Who likes a hotter climate? Northwest mosquitoes

A warming climate has meant better conditions for mosquitoes in much of the Northwest.

A report from the nonprofit Climate Central says mosquito season is now 32 days longer in Seattle than it was 40 years ago.

The season with sufficient warmth and humidity for mosquitoes has grown two weeks longer in Portland and Eugene.

The group analyzed temperature and humidity record at 242 locations in the United States between 1979 and 2022. In nearly three-fourths of the studied locations, mosquito season — with temperatures between 50 and 95 degrees Fahrenheit and relative humidity of at least 42% — increased over that period. In only one-fourth of locations, mostly in the South, where temperatures often exceed 95 degrees, did mosquito season shrink.

“With changing climate, there's winners and losers, and a lot of pest species are winners,” said Aimee Code, who directs the pesticide program at the Portland-based Xerces Society for Invertebrate Conservation. “The warmer the weather, the faster they reproduce.”

Shifting away from the fossil fuels that are heating the planet’s climate would be the surefire way to make the Northwest less amenable to mosquitoes again.

Until that happens, there are ways to fight back against nature’s buzzing bloodsuckers, and they don’t have to involve harmful pesticides.

“These mosquitoes, they're terrible fliers,” Code said. “Just put out a box fan — you don't need to introduce a chemical into the mix.”

A box fan may blow a nuisance into someone else’s yard, but Code says the first line of defense needs to focus on mosquitoes before they are able to fly. Young mosquitoes hatch after 4 to 7 days from stagnant bodies of water, as small as half an inch of water in a soup can left out in the rain.

“Go out there and find that stagnant water that you can dump to avoid those mosquitoes even getting a hold,” Code said.

Code says insecticide spraying and electric bug-zappers are ineffective, since they only kill adults, and they can kill many beneficial insects, from butterflies to honeybees, as well.

In the Northwest, only Spokane has experienced a decreasing mosquito season, dropping by 8 days over the past 43 years, reflecting an increasing frequency of extremely hot conditions in which neither mosquitoes nor humans thrive.