University of California, Davis, researchers found mosquitoes under a Marin County apartment that had developed a tolerance to pyrethroids, a common agricultural pesticide. Mosquitoes have shown resistance to the chemical in Africa and Asia, but this is the first instance in North America, experts said.
The mosquitoes, called culex pipiens, are one of the most common house mosquitoes in North America and are the same species as those that spread West Nile across the country last year.
Entomologists say the increased resistance may allow mosquitoes to spread diseases like West Nile virus more easily. And as resistance increases, even the strongest, most toxic pesticides may prove useless.
"If we find resistance building and are able to switch to other control strategies, we can nip it in the bud," said Bruce Hammock, one of the authors of the report recently published on the Internet by Pest Management Science. "If we continue to use the same insecticides and not use them very carefully, the product then becomes useless."
California is better equipped than many states to fight pesticide resistant mosquitoes, said Roger Nasci, an entomologist with a branch of the Centers for Disease Control in Fort Collins, Colo.
"The situation in California is quite good because of a network of very integrated mosquito control systems," he said. "A lot of states are now playing catch-up."
On the other hand, the state's dependence on pyrethroids has curbed the development of alternatives, said John Edman, a professor of medical entomology at UC Davis and an authority on the spread of diseases like West Nile in California.
"It's clearly worrisome," said John Edman. "You don't want to be too alarmist, but it is an issue that really has the be addressed."
Farmers have used pyrethroids, a less toxic alternative than other chemical pesticides, for decades. Mosquitoes exposed to the chemical while living in irrigated fields are one reason why the insect has become resistant.
Jim Wandersheid, district manager of the Marin-Sonoma Mosquito and Vector Control District, said his center is vigilant about eradicating mosquito populations early by killing the larvae that collect in hot tubs, septic tanks, and water pipes.
"California probably has the best mosquito control in the world," he said.