With Idaho summer heat, expect more mosquitoes — and with them, West Nile virus

Idaho residents hoping for reprieve from the heat wave may soon find themselves exposed to a new summer danger — West Nile virus.

Peak mosquito season runs from the end of June until early September. And the Treasure Valley will likely see a rise in mosquito numbers as the summer progresses, said Adam Schroeder, director of Ada County Weed, Pest and Mosquito Abatement.

“We’re right in the heat of it now,” Schroeder told the Idaho Statesman.

This year’s cool spring slowed early mosquito development, but as overnight temperatures remain consistently warm, the creatures will come out with greater force.

Ada County’s increase in mosquito numbers is not extraordinary at this point, Schroeder said. And as of July 22, Idaho has not yet detected West Nile virus, which can cause serious illness in humans and other animals.

But with the recent heat, it’s fairly likely we’ll see West Nile virus soon, James Lunders, district director of the Canyon County Mosquito Abatement District, told the Statesman.

“We usually see it after we’ve had a long period of hot weather,” Lunders said.

How mosquito populations change over time

There are generally two peaks in mosquitoes season, Lunders said.

The peak for floodwater mosquitoes is already behind us, but the peak for Culex mosquitoes — which spread disease like West Nile virus — will likely hit in late August and early September, Lunders said.

Beyond seasonal trends, it’s not easy to forecast mosquito surges. Local environmental conditions, weather patterns, and human impacts such as overwatering influence mosquito populations differently each year, Schroeder said, and Ada County hasn’t seen any general increases or decreases in mosquito patterns over the last decade.

But officials anticipate some tried-and-true mosquito magnets.

Stagnant water, for example — often caused by flooding and overwatering — gives mosquitoes the chance to reproduce. Long periods of warm weather allow mosquitoes greater opportunity to complete their life cycle. That’s why mosquito season in the Southern U.S. is nearly year-round.

Urban development has changed mosquito habitats in the Treasure Valley. Some areas that used to be irrigated pastures are now subdivisions, Lunders said, so there’s been a shift from floodwater mosquitoes to Culex mosquitoes — which are closely associated with man-made structures, such as storm drains and retention ponds.

West Nile virus a concern in Treasure Valley

Not all mosquitoes spread diseases, Thomas Turco, lecturer in the School of Public and Population Health at Boise State University, told the Statesman. Only the female mosquitoes bite. They inject their virus and saliva into a victim, and then eat a blood meal to take the nutrients needed to lay eggs.

But from a public health point of view, mosquitoes are likely the deadliest vector of diseases out there, Turco said. The World Health Organization estimated that mosquitoes cause about 725,000 deaths a year worldwide.

Mosquito abatement districts across Idaho run surveillance programs to trap and test mosquitoes for diseases common to the local area — particularly West Nile virus, which is the leading cause of mosquito-borne disease in the continental U.S., according to the Centers for Disease Control and Prevention.

Although most people infected with West Nile virus don’t develop symptoms, infection can be severe, causing fever or inflammation of the central nervous system.

Two people in Ada County died from West Nile virus last year, Schroeder said. Last year was the worst on record for the virus in Ada County, with 107 positive pools of mosquitoes detected.

“We had a hot, dry summer mixed with some overwater, and that’s what Culex mosquitoes love,” Schroeder said.

But Ada County is not seeing anything close to that kind of West Nile virus pressure yet.

Positive pools tend to come out of Canyon County and Gem County one to two weeks before Ada County, Schroeder said. That’s because the disease typically tends to move from west to east, corresponding with bird migrations.

Birds are the primary host for the virus, Schroeder said. Mosquitos bite infected birds and then bite humans and horses.

Eradicating mosquitoes? It’s complicated

If public health experts had the power to make all the mosquitoes go extinct with the flip of a switch, would they do it?

“I absolutely would,” Lunders said. “They cause a great deal of disease and also cause a ton of economic harm on agriculture.”

None of the mosquito species Canyon County Mosquito Abatement deals with are efficient pollinators, and no species rely on them as a food source, he added.

Turco had another perspective.

“You start messing with the ecosystem where they are in, you can have a problem,” Turco said. “It’s all part of that balance.”

In the meantime, there are other ways to deal with the pesky creatures. Schroeder recommends the “Three D’s — dress, defend and drain.”

People can wear long-sleeved clothing during dawn and dusk — when mosquitoes like to come out — drain all standing water sources on their property, and use mosquito repellent.