High tides have been one of the factors leading to the recent mosquito population boom

Imperial Beach mayor Serge Dedina and mayor pro tem Brian Bilbray must be hearing that ultra-high-pitched buzz and feeling the sting of an unusually large swarm of mosquitoes this fall. Both went on the record asking the county to explain why.

The answer from the Vector Control wing of the county's Department of Environmental Health is simple: coastal salt marshes on both the north and south ends of town have more standing water than usual, a result of unusually high tides.

"When you get an unusually high tide there, it can allow water into low-lying areas that don't always have water and if it stands seven to ten days without percolating into the ground, it's there long enough for a full mosquito-breeding cycle," says Chris Conlan, a supervising vector ecologist for the county.

Vector-control officers have a pretty good idea of the usual standing-water spots in the Tijuana Estuary and the River Valley to the south, and the salt marshes of the San Diego Bay National Wildlife Refuge at the bottom of San Diego Bay — the north end of I.B. But high tides and unusual weather have created new breeding areas.

"Heat, humidity, and rain put out the welcome mat for breeding," Conlan said.

Neither Dedina nor Bilbray responded to multiple calls and emails for this story.

The rest of the county is having a normal year, mosquito-wise — but conditions around I.B. are just right for a rare infestation.

There are about two dozen species of mosquito in the area, including the recently arrived Aedes aegypti (which has been known to carry yellow fever) and several of the known West Nile virus carriers, he said. (Odds of contracting yellow fever are low because a mosquito has to bite an infected person — or monkey — in order to infect anyone else.)

Because the salt marshes on both sides of I.B. are managed by the U.S. Fish & Wildlife Service, there are restrictions on what vector control can do to treat there — for example, they can't use their helicopter and there are areas where the vector control officers can't drive in and have to go on foot.

"We've worked very hard to develop a good working relationship with Fish & Wildlife — we do have to notify them if we're going into sensitive areas, and they will sometimes accompany us," Conlan said. And results from their efforts don't show up for seven to ten days, he said.

The techs use a commercial mosquito killer that combines toxins from Bacillus thuringiensis and Bacillus sphaericus. It is believed to be harmful only to mosquitoes and remain viable for just a few days.

"We're killing the larval stages, so anything at the pupal stage or already flying around is going to finish its life cycle," Conlan said. "With the tides, we've seen short waves of sizable infestations."

But the biggest problem is the many hundreds of acres of coastal marsh that are getting soaked by unusually high tides, creating new spots for mosquitos to breed.

"Our people are working by tide charts, and it's a very low window," Conlan said. "It's a mad scramble to get in there when the tide goes out to get a good look and treat."