Fall Pests in Toronto: Why They Come Indoors and How to Stop Them

Every September, Toronto homes fill up with bugs that were not there in August. Cluster flies on the windows, boxelder bugs on the warm south wall, spiders in the basement, and mice in the walls. It is not random. Cold nights push overwintering pests toward the warmest, driest shelter they can find, and your house is it. This guide covers the six fall invaders Toronto sees most, why they show up, and the sealing that keeps them out. Most of it is prevention you can do yourself. Some of it needs a pro.

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Why do pests come indoors in the fall?

Because they are trying not to freeze. As nights drop below roughly 10 degrees, insects that survive winter as adults look for a protected place to wait it out, a behaviour called overwintering. Your house radiates heat, has countless small gaps, and stays dry, which makes it the best shelter on the block. Mice are chasing the same thing, warmth and food, and a mouse only needs a gap the width of a pencil to get in.

The fall invasion is an entry problem, not a spray problem. These pests are coming in from outside, so the fix is closing the gaps they use, not fogging the rooms they end up in. That is why the checklist at the bottom of this page matters more than any product.

Cluster flies

If you have slow, sluggish flies collecting on sunny upstairs windows in October, those are cluster flies. They are larger and slower than a housefly, and they gather in numbers on the warm side of the house, then crawl into wall voids and attics to overwinter. They do not breed indoors and they do not carry disease the way a housefly does, but they come back to the same house year after year, because they leave a scent that draws next year's flies to the same spot.

You cannot fix cluster flies by killing the ones on the window. They are entering through gaps around the roofline, soffits, and window frames on the sunny side. Sealing those gaps in late summer, before they arrive, is the only real prevention. Once they are in the walls, a licensed technician can treat the entry points, which is worth doing to break the annual cycle.

Boxelder bugs

Boxelder bugs are the half-inch black bugs with orange or red lines that pile up on warm south and west walls on sunny fall days. They cluster on the outside of the house first, then work their way in through cracks to overwinter. They do not bite, do not breed indoors, and do not damage the house, but crushing them stains surfaces and leaves an odour, so vacuuming beats squashing.

Like cluster flies, they are a sealing problem. Caulk around window and door frames, utility penetrations, and cracks in siding on the sunny walls keeps them out. Removing a nearby boxelder or maple tree is sometimes suggested, but that is a big step for a harmless nuisance, and sealing is the practical answer.

Stink bugs

The brown marmorated stink bug is a shield-shaped brown bug, about the size of a fingernail, that has spread through Ontario and now shows up on Toronto homes in fall looking for the same overwintering shelter. The name is earned: crush one and it releases a strong odour, so vacuum them into a bag and empty it outside rather than squashing them.

They are a nuisance in the home, not a health or structural threat. The prevention is identical to the other overwinterers, seal the gaps around windows, doors, vents, and utility lines, and fix torn screens. A shop vac handles the ones already inside.

Spiders

Spiders get more visible in fall for two reasons. Some species mature and mate in autumn, so you see bigger adults, and others move toward the warmth of the house as it cools. Most Toronto spiders are harmless, and they eat other insects, so a few in the basement or garage are doing you a small favour. This is one case where the honest answer is often to leave them be.

Call for spider control when you are finding webs and egg sacs in living areas, when the population is climbing, or when you simply do not want them, which is a fair reason too. Fewer insects overall means fewer spiders, so sealing out the fall invaders they feed on cuts the spider count as a side effect. Our spider control page covers which Toronto spiders are worth a second look and which are not.

Silverfish

Silverfish are the teardrop-shaped, silvery, wingless insects that dart out when you move a box in the basement or open a bathroom cabinet at night. They do not come in from outside the way the overwinterers do. They are already in the house, in the damp, dark spots, and they get noticed in fall as people clean and store things. They feed on paper, cardboard, book bindings, and starchy items, and heavy numbers can damage stored documents and clothing.

Silverfish are a moisture problem. They need humidity, so the fix is drying things out: a dehumidifier in the basement, fixing leaks, ventilating the bathroom, and storing paper and fabric in sealed plastic bins instead of cardboard. Cut the damp and you cut the silverfish. A persistent, spreading population is worth a professional treatment.

Mice in the fall

Mice are the fall invader that actually costs you. Unlike the overwintering bugs, mice breed indoors all winter, chew wiring and insulation, and contaminate food, so a couple in October becomes a colony in the walls by February. September through December is when they move in, through the same pencil-width gaps the bugs use, plus the bigger ones around the garage, gas line, and dryer vent.

Mice need trapping plus sealing, not bait alone, because bait clears the current mice and leaves the entry open for the next. Our mouse control page walks through the whole approach and the cost. Toronto's property standards bylaw actually requires properties to be kept free of rodents, and sealing is how you meet it.

The fall exclusion checklist

Nearly every fall pest, bug or mouse, gets in through a gap. Close the gaps and you solve most of the problem before it starts. Work through this in late summer or early fall:

• Seal gaps around utility penetrations. Where the gas line, water line, cable, and dryer vent enter the house, pack the gap with steel wool for mice and caulk over it for insects.
• Fix door sweeps and thresholds. A worn garage or exterior door sweep is a highway. A pencil fits under it, so a mouse does too.
• Caulk window and door frames on the sunny south and west walls, where cluster flies, boxelder bugs, and stink bugs concentrate.
• Screen the vents. Cover attic, soffit, roof, and weep-hole vents with fine metal mesh or quarter-inch hardware cloth so pests cannot crawl through.
• Repair torn window and door screens before you run out of warm evenings to use them.
• Cut the moisture. Run a basement dehumidifier and fix leaks to keep silverfish and other damp-lovers down.
• Manage the outside. Move firewood away from the wall, trim branches touching the roof, and clear leaf piles against the foundation where pests shelter before moving in.

The products a professional uses to treat entry points are all registered by Health Canada's Pest Management Regulatory Agency, which evaluates risk to children and pets before approving them, and are applied to the label.

When does a fall pest problem need a pro?

Most of the overwintering bugs, cluster flies, boxelder bugs, stink bugs, are a nuisance you can vacuum and seal against yourself. Call a pro when the numbers are large every year despite sealing, when the entry points are on a roofline or soffit you cannot safely reach, or when you want to break the annual cluster-fly cycle with a treatment of the entry points before they arrive.

For silverfish, get help when a moisture fix does not knock the population down. For spiders, call when they have moved into living areas or you just want them gone. When you are not sure what you are seeing, tell us and we will identify it before anyone books a treatment.

Common questions

Reviewed August 2026.

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