Fall Pest Invaders: Why They Show Up Every Autumn

What are cluster flies and why are they in my house?

Cluster flies are a group of true flies (Pollenia) that get into houses through cracks in walls and roofs to spend the winter. They are familiar here: a 2022 review in The Canadian Entomologist notes they were reported as a common household pest in Ontario and Québec from 1951 onwards.

The same review says at least one North American cluster fly species is a parasitoid of earthworms, and that several hundred flies may gather in any one cluster. In early spring, hundreds of them may die inside a home before they find their way back outside.

Are stink bugs and conifer seed bugs in the house harmful?

Brown marmorated stink bugs are mostly a nuisance, but western conifer seed bugs are not always harmless: a 2005 paper in The Canadian Entomologist describes damage to common plumbing materials caused by overwintering western conifer seed bugs. Ontario's agriculture ministry says brown marmorated stink bugs do not bite and do not reproduce inside structures, and both bugs follow the same fall pattern as cluster flies. Brown marmorated stink bugs can gather indoors in large numbers through the fall and winter, and the ministry says they can be removed with a vacuum cleaner or shop vac, as long as the bag or contents are thrown out promptly because the odour will persist.

Why are boxelder bugs gathering on my house?

They are getting ready for winter: in late summer or early fall boxelder bugs stop feeding, leave their host trees and move into buildings, including homes. Boxelder bugs feed on boxelder trees, and a 2025 study in the journal Insects says they mainly gather on the outside of south- or west-facing surfaces, clustering under siding, metal cladding or brick veneer. The same study says that, depending on the gaps in a building's exterior, they may move deeper into the structure and end up in living spaces, where they are mainly a nuisance.

When should you treat your house for fall bugs?

Start before the fall: Ontario's agriculture ministry says shorter days in August and September trigger new stink bug adults to move to overwintering sites, and that sealing cracks before the fall is the most effective way to keep them out.

For cluster flies and boxelder bugs, a residual product goes on the south- and west-facing walls, window and door frames, soffits and foundation gaps before they bunch up on the walls. Seal the entry points with weather stripping and caulk at the same visit. For stink bugs, the ministry does not recommend insecticides, so sealing those gaps is the main defence. For the rest of the year, see why Ontario pest control is now a year-round job.