Come September, Scarborough homes start filling with bugs that were nowhere to be seen in August. Cluster flies pooling on the windows, boxelder bugs baking on the warm south wall, spiders down in the basement, and mice moving into the walls. None of it is chance. Cold nights drive overwintering pests toward the warmest, driest shelter within reach, and in a Woburn bungalow or a Guildwood back-split, that shelter is your house. This guide runs through the six fall invaders Scarborough sees most, why they turn up, and the sealing that keeps them out. Most of it is prevention you can do on your own. Some of it takes a pro.
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Why do pests come indoors in the fall?
Because they are trying not to freeze. Once nights slip below roughly 10 degrees, insects that spend winter as adults go looking for a sheltered spot to wait it out, a behaviour called overwintering. Your house throws off heat, is riddled with small gaps, and stays dry, which makes it the best shelter on the street. Mice are after the same two things, warmth and food, and a mouse only needs a gap the width of a pencil to get inside.
The fall invasion is an entry problem, not a spray problem. These pests are working their way 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 foot of this page counts for more than any product on a shelf.
Cluster flies
Slow, sluggish flies gathering on sunny upstairs windows in October are cluster flies. Bigger and lazier than a housefly, they mass on the warm side of the house, then creep 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 return to the same house year after year, because they leave behind a scent that pulls next season's flies to the very same spot.
Killing the ones on the glass does nothing for the problem. They are coming in through gaps along the roofline, the soffits, and the window frames on the sunny side, which on many Scarborough two-storey detached homes is a stretch you cannot safely reach. Sealing those gaps in late summer, before the flies arrive, is the only real prevention. Once they are into the walls, a licensed technician can treat the entry points, which is worth doing to break the yearly cycle.
Boxelder bugs
Boxelder bugs are the half-inch black bugs marked with orange or red lines that heap up on warm south and west walls on bright fall days. They gather on the outside of the house first, then push in through cracks to overwinter. They do not bite, do not breed indoors, and do no damage to the building, but crushing them stains surfaces and leaves an odour, so a vacuum beats a shoe.
Like cluster flies, they come down to sealing. Caulk around window and door frames, utility penetrations, and cracks in siding on the sunny walls keeps them out. Taking out a nearby boxelder or maple, common enough on Scarborough's mature treed lots, gets suggested now and then, but that is a heavy 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 across Ontario and now turns up on Scarborough homes each fall chasing the same overwintering shelter. The name is well earned: crush one and it puts out a strong odour, so vacuum them into a bag and empty it outdoors rather than squashing them.
They are a nuisance inside the home, not a health or structural threat. Prevention is the same as for the other overwinterers: seal the gaps around windows, doors, vents, and utility lines, and patch torn screens. A shop vac takes care of any already indoors.
Spiders
Spiders get more noticeable in fall for two reasons. Some species reach adulthood and mate in autumn, so you see larger ones, and others drift toward the warmth of the house as the weather cools. Most Scarborough spiders are harmless, and they eat other insects, so a few in the basement or garage are quietly working in your favour. This is one case where the honest answer is often to leave them where they are.
Call for spider control when webs and egg sacs start showing up in living areas, when the count keeps climbing, or when you simply want them gone, which is a fair reason on its own. Fewer insects overall means fewer spiders, so sealing out the fall invaders they feed on trims the spider count as a side effect. Our spider control page covers which Scarborough spiders are worth a second look and which are not.
Silverfish
Silverfish are the teardrop-shaped, silvery, wingless insects that dart out when you shift a box in the basement or open a bathroom cabinet at night. They do not come in from outdoors the way the overwinterers do. They are already in the house, in the damp, dark corners, and they get noticed in fall as people clean and stow things away. They feed on paper, cardboard, book bindings, and starchy items, and heavy numbers can chew through stored documents and clothing.
