Archived News


March 8, 2006

Editor's note: The July announcement of two human cases of West Nile virus in Yolo County and the confirmed presence of the virus in birds on the UC Davis campus have renewed interest in prevention of West Nile virus as temperatures soar in California. The tips below were published in March 2006.

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

Medical entomologists at the University of California, Davis, and mosquito abatement districts are adding another “B” word: Beware.

Mosquito season, and with it the West Nile virus (WNV), is arriving earlier this year, warn medical entomologists at the University of California, Davis and mosquito abatement district officials.

Culex mosquitoes, the principal carriers or vectors of WNV, 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 WNV 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 WNV in all 58 counties.

Last year’s WNV 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’s 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.

“Excess water,” Washino cautioned, “can result in heavy mosquito breeding.”  

The female mosquito, the designated drinker of the insect world, is out for blood and water. She needs blood to develop her eggs, and water to lay her eggs. The eggs hatch into larvae or wrigglers, and the larvae molt into pupae or tumblers, all aquatic stages. Adult mosquitoes emerge from the pupae and are ready to mate within one or two days.

Mosquito vector control districts throughout the state are gearing up for the onslaught.

“It’s important to take preventive measures now because if we can reduce the numbers of the first generation of mosquitoes, it can prevent mosquito populations from reaching their full potential later on in the season,” said Dave Brown, manager of the Sacramento-Yolo Mosquito and Vector Control District. “WNV is here to stay, so we are going to have to be diligent at keeping mosquito populations low to avoid transmission of the virus.”

First discovered in the West Nile district of Uganda in 1937, West Nile virus was identified in New York in 1999 and in California in 2002. For the last two years, California’s WNV deaths and infection rates led the nation. Sacramento County, the nation’s “hottest hot spot,” tallied a record 175 human cases in 2005.

Statistics throughout the United States last year showed 2949 cases of WNV and 116 deaths, according to the Centers for Disease Control and Prevention (CDC).

Although most WNV infections are mild, with flu-like symptoms, WNV can cause severe infections that may include neck stiffness, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and rarely, death, Lanzaro said. The most serious is encephalitis (inflammation of the brain).

CDC officials estimate that about one in five people bitten by an infected mosquito will become ill, and less than one percent of infected individuals will require hospitalization.  People over 50 and those immune-compromised are most susceptible to West Nile disease.

The mosquito-borne disease also infects and kills horses and birds.

The Culex female, which lives about three weeks, can lay approximately 250 eggs in her lifetime. The egg stage lasts one to two days; larva stage, about 7 to 12 summer days; and pupa stage, two to 3 summer days.

Brown said WNV prevention starts with “eliminating standing water on your property.”  

Anything that can hold water for more than a few days can become a breeding site, the mosquito experts said. This includes the more obvious rain barrels, recycle bins, uncovered trash cans, flat roofs, roof gutters, street gutters, ditches, catch basins, wheelbarrows, fountains,  plastic wading pools, and livestock watering troughs, but also window boxes, pet watering bowls, lawn ornaments,  tree holes, flower pot saucers, ashtrays, candleholders,  old tires,  table umbrella bases, tin cans, jars, bottles, and children’s toys, such as wagons, beach buckets and tire swings.

Other spots include tire tracks or ruts; plastic pipes and pipefittings; tarp covers over boats, recreational vehicles, woodpiles and grills; unmaintained swimming pools, spas, hot tubs and septic tanks; covers of swimming pools and hot tubs; backyard or back-porch aquariums; tops of light fixtures; tops of heating and air conditioning units,  low spots in lawns (from overwatering or rain); and leaky areas beneath faucets.  

When people irrigate their lawns or gardens,” Brown said, “they should avoid using so much water that it runs off into the roadside ditch or catch basin. Irrigating only what needs to be irrigated saves precious water and eliminates mosquito breeding, which can turn into a cost savings for everybody concerned.” 

Favorite breeding spots include old water-filled tires. Residents should remove them from their property or drill holes in them. This includes drilling holes in children’s tire swings and in tires used for landscaping retaining walls.

Other tasks that residents can do to help eliminate mosquito breeding sites include:

  • Clean or hose out birdbaths and fountains at least once a week, preferably twice a week
  • Stock fish ponds with mosquitofish (Gambusia affinis), which eat the larvae
  • Remove excess vegetation around ponds and yards; females like to lay their eggs in algae at the edge of the pond, and adult mosquitoes like to rest in dense shrubbery
  • Use dunks (doughnut-shaped pellets) or Bacillus thuringiensis subsp. israelensis  (BTI) to kill mosquito larvae in larger ponds. Both are non-toxic to animals
  • Clean clogged roof gutters, which can produce millions of mosquitoes
  • Overturn wheelbarrows and plastic wading pools to prevent water accumulation
  • Drill holes in permanent backyard containers to drain any water
  • Keep water fresh in pet bowls
  • Maintain swimming pools and spas with chemicals and filters
  • Remove trash such as discarded tires, cans, cups, tin foil, plastic and paper
  • Adjust tarps over vehicles or firewood to allow water runoff
  • Fill tree holes with dirt or cement.
  • Empty any excess water in dumpsters, trash cans and recycle bins and overturn the lids
  • Make sure window and door screens are “bug tight”
  • Replace outdoor lights with yellow “bug lights,” which tend to attract less mosquitoes than ordinary lights
  • Screen back porches if you like to sit outside in the open air  

Even the plastic sheeting that landscapers place under bark or rock to prevent weeds from poking through can be a “water bed” for eggs, larvae and pupae, the mosquito experts said.  The sheeting should be replaced with a landscape fabric that prevents weeds yet allows drainage.

Although Culex mosquitoes do not lay their eggs in fast-moving creeks, they can and do lay their eggs in water-filled tin cans and other trash thrown in the water. Earth Day (April 22) should be observed every day, the mosquito experts agreed.

Brown recommends the 7 D’s of West Nile virus precautions and protection: Dawn, Dusk, Drain, Dress, Defend, Door, DEET and District.

Dawn and Dusk: Avoid spending time outside at dawn and dusk when mosquitoes are most active.

Drain: Empty standing water on your property

Dress: Wear shoes, socks, long pants, and a long-sleeved shirt outdoors when mosquitoes are the most active. Wear loose clothing of a thick material.

Defend: Use effective repellents such as DEET, Picaridin or oil of lemon eucalyptus, as recommended by the EPA. It is crucial to follow the label instructions.

Door: Be sure your door and window screens are tight-fitting so mosquitoes can’t pass through.

DEET: This insect repellent, for adults, is effective on your clothing, but should be used sparingly on exposed skin. Be sure to follow the label and read the precautions on the EPA Web site,

District: Mosquito vector and control district personnel are on hand to address any mosquito problem

Since the State Legislature authorized the formation of mosquito abatement districts in 1915, some 61 mosquito and vector control agencies now serve the people of California, according to Christopher Voight, executive director of the Mosquito and Vector Control Association of California, Sacramento.  

The California districts incorporate a three-pronged attack—surveillance, public education and mosquito control. The agencies monitor and control larval and adult mosquito populations; monitor activity of vector mosquitoes and disease agents, and provide information to the communities.

More information: 

UC Mosquito Research Program Web site at or call (530) 752-6983

Mosquito Vector and Control Association of California Web site at or call (916) 440-0826

Mosquito abatement districts throughout the state, including the Sacramento-Yolo Mosquito and Vector Control District at for Equine Health