When does mosquito season end in NY? And when does West Nile virus peak?

The warmest days of summer are likely behind us, but September remains warm — and warm enough for mosquitoes. How long will they last this year?

The flying pests (and important pollinators) typically stick around into October. And the effects of climate change could lengthen that season.

Looking at long-term data from field sites in Suffolk County, it does appear that mosquito seasons are increasing in duration, said Courtney Cuinn Murdock, associate professor at Cornell University’s entomology department.

“We have noticed over time that mosquito season is lengthening and when you think about how climate change is affecting climate in this area of the world — temperatures are warming, especially over winter, and seasons are becoming wetter,” Murdock said. “... Depending on the mosquito species and its specific life history, those changes could be positive because it could influence if mosquitoes are overwintering. Warmer winters might increase their ability to make it through the winter.”

Mosquitoes can also emerge earlier if spring or winter are warm. Wetter conditions can establish more habitat for species that have an aquatic juvenile stage.

More: Serious viral disease EEE found in upstate NY horse. What to know about the human health risks

Mosquito season typically starts in May and runs through October, the peak of surveillance at the Suffolk County project. The research focuses on aedes albopictus, the Asian tiger mosquito, which is an invasive species, and culex pipiens, the common house mosquito, which are the main West Nile virus vector. The long-term data shows a decline in populations for mosquitoes once October arrives, even for those that overwinter as adults.

Mosquitoes typically overwinter in warm, sheltered locations like stormwater sewers in urban areas.

West Nile Virus peaks in the late summer and early fall

Despite the decline of mosquito populations later in the year, cases for West Nile Virus typically peak around mid- to late-September, based on the three-year rolling average of 2021-23 used by the New York State Department of Health. The week of Aug. 18 to 24 saw the most human cases of the virus so far this year outside of New York City, with seven: two in Nassau County, four in Suffolk County and one in Westchester County. There is no mosquito surveillance, but also no West Nile Virus reported, in Monroe County this year.

While there haven’t been any cases in the Rochester area in 2024, people should be vigilant of contact with mosquitoes during this time of year, Murdock said.

More: Mosquitoes have emerged in New York. How to reduce the common pests where you live

“It’s transmitted among different bird species in the area and has to reach certain levels before it spills over into the human population depending on the region of the country,” she said.

Why do West Nile virus cases peak in the transition to fall? Bird migration could play a role. West Nile virus is first amplified in avian populations, Murdock said, and culex mosquitoes are very likely to feed on birds.

A major outbreak of West Nile virus in the New York City area that killed birds and caused an increasing number of human cases in 1999 was traced back to urban areas near wetlands where migratory birds, avian mosquitoes and humans congregated, according to a 2000 study .

— Steve Howe reports on weather, climate and the Great Lakes for the Democrat and Chronicle. An RIT graduate, he has covered myriad topics over the years, including public safety, local government, national politics and economic development in New York and Utah.

This article originally appeared on Rochester Democrat and Chronicle: When does mosquito season end in NY? And when does West Nile virus peak?