Why mosquito season is longer this year in some parts of Canada

TORONTO — Canadians who were relieved to be out of mosquito season might be surprised to learn the pesky insects are not gone just yet.

Experts say mosquitoes seem to be living longer this fall in parts of southern and eastern Ontario and Eastern Canada, including Nova Scotia, because of warmer-than-usual temperatures.

Mosquitoes typically thrive in warm and humid environments and these conditions have been extended by this year’s strong El Nino — a natural phenomenon that causes global temperatures to spike every few years.

“As long as those mosquitoes haven’t been exposed to temperatures low enough … so cold that it damages them and kills them, then they’ll just stick around,” said Laura Ferguson, an assistant professor in the biology department at Acadia University who studies how ticks and mosquitoes respond to environmental change.

Ferguson, who has been bit by mosquitoes in recent weeks, said overnight temperatures have to dip below freezing for a consistent period of time to have an effect on the insect.

Bill Merryfield, a research scientist at Environment and Climate Change Canada, said El Nino usually affects Canada toward the end of the year, causing unusually mild fall and winter temperatures.

“This year’s event is truly exceptional because this warming has already exceeded three degrees, which is well above that of any previous event and is expected to go even higher before the usual peak of El Nino in November or December,” he said, referring to Pacific Ocean temperatures tied to the weather phenomenon.

“That would consist of warmer-than-usual temperatures coast to coast” in Canada, Merryfield said.

Warmer temperatures may reflect a “new normal” for autumn and winter seasons in Canada as the planet continues to warm at a faster rate than previous years, he added.

In Toronto and surrounding areas of southern Ontario, where mosquitoes were present into late October, temperatures reached the low 20s at the end of the month and in the first week of November. The Weather Network said Toronto hasn’t recorded any temperatures below or at freezing since May 3 this year.

Researchers are still waiting to see how this year’s “super” El Nino will affect mosquito populations next year during warmer seasons, especially across Canada’s 80 diverse mosquito species. Ferguson said this type of weather event could influence how the mosquitoes survive a warmer winter and the size of their populations next spring.

“We definitely expect that they’ll have a longer active season, particularly if spring comes earlier or fall is later and warmer,” she said. “We think we’re seeing some evidence of that.”

Besides the bites and itching that follows, experts are concerned about the possibility of increased disease transmission if mosquitoes’ life spans are prolonged because of unusual weather patterns that lead to a boom in their populations. Temperatures in Canada and around the world are already elevated due to human-caused climate change.

The longer mosquito season is extended in Canada, the more those risks for virus transmission to humans and animals remain, said Carleton University PhD candidate Marc Avramov, who focuses on mosquito-borne diseases such as West Nile virus and eastern equine encephalitis.

“We stop worrying about that as soon as those colder fall conditions start appearing,” he said.

Avramov said new mosquito species could also make their way into Canada, including some that can adapt more easily in the wintertime, and could introduce new emerging diseases if mild weather conditions are sustained.

“That’s one of the risks we always keep an eye out for,” he said.

This report by The Canadian Press was first published Nov. 4, 2026.

Sadeen Mohsen, The Canadian Press