Mosquito season, and with it West Nile virus, is arriving earlier this year, warn medical entomologists and mosquito abatement district officials.

Standing water from winter storms, warmer temperatures and longer daylight hours spell two "B" words for mosquitoes — breeding and blood-sucking.

Now, medical entomologists at UC Davis and mosquito abatement districts are adding another "B" word: Beware.

Culex mosquitoes, the principal carriers or vectors of West Nile are usually the most active in California from April through October, but the unseasonable springlike weather awakens them like an alarm clock from their winter semi-hibernation, said medical entomologist Gregory Lanzaro, director of the UC Mosquito Research Program, director of the UC Davis Center for Vectorborne Diseases, and a professor of entomology at UC Davis.

"These conditions are like their wake-up call," Lanzaro said. "The mosquitoes that were infected with West Nile before they went into their semi-hibernation or diapause, still have the virus. They're loaded and ready to go."

The disease, transmitted by the bite of an infected mosquito, last year killed 18 people in California and infected more than 900 others throughout the state. Health officials found the virus in all 58 counties.

Last year's West Nile outbreak in California was not an isolated case, said medical entomologist Robert Washino, chair of the UC Davis Department of Entomology and a 32-year member of the Sacramento-Yolo Mosquito Vector Control District Board. "It's a preview of what's to come unless we take proactive actions."

"This year we're heading for a very high mosquito population," Washino predicted. Long-term studies show that a high mosquito population accompanies a trio of specific weather conditions: heavy snowpacks in the Sierra Nevada mountains, a series of hard-hitting storms in the valley with abnormally high rainfall, and springlike weather.

That's why it is important to empty, turn over, throw out or cover all containers accessible to mosquitoes, and to take precautions with fish ponds, bird baths and fountains, the entomologists said.