What are fall invaders?

There is no genus or family called 'fall invaders.' The term is used by extension entomologists and pest professionals as shorthand for a specific, shared behavior: as days shorten and nights cool in late summer, several unrelated insects enter a hormonally programmed dormancy called diapause and go looking for a sheltered place to spend winter. UNH Cooperative Extension's seasonal insect invaders fact sheet groups them precisely because of this shared habit, not because they're related to one another — a cluster fly, a stink bug, and a lady beetle have almost nothing in common biologically except that each independently evolved to overwinter in tight, dry crevices, and each now treats modern siding, soffits, and attic vents as a stand-in for the rock faces and bark crevices their ancestors used before houses existed.

That's the whole category: not a family tree, but a seasonal appointment every one of these insects keeps on roughly the same New Hampshire calendar.

UNH Cooperative Extension names five insects as the state's principal overwintering home invaders: cluster flies, the brown marmorated stink bug, the multicolored Asian lady beetle, the boxelder bug, and the western conifer seed bug. Three of the five — the lady beetle, the stink bug, and the conifer seed bug — are documented as relatively recent arrivals to New Hampshire, which is part of why the state extension service tracks and names them as a group rather than leaving homeowners to look each one up separately. All five share the same fall aggregation pattern: they mass on sun-warmed, south- and west-facing walls on warm afternoons in September and October, then work their way into cracks, vents, and siding gaps ahead of the first hard frost.

Calling something a fall invader isn't just descriptive — it's a set of candid predictions extension sources back up consistently. None of these five reproduce indoors; the attic and wall void are a roost for winter, not a nursery, so there's no growing colony to eliminate. None damage the structure of the home. None spread disease to people. What you're left with is a nuisance, full stop — with one partial, unsoftened exception: the Asian lady beetle genuinely does nip on occasion, actually does reflex-bleed a staining fluid when disturbed or crushed, and is a documented indoor allergen in sensitized people. That exception doesn't undo the category; it's simply the one member of it that deserves a slightly more serious response than 'ignore it and vacuum.'

The other predictable pattern is the spring one: insects that show up at a sunny window in February or March are, in every documented case, last fall's same population reactivating with the warmth — not a fresh invasion and not evidence that whatever you did last September failed.

Homeowners reasonably ask whether crickets, centipedes, spiders, and mice belong on this list too, since they also show up more in fall. They're adjacent, not identical. House centipedes are driven by moisture and available prey and can be present indoors year-round in a damp basement — a humidity problem, not a photoperiod one. Camel crickets share that same moisture-driven story; field and house crickets that chirp indoors are closer to the true fall-invader pattern, wandering toward warmth as nights cool. House spiders mostly never left — the fall surge is largely sexually mature males searching for mates, not a cold-weather migration indoors. And mice are the sharpest break from the category: they enter for the same seasonal reason but then do the three things this category is defined by NOT doing — they breed indoors, they damage wiring and insulation, and they can carry disease. Treating a mouse like a stink bug is the single costliest misclassification a homeowner can make in this hub.