Another drought hazard: West Nile Virus

Alongside water rationing, lawn death and empty swimming pools, the ongoing drought also brings a surprise nuisance: more mosquitoes.

Phil Smith, district manager for the Marin/Sonoma Mosquito and Vector Control District, reports an unusually high number of the airborne pests in Sonoma County, saying that drought conditions exacerbate egg-laying due to conditions caused by lower water levels.

In an email, Smith said the most dangerous outcome of the increased mosquito population is the crowding of bird and insects in urban areas. Dead birds that carry diseases like West Nile Virus are transferred to the mosquito whenever the insect draws blood, and the infected mosquito can transmit the disease to humans through biting.

“To date, we’ve had 801 cases of West Nile in California, compared to less than half that number in 2013,” Smith said.

California’s Department of Public Health advocates following “the three D’s” to reduce the chance of contracting West Nile and other mosquito-borne diseases: apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus or IR3535; wear proper clothing at dawn and dusk, the ideal feeding time for mosquitoes; and drain any standing water at or near your living areas, such as upright tires, flower pots and buckets.

Vector Control gathers data by laying traps for mosquitoes, either through carbon dioxide, which attracts them, or another attractant that focuses on females who recently drew blood and are ready to lay eggs.

According to assistant director Erik Hawk, Vector Control has more than 1,800 sample locations in Petaluma, and their weed abatement program uses spray trucks that drive to the spawning areas and a breeding program that rears mosquito-eating fish, the insect’s natural predator.

Mosquitoes reproduce by laying their eggs in still water. Before the drought it was believed that lower water levels would actually reduce the mosquito population, but, in fact, the opposite has occurred, state officials say. The California Department of Health’s investigation into the rising West Nile Virus cases in the state concluded that the amount of still water mosquitoes have access to is actually on the rise. One possibility, Smith says, is when it comes to mosquito growth, you can’t beat the heat.

“High temperatures speed up the breeding cycle, which can go from egg to adult in as little as five days in warm weather,” Smith said.

Another theory holds that, because the drought partially dries up streams and creeks, mosquitoes gain access to new sources of still water. Along with that comes the loss of fish life in those creeks and streams, which reduces the predator population for mosquitoes, giving the larvae and the newly-hatched adults a better chance to leave their birthplace and bite away.

Smith also warns that along with certain water-conservation methods comes mosquito conservation. Rain barrels, for instance, are seeing more use for household water conservation, and if those barrels are left uncovered, it could create a perfect mosquito breeding environment in backyards. The same holds true for uncovered hot tubs and pools.

Even construction causes mosquitoes to appear out of seemingly thin air. Smith said artificial wetlands, like Shollenberger Park, become ideal breeding grounds.

The still water and high temperature of such marshes, combined with periodic high tides, can force mosquitoes to hatch early, Smith said, and marsh mosquitoes can fly many miles before settling down after their first meal.

(Contact William Rohrs at william.rohrs@arguscourier.com.)

High temperatures speed up the?breeding cycle, which can go from egg to adult in as little as five days?in warm weather

Phil Smith