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The Bird-Bug Balance

Back in the bad old days of DDT, mosquito control was a simple, if deadly, matter. Now it's often a balance of biological controls, like introducing mosquito-eating bacteria and minnows into flooded agricultural fields. Complicating the mix is the new emphasis on rehabilitating wildlife habitat, some of which takes the form of seasonally flooded farm fields. What's good for birds is also good for bugs. And that's not always good for humans.

This fall, the San Joaquin County Mosquito and Vector Control District will be working with other agencies, Ducks Unlimited and several Delta farmers on a series of studies aimed at finding a flooding regime that maximizes waterfowl habitat and minimizes mosquito breeding.

On Staten Island, biologists will be gauging the effects of water depth on mosquitos by flooding one field with 4-8 inches of water and another with over 18 inches. "We're looking at the possibility that the wind and wave action that occur in deeper water are less conducive to female mosquitoes laying eggs," says the District's John Stroh.

Just west of Stockton, on Rindge Tract, the study will focus on three pieces of ground all flooded at the same depth but given different vegetative treatments. One tract will be mowed or disked. Having nothing on the ground is ideal for mosquito abatement, although undesirable for birds.

A second tract will contain cornstalks and other agricultural residue, which is more beneficial to farmers and wildlife. A third will be mowed or disked in strips. The biologists speculate that this patchwork vegetation will leave room for mosquito fish to breed and eliminate large numbers of mosquitos, while still attracting waterfowl.

The project, which will continue until the fields are drained in the spring, isn't merely a matter of itchy arms and legs. Encephalitis has made a reappearance in the Central Valley according to lab studies of exposed chickens, and officials are under the gun to control the vectors. Ducks Unlimited, because of its longstanding ties with the farming community, has been crucial in the mosquito abatement district's dealings with landowners. The conservation group has, for many years, been giving technical assistance to farmers who want to flood their fields to increase waterfowl habitat, says spokesperson Bill Harrell.

"We had about 20,000 acres in the Delta enrolled in the program last year," Harrell says. "Next year, we'll be using information from the studies to give farmers a clear picture of water guidelines."

Contact: John Stroh (209)982-4675

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