Cluster Flies, Stink Bugs and Ladybugs in a Maine House

Every Maine winter produces the same call: it is February, the sun is out, and there are twenty insects at the window in an upstairs bedroom. Nothing is wrong with the house, nobody is unhygienic, and no infestation is under way. What is happening is that insects which moved into the building’s walls in autumn have been woken up by a warm wall.

The Board of Pesticides Control’s Got Pests? service describes the cluster fly pattern precisely: they usually appear in late fall or early winter and again on warm, sunny days in early spring, gathering in numbers at windows — often in rooms that are not regularly used. If that sounds like your spare room, read on.

Which one is it

None of these damages the structure, the food in your cabinets, or your family. They are a nuisance, and the honest answer to “what do I spray” is usually “nothing, indoors.”

The five species, briefly

Cluster flies. UMaine Cooperative Extension’s Pest Management Fact Sheet #5010 explains that cluster flies spend the summer unnoticed as parasites of earthworms, can run to four or more generations a season, and are most abundant where earthworms are — grassy areas with good soil and moisture, and old farm ground where manure was stored. That is why a farmhouse on a field gets them by the hundred and a house in town gets a handful. In late summer the adults search for protected overwintering sites such as attics, wall voids and loose bark, and the fact sheet records, from casual observation, that they seem to favor light-colored buildings.

Brown marmorated stink bugs. The Maine Department of Agriculture, Conservation and Forestry notes this invasive species was first discovered in Pennsylvania in 2001 and has done heavy crop damage in mid-Atlantic states. In Maine its main showing so far is indoors: the department has verified overwintering reports from 90 towns in 13 counties, and it warns that homes often see the bug in large numbers two to three years before crop damage becomes noticeable.

Boxelder bugs. Got Pests? describes them as pests because they congregate in large numbers trying to get into buildings in the fall and trying to get out in the spring, gathering on the south and west sides of buildings on sunny days. They cause little damage even to the boxelder and maple trees they feed on.

Western conifer seed bugs. UMaine Extension says these come indoors at the end of summer and in early fall looking for a warm, protected place to winter. They do not harm the wood of a home. Extension does flag one unusual exception — occasional reported damage to PEX plumbing and radiant heating pipe, documented in a Cornell study — which is worth knowing if you have a radiant floor and a lot of these bugs.

Multicolored Asian lady beetles. UMaine Extension calls this a common ladybug that becomes a household nuisance each fall in cold climates as adults seek warm places to spend winter. It was deliberately released in the U.S. as far back as 1916 to control aphids and scale, and one of its many nicknames, the Halloween lady beetle, comes from adults typically moving indoors in October. The large black W behind the head separates it from Maine’s native lady beetles.

Why they are in the wall in the first place

From late summer into autumn these insects look for a sheltered place to spend winter, and a heated building radiating warmth on a cool afternoon is the most attractive thing in the landscape. They gather on the sunny side and work into the first gap they find: around window frames, under lifted siding, behind fascia, through unscreened soffit and ridge vents, and around utility penetrations. UMaine’s cluster fly fact sheet makes the point directly — where siding is tight, the flies have less opportunity to get into the structure.

Once inside the void they go quiet. Then something warms the wall. The cluster fly fact sheet describes flies becoming active on warm days in early winter or when the heat is turned on, apparently only after a spell of cold, and moving toward warmth and light into the living space rather than back out the way they came. That is why it is always a window, and always in the middle of winter.

What to do now, in February

1. Vacuum them. UMaine Extension calls the vacuum cleaner a quick and effective way to reduce a cluster fly population in the home. Empty the canister outside.
2. If they are persistent at one window, check the weatherstripping and the gap where the frame meets the wall — that is often the route from the void into the room.
3. Do not spray them. They are emerging from inside a void, so an application at the window treats the ones you can see and nothing else, for weeks.
4. Keep a note of which rooms and which walls. That is the map for the work in late summer.

Handle lady beetles and seed bugs gently, or vacuum them, rather than crushing them: Extension notes lady beetle secretions stain furniture, curtains and rugs, and seed bugs smell when crushed.

What to do before autumn, which is the actual fix

UMaine Extension’s advice for cluster flies is to “build them out”, and the same work keeps every species on this page outside. On the sunny side of the building first:

• Caulk cracks and seal around window and door frames, and replace failed weatherstripping.
• Screen soffit, ridge and gable vents, and repair any screening that has rusted — Extension names soffit vent screening specifically.
• Close the gaps around dryer vents, bath fan terminations, hose bibs, cable and electrical entries, and mini-split line sets.
• Repair lifted or damaged siding, fascia and trim where a gap has opened.
• Where pressure is heavy, a timed exterior application to the resting areas reduces how many get in at all. The cluster fly fact sheet puts that window at mid to late August, and notes many people hire a professional for an exterior barrier. Applied in November, after the insects have settled in, it is a waste of money.

That last item is licensed work. Maine’s stated policy, printed on every Got Pests? page by the Department of Agriculture, Conservation and Forestry, is to minimize reliance on pesticides and use them only as a last resort within integrated pest management — which, for overwintering insects, means sealing comes first and spraying a distant second.

The pattern worth noticing

Every one of those repairs is the same repair that keeps mice out, and the same one that stops carpenter ants finding wet wood. A Maine house tightened for one of these problems is tightened for all three, which is why exclusion is the recommendation this site keeps returning to. See rodent exclusion for how entry-point sealing works as a service, and the Maine pest calendar for where the late-summer sealing window sits in the rest of the year.

Updated 2026-10-04.