Mosquito On The Menu At The Food-chain Diner

Pest-eating Species Used For Insect Control

August 27, 1991|By John A. Morris | John A. Morris,Staff writer

Mike Norris and Alan Revell strapped on bright-yellow hip-waders andstepped gingerly out into the middle of an Arnold storm-water control pond. They stretched a brown seine net down into the greenish, murky water and dragged it back to shore.

On dry ground, the net spilled open to reveal hundreds of tiny, silver fish.

"This pond was a constant source of mosquitoes until we put thesein there," said Norris, a supervisor with the county Department of Public Works mosquito control program. "Since then, we haven't seen any mosquitoes at all."

That's because the 2 1/2-inch-long, flat-headed fish -- gambusia holbrooki -- eats mosquito larvae before they can mature.

The county is experimenting with the gambusia, also known as the mosquitofish, in its storm water control ponds. They hope touse the fish to cut back on mosquito populations, rather than douse neighborhoods with chemical pesticides.

The Department of Public Works has used Cythion, a chemical pesticide with a skunk-like odor, to control mosquitoes in county neighborhoods since 1972, Norris said.

Although the least toxic pesticide Norris said the county has been able to find, the spraying draws some resident complaints. It smells bad and can cause ill health effects if breathed directly into the lungs, he said.

"It's a pesticide. We're not spraying water from those county trucks," Norris said.

To cut back on the use of pesticides, the county is seeking alternatives, including a bacteria and the mosquitofish, that attack the larvae. Cythion is used specifically to kill airborne adults.

The county's pilot mosquitofish program is part of a statewide experiment that began last year, said Stanley Joseph, chief of the state Department of Agriculture's mosquito control section. The performance of the mosquitofish is being evaluated at more than 150 sites, including storm water ponds and sewage lagoons around the state, he said.

So far, so good, Norris said. An adult mosquitofish can eat 1,000 insect larvae a day.

Norris said the mosquitofish can survive in the poorest quality water, making them idealfor the storm water ponds. The county dropped 100 mosquitofish into the storm water pond off Finneans Run in Arnold last year. Less than 18 months later, thousands thrive.

Using excess mosquito fish fromthat pond, the county has stocked seven others in Lake Shore, Pasadena, Severn and Foreman's Corner. Yesterday's harvest was destined fora pond in the Greenhaven community.

Still, the mosquitofish -- which has been used in some states for decades, and has been recommended by the World Health Organization -- has its critics.

While there's no doubt the gambusia gobbles up mosquito larvae, unchecked it caneat good bugs, fish eggs and young fish, such as largemouth bass andcarp. In Arizona, for example, it all but destroyed the gila topminnow, a fish that also eats mosquitoes. State officials there are trying to replace the gambusia with more topminnows in their mosquito-control efforts.

"I don't want to make them sound like baby piranha, but they are aggressive fish," said Mike Hirshfield, senior science adviser to the Chesapeake Bay Foundation. "They'll eat anything they can wiggle down their tiny little throats."

James Williams, a research biologist at the National Fishery Research Center in Gainesville, Fla., said the U.S. Fish and Wildlife Service has not recommended using gambusia for mosquito control in decades. He told the Associated Press that he was not aware of any scientific studies showing that their use significantly decreased mosquitoes.

County and state officials say the benefits appear to outweigh the dangers, at least in Maryland.

Much of the controversy has centered around another subspecies of mosquitofish, the gambusia affinis, which was barred from use in Maryland by the Department of Natural Resources, Joseph said. The gambusia holbrooki is native to this state.

"Since they are cannibalistic, there is concern they will eat the young of more desirable fish," Joseph said. "But, hey, it's a fish-eat-fish world. It works both ways. We've put them in ponds and seen other fish eat them."

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