asian tiger mosquito (Aedes albopictus), an introduced species, is also an efficient vector. It’s now abundant in the D.C. area and many other parts of the United States. Because Asian tiger mosquitoes and some Culex species breed in tiny amounts of standing water, area governments urge residents to tip over flowerpots, frequently change birdbath water, discard old tires that might collect water, and clean gutters to keep vectors of West Nile at bay.
—Howard Youth
ZooGoer 33(2) 2004. Copyright 2004
Friends of the National Zoo.
All rights reserved.