Silverfish are a moisture problem, and the damp postwar basements common across Scarborough suit them well. They need humidity, so the fix is drying things out: a dehumidifier in the basement, repaired leaks, a ventilated bathroom, and paper and fabric stored in sealed plastic bins instead of cardboard. Cut the damp and you cut the silverfish. A stubborn, spreading population is worth a professional treatment.
Mice in the fall
Mice are the fall invader that genuinely costs you. Unlike the overwintering bugs, mice breed indoors right through winter, gnaw wiring and insulation, and foul 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, the gas line, and the dryer vent, which the older foundations on Scarborough's postwar bungalows offer plenty of.
Do not sit on mice the way you can sit on a few boxelder bugs. A handful caught early is a small job. An established colony in the walls and attic is a large one. Fall is the cheapest time to deal with them.
Mice need trapping plus sealing, not bait on its own, because bait clears the current mice and leaves the entry wide open for the next lot. Our mouse control page walks through the whole approach and the cost. Toronto's property standards bylaw, which applies across Scarborough, 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 head off most of the problem before it starts. Work through this in late summer or early fall:
- Close up the utility penetrations. Wherever the gas line, water line, cable, and dryer vent pass into the house, stuff the gap with steel wool against mice and caulk over the top against insects.
- Sort the door sweeps and thresholds. A worn sweep on the garage or an exterior door is an open lane. If a pencil slides under it, so does a mouse.
- Caulk the window and door frames on the sunny south and west walls, the side where cluster flies, boxelder bugs, and stink bugs pile up.
- Mesh the vents. Fit attic, soffit, roof, and weep-hole vents with fine metal mesh or quarter-inch hardware cloth so nothing can crawl in.
- Patch torn window and door screens while there are still warm evenings left to enjoy them.
- Dry it out. A basement dehumidifier and repaired leaks keep silverfish and the other damp-lovers in check.
- Tidy the exterior. Shift firewood off the wall, cut back branches resting on the roof, and rake up the leaf piles banked against the foundation where pests hole up before coming in.
The products a professional uses to treat entry points are all registered by Health Canada's Pest Management Regulatory Agency, which weighs the 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. Bring in a pro when the numbers stay large every year despite sealing, when the entry points sit on a roofline or soffit you cannot safely reach, or when you want to break the annual cluster-fly cycle by treating the entry points before the flies arrive.
With mice, do not wait and watch. Bugs on a window can hold until the weekend. Mice breeding in the walls cannot.
For silverfish, get help when a moisture fix does not knock the population back. For spiders, call when they have spread into living areas or you just want them gone. When you are not sure what you are looking at, tell us and we will identify it before anyone books a treatment.
Common questions
Why do I get the same bugs on my windows every fall?
Those are overwintering pests, mainly cluster flies and boxelder bugs, moving indoors to sit out the cold. Cluster flies leave behind a scent that pulls the next generation back to the very same house, which is why the pattern repeats each year. Sealing the entry points is what breaks it.
Are cluster flies and boxelder bugs dangerous?
No. Neither one bites, breeds indoors, or harms the house. They are purely a nuisance. What makes them worth dealing with is the volume, plus the stain and smell when they get crushed. Reach for the vacuum, not your shoe.
Do I need to spray for fall invaders?
Usually no. Since these pests are entering from outside, closing the gaps they use is the genuine fix, not fogging the rooms where they gather. For heavy, year-after-year cases a pro treats the exterior entry points instead.
How do I stop mice from getting in this fall?
Pack every gap wider than a pencil with steel wool and hardware cloth, sort out the garage and exterior door sweeps, and keep food in sealed containers. If scratching or droppings have already turned up, trap and seal now rather than leaving it.
When do fall pests start showing up in Scarborough?
Typically September, as the nights turn cold, carrying through to December for mice. The window to seal is late August into early September, ahead of the move-in, since prevention beats eviction by a wide margin.
Should I just leave the spiders alone?
Frequently, yes. Most Scarborough spiders are harmless and they prey on other insects. Step in when they spread into living areas, when the count keeps rising, or when you simply would rather not share the house with them.
Reviewed August 2026.
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