Stink Bugs and Boxelder Bugs: Why They Move Into Montreal Homes in Fall

Every fall, as soon as the nights turn cold, many Montreal homeowners notice insects gathering on their outside walls, then around windows and in warm rooms. The two most common fall visitors are the brown marmorated stink bug and the boxelder bug. They are not looking for food in your home: they are looking for a place to spend the winter.

How to recognize them

Brown marmorated stink bug

The brown marmorated stink bug is about 1.5 cm long, with a greyish-brown shield-shaped body, pale bands on its antennae and alternating light and dark markings along the edge of its abdomen. It arrived in Quebec about ten years ago and is becoming more common. When crushed or disturbed, it releases a strong, unpleasant smell, hence its name.

Boxelder bug

The boxelder bug is longer and narrower, black with red or orange lines on its back. It often clusters on sunny south- or west-facing walls, especially when boxelder (Manitoba) maples grow on or near the property.

Why they come inside

Both insects overwinter in a dormant state. In the fall, they gather on warm surfaces and then squeeze through any opening: gaps around windows and doors, torn screens, vents, siding joints, and holes around wires and pipes. Once inside walls or the attic, they stay still until a warm day or the furnace wakes them up. That is why you may see a few near windows in the middle of winter.

Are they dangerous?

Good news: neither one bites people, spreads disease, damages structural wood or breeds inside your home over the winter. The real nuisance is their numbers, the stink bug's odour and the small stains crushed insects can leave on curtains and light-coloured walls.

What to do if they are already inside

• Vacuum them up instead of crushing them, then empty the bag or canister outside into a sealed bag.
• Skip indoor insecticide sprays: they do little against insects hidden in walls and leave residue behind.
• Note where they show up (a window, an outlet, a ceiling light): it often points to where they are getting in.

Keeping them out: prevention beats treatment

The best time to act is late summer and early fall, before they gather on your walls:

• Seal cracks around window and door frames with a suitable caulk.
• Repair screens and install door sweeps, including on the garage door.
• Cover vents with fine mesh and check where wires and pipes enter the house.
• Keep debris and woodpiles away from the foundation.

For heavily exposed homes, a professional exterior perimeter treatment applied at the right time in the fall can greatly reduce how many insects make it inside. The technician will also point out the entry points to seal.

When to call an exterminator

Call if they come back in large numbers every year, if you find them in several rooms, or if you are not sure what you are looking at. Send us a photo or check our Identify a pest page. ExtermExpress serves Montreal, Laval, the North Shore and the South Shore, 24/7: (514) 295-7696.