Putting the Bite on Mosquitoes

Calvert County distributes the eastern mosquitofish, Gambusia holbrooki, for free to gobble up larval mosquitoes in ponds and other contained areas. (By Pouya Dianat -- The Washington Post)
By Dan Morse
Washington Post Staff Writer
Thursday, August 9, 2007

Early August, the dead of a sweltering summer, a time to wish the worst kind of evil upon mosquitoes. Hear, then, what professionals have to say about Gambusia holbrooki , a tiny but hungry fish that is sometimes turned loose in the insect's breeding ground:

" The piranhas of the minnow world."

-- Dan Schamberger,

Maryland Department of Agriculture

" Anything smaller than them is on the menu."

-- Joe Conlon,

American Mosquito Control Association

" It's a feeding frenzy."

-- Barbara Carlough,

Calvert County Mosquito Control Division

To pesticides, bug spray and citronella oil, add the mosquitofish, one of the lesser-known weapons of insect control. Deployed in backyard ponds, large birdbaths and elsewhere, the fish is celebrated by its proponents as a smart bomb in the war between man and mosquito.

In Maryland, state agricultural workers have delivered about 10,000 of the fish this year, mostly on the Eastern Shore. They supplied stock to a Montgomery County homeowners association that wanted to torpedo a storm-water management pond, and they plan to deliver to ponds in a neighborhood in the Annapolis area.

Virginia's York County, just east of Williamsburg, breeds mosquitofish for stocking in storm-water ponds and elsewhere. Arlington County does not supply the fish, but a county environmental health official recommends that homeowners try them ornamental backyard ponds.

In the District, officials have placed the fish into two contained ponds at the U.S. Armed Forces Retirement Home as part of a pilot project. "They're very effective," said Carl Profater, head of the city's West Nile virus monitoring program. "They're very voracious."

Mosquitofish, as Gambusia holbrooki and a related species are commonly known, have been quietly enlisted for decades for insect control. They are hardy enough to thrive in New York City's wastewater treatment plants. Proponents say the method has potentially wide appeal because of concerns over West Nile and chemical insecticides.

The fish, which generally grow no longer than two inches, attack mosquitoes in their larval stage, before they are capable of flight. Each fish eats up to seven times its body weight a day, according to Jim Wanderscheid, head of mosquito control for two counties north of San Francisco, where government workers deliver about 100 batches of mosquitofish a day.

Still, experts are divided over how well the mosquito-killing fish get the job done. "Their efficacy in many situations appears questionable at best and is likely to result in a false sense of security," says the Virginia Department of Game and Inland Fisheries.

The whole idea of mosquitofish makes that department uneasy. For starters, someone could release Gambusia affinis , known as the western mosquitofish, which experts say isn't native to Virginia or Maryland, potentially altering habitat. The department also worries about Gambusia holbrooki , the eastern mosquitofish, which is used in the Washington area and is native to eastern and central Maryland and eastern Virginia. That fish could find its way to certain bodies of water -- small salamander and frog breeding ponds, for example -- and do damage.

But others in the commonwealth swear by mosquitofish, as long as the species is native and correctly deployed.

York County raises Gambusia holbrooki in a series of metal tanks designed as cow troughs. Because adults will eat their young, workers try to keep baby mosquitofish separated. For mosquito control, the county recommends different concentrations, depending on the situation.

Large birdbath: 10 fish.

Sewage lagoon: 1,000 per acre.

Ditch: 1 fish per yard of ditch.

York is the only county in Virginia licensed by the state to distribute gambusia, according to the Virginia Department of Game and Inland Fisheries.

But residents in Northern Virginia are getting their hands on them. If they call into the Arlington County Environmental Health Bureau, mosquito disease specialist Aftab Hussain recommends that they try the fish -- as long as application is limited to small, contained water bodies.

Calvert County in Maryland has found that the fish, which it supplies to certain locations upon request, flourish in a storm-water runoff pond by the public library in Prince Frederick. Mosquito control workers stop by several times a year, ready with a net. They deliver their catch to homeowners or store the fish in temporary quarters -- a 55-gallon plastic drum outside their office -- for later use.

Bill Clay, head of the county's broader mosquito control efforts, said one key to the fish population's success, no matter where they are, is that there be adequate plant cover to shelter the babies from their hungry parents.

Ruth Reid, 74, received a batch of the county's fish about six years ago. She and her husband, Jesse, enjoy evenings outside their Huntingtown home, often on a bench next to a backyard pond brimming with goldfish. Fountains gurgle gently nearby.

The mosquitofish did their job, Reid said, but eventually died out for reasons that remain unknown. Late last month, Reid called for replacements.

Two county workers visited to assess her pond and returned with a new batch of fish. Worker Sonja Gatton tilted a red cooler and slid about two dozen mosquitofish into Reid's pond. No immediate frenzy ensued, a condition that Gatton and her co-worker attributed to the stress of travel.

When the fish do hunt, they tend to do so in schools, their snouts perfectly positioned to pick off their prey. "They just poke their little mouths right up to the surface, and you can see the water ripple," Reid said.

Though the number of West Nile cases in the Washington region has fallen sharply in recent years, concerns persist here and throughout the country. That may be one reason that, according to a leader in the fish farming industry, mosquitofish are more popular than ever.

"It's definitely something that's been growing," said Mike Freeze, an Arkansas fish farmer and vice president of the National Aquaculture Association.

Ken Holyoak, owner of Ken's Hatchery and Fish Farms, Alapaha, Ga., raises mosquitofish in more than 100 acres of ponds. He ships them overnight, carefully packaged, anywhere in the country. Holyoak said one of the fish's positive attributes is that the population will sustain itself. "They're a bunch of sex maniacs," he said.

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