High water to fuel bumper crop of mosquitoes

By Marissa Harshman, Columbian Health Reporter



Get the mosquito repellent ready.

Receding rivers and warming temperatures are combining to create an ideal mosquito hatching environment in Clark County.

Pesky floodwater mosquitoes, which are a nuisance but carry no diseases, lay their eggs in the mud of riverbanks.

The flooding Columbia River had been keeping the eggs covered with cold water, which is not conducive to hatching. But as the water level drops and temperatures rise, the water warms and the eggs hatch, said Ken McMillan, crew supervisor for the Clark County Mosquito Control District.

“I suspect we’re going to have somewhat of a bad year,” McMillan said. “But not as bad as last year near the river or Government Island.”

Unlike last year, the Port of Portland recently sprayed Government Island, which should curb the number of mosquitoes there and along the northern edge of the Columbia River, he said.

High water levels across the county also mean the agency is playing catch-up with its preventative measures. Flooding has made it difficult for crews to reach areas where floodwater mosquitoes thrive, McMillan said.

As a result, the mosquito control district is relying more on helicopters to spray hard-to-reach areas. In a typical year, helicopters are used twice. This year, helicopters have gone up in the air three times, McMillan said.

“The river has been over flood stage for over a month, and it’s still really high when normally it’s a lot lower,” McMillan said.

“We don’t know how long the river’s going to be this high, but until it drops, we’re going to have problems,” he added.

By this time of year, the crews should be wrapping up floodwater mosquito treatment, and targeting storm drains and retention ponds that serve as breeding habitat for mosquitoes that carry West Nile virus, he said. Crews have begun preventative efforts for the disease-carrying variety, McMillan said.

West Nile virus has not yet been detected in mosquitoes, other animals or humans this year in Washington. In 2010, West Nile was detected in 130 mosquito samples from Grant, Yakima and Benton counties. In addition, two people in other parts of the state contracted the virus, according to the Washington State Department of Health.

State and local health departments are encouraging residents to be proactive to prevent mosquitoes from breeding.

Emptying stagnant water in cans, tires and buckets will eliminate mosquito habitats. Changing water in birdbaths, ponds and animal water dishes, and properly maintaining swimming pools also will cut back on breeding areas for mosquitoes, according to the health department.

People enjoying outdoor activities should use mosquito repellent with DEET, Picaridin or oil of lemon eucalyptus. When possible, health officials suggest wearing long-sleeved shirts, pants, shoes and socks when outside.

Marissa Harshman: 360-735-4546 or marissa.harshman@columbian.com.

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