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The drought, which might fuel optimism regarding mosquitoes and the spread of West Nile virus, may actually be promoting growth of the insect's population, partly because of the widespread use of open rainwater collection containers.

That's the word from the Marin/Sonoma Mosquito & Vector Control District, which has seen a more than 20 percent rise in service requests for mosquito problems this season, compared to the same time last year, district spokeswoman Nizza Sequeira said.

District field personnel also have reported finding mosquito larvae in rain barrels and other containers in use without the kind of fine-mesh screens commonly employed to keep the insects out, district personnel said.

The fact that heavy mid-February rains were followed by warm weather also accounts for a sudden and early mosquito population boom, Sequeira said.

"We had the rains, and it warmed up, and they just blew up," said Jeff Petersen, a vector control technician with the district. Since then, "it's been steady."

Mosquitoes, pesky even without the potential for certain species to carry West Nile virus and other diseases, breed in standing water and thrive when it's warm. Population changes are directly tied to weather.

Unfortunately, some people doing the right thing by saving water to offset the lack of rain may be contributing, inadvertently, to a bumper crop of bugs.

State health officials recommend all rain catchment basins or containers be covered or screened.

Even without any notable change from seasonal weather patterns, it's a good idea to keep a running inventory of potential breeding spots and take measures to avoid helping mosquitoes reproduce, officials said.

The local vector control district, funded by property taxes, provides a variety of free interventions, including small mosquitofish that prey on the larva that can be deployed in decorative ponds to keep mosquito populations down.

There are also hormone-based pellets that prevent larvae from maturing into adults and are safe to use in rain collection receptacles and other locations.

A type of naturally occurring bacteria that is lethal to the larvae is often used in open waterways, as well.

Petersen said he was checking ponds at Rincon Valley Community Park on Tuesday when some passers-by alerted him to a nearby storm drain. He saw mosquito eggs, pupae and larvae, and treated the water with the bacteria product.

Later, he delivered several mosquitofish fry to Anita Selinger-Finell for the reed-filled pond in the garden behind her home in Santa Rosa's South Park neighborhood.

Selinger-Finell said she's been ensuring the pond has fish every year since buying the house eight years ago, and had on her calendar May 1 to call and request some.

"I feel it's my duty," she said.

Sequeira said old, sometimes crumbling concrete septic tanks also provide breeding grounds for a particular kind of nocturnal mosquito that enters through cracks in the concrete or through venting pipes. That species is one of the West Nile vectors, she said.

West Nile virus, which can sometimes be fatal to humans, has yet to ramp up in any significant way this season, a spokesman for the California Department of Public Health said.

But it has been detected in six counties so far, including Lake County, where the discovery of three dead, infected birds in early April suggested a possible rise in incidence, a vector control official said at the time.

But the statewide forecast remains unclear, and the overall virus activity appears to be roughly on par with last year at this time, said Corey Egel, a spokesman for the state health agency.

There have been no human cases thus far this year, he said.

Service requests can be submitted to the Marin/Sonoma Mosquito & Vector Control District online at www.msmosquito.com or by calling (707) 285-2200.

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com.