Why Mosquito Season Still Hasn't Ended in November

You raked the leaves in a toque and got a mosquito bite while you were at it. If that combination feels wrong, you are not imagining things, and the scientists studying it have noticed too.

Mosquito season is running longer than usual this fall in parts of southern and eastern Ontario and Eastern Canada, including Nova Scotia, because of warmer-than-usual temperatures, the Canadian Press reported on November 4. In Toronto and the surrounding region, mosquitoes were still active into late October, helped along by temperatures in the low 20s at the end of the month and into the first week of November. Laura Ferguson, an Acadia University biologist who studies how ticks and mosquitoes respond to environmental change, told CP she had been bitten within the past few weeks.

Key takeaways

Mosquitoes are staying active into November in parts of Ontario, Nova Scotia, and other parts of Eastern Canada because of an unusually warm fall.

The Weather Network said Toronto had not recorded a temperature at or below freezing since May 3 this year.

Mosquitoes stay active until overnight temperatures drop below freezing for a consistent stretch, so one frosty night is not enough.

A researcher at Carleton University said a longer season keeps the window open for viruses such as West Nile to spread.

Until your area gets a run of freezing nights, repellent at dusk and dawn and tipping out standing water still make sense.

Why has a warm fall kept them around?

Mosquitoes thrive in warm, humid conditions, and this year those conditions have simply not ended on schedule. Part of the explanation is El Niño, the natural climate pattern that pushes global temperatures up every few years. Bill Merryfield, a research scientist at Environment and Climate Change Canada, told CP that El Niño typically affects Canada toward the end of the year, bringing unusually mild fall and winter temperatures.

He described this year's event as exceptional and said a warm fall like this one may reflect a new normal as the planet keeps warming. The longer trend matters too, since temperatures in Canada and around the world are already elevated because of human-caused climate change.

How many freezing nights does it take?

The practical detail worth knowing is what actually ends mosquito season. Ferguson told CP that overnight temperatures need to dip below freezing for a consistent period before mosquitoes are damaged enough to die off. A single frosty night does not do it, and much of Eastern Canada has not yet strung together the run of freezing nights required. The Weather Network noted that Toronto had not hit freezing at all since May 3. Ticks follow a similar late-season pattern, as our report on rising tick numbers in Canada shows.

Canada is home to about 80 mosquito species, and researchers are watching how this year's unusually strong El Niño will shape their populations next spring. Ferguson said she expects a longer active season whenever spring arrives earlier or fall runs later and warmer.

Should late-season bites worry you?

For most people, late-season bites are an annoyance more than a danger, and the odd November bite is nothing to lose sleep over. There is a health angle underneath the novelty, though. Marc Avramov, a Carleton University PhD candidate who studies mosquito-borne illnesses such as West Nile virus and eastern equine encephalitis, told CP that a longer season keeps the window open for those viruses to spread among animals and people. Ontario's surveillance showed the same pattern earlier this year, as our report on West Nile in Ontario mosquitoes describes.

Normally, cold fall weather is the signal that the worry is over for the year, and this fall that signal is late. Our guide to the symptoms that can follow a mosquito bite explains which signs mean it is time to see a doctor, and the same awareness applies to anyone travelling to places where mosquito-borne illness is more common.

Do you still need insect repellent?

Until your area gets its run of freezing nights, keep the summer routine within reach, and that includes insect repellent if you will be outside around dusk or dawn. Health Canada's guide to personal insect repellents explains which products are approved and how to use them safely, including on children. The steps below take very little effort.

Use an approved repellent outdoors. Choose a product containing DEET or icaridin and follow the label, especially if you will be outside around dusk or dawn.

Tip out standing water. Empty flowerpots, wheelbarrows, gutters, and bird baths, since even one warm week gives mosquitoes a place to breed.

Cover up on mild evenings. Long sleeves and pants cost nothing and cut down on bites when the air is still and warm.

Keep the screens on a little longer. Leave window and door screens in place until the overnight temperatures stay below freezing.

Watch for symptoms after bites. Fever, a severe headache, or unusual tiredness in the weeks after a bite deserves a call to a health professional.

Keep some perspective while you do it. The overwhelming majority of mosquito bites in Canada lead to nothing more than an itchy welt, and the season will end once winter finally settles in.

Frequently asked questions

When does mosquito season end in Canada?

Mosquito season usually winds down once overnight temperatures stay below freezing for a consistent stretch, which in much of southern Canada happens by mid to late fall. A single frost is not enough. This year, unusually warm weather has kept mosquitoes active into November in parts of Ontario, Nova Scotia, and other parts of Eastern Canada.

Can you still get sick from a mosquito bite in November?

The risk is low, though it does not disappear while mosquitoes remain active. Researchers have noted that a longer season keeps the window open for viruses such as West Nile to spread. If you develop a fever, severe headache, stiff neck, or unusual tiredness after bites, contact a health professional and mention when and where you were bitten.

Is it safe to use repellent on children in the fall?

Health Canada's guidance on personal insect repellents applies all year, so the same rules hold in the fall as in summer. Products with DEET or icaridin are approved for use on children, with age limits and concentrations listed on the label. Follow the instructions closely, apply it for children yourself, and keep it away from their hands and eyes.

Your next step

If you develop a fever, a severe headache, or unusual fatigue in the weeks after mosquito bites, or you have questions after travelling somewhere mosquito-borne illness is more common, it is worth getting checked properly. The Medimap Health Hub has more seasonal health guides, and you can compare walk-in clinics near you and book a same-day or next-day visit.

Disclaimer: This article is for general information only and is not medical advice. It does not replace a conversation with a doctor or other qualified health professional who knows your history. If you think you are having a medical emergency, call 911 or go to the nearest emergency department.