Cluster flies and boxelder bugs are autumn invaders that enter buildings through small cracks to spend the cold months in wall voids and attics. Michigan State University Extension advises sealing those entry points before September, because treatment indoors works poorly against insects hidden in spaces that cannot all be reached.
Key takeaways
- Michigan State University Extension says boxelder bugs are the most common fall invader in Michigan.
- Boxelder bugs do not bite, do not lay eggs indoors and do not eat fabrics or stored food.
- Cluster flies are usually noticed in winter, when sunlight warms the south or west side of a building.
- Cluster flies can fit through a crack about the thickness of a credit card, so small gaps matter.
- Insecticides applied indoors tend to disappoint because the insects hide in spaces that cannot be fully treated.
- The big invasion months are September and October, so repairs should be finished before then.
Why do these insects pick the sunny walls?
Cluster flies come out of the wall voids where they spent the cold months when the sun warms a building. Boxelder bugs gather on windowsills in autumn and winter after finding their way inside.
Michigan State University Extension describes cluster flies as usually noticed in winter, when sunlight warms the south or west side of a building. The warmth brings them out of the wall voids where they spent the cold months, and they gather on windows and walls. That is why a sunny room in January can suddenly show a cluster of flies after weeks with none. The cluster fly control page explains how a technician looks for the entries on those sides of a building.
Boxelder bugs follow a different path. They enter buildings in autumn to overwinter in attics and wall spaces, and then they collect on windowsills during warm spells. Seeing many insects in a window is a sign that they have already found their way in, not that they are breeding inside. Knowing the difference helps a homeowner decide where to look for the entry points, which are usually outside the room where the insects are seen.
What do these insects do once they are inside?
Boxelder bugs do not bite, do not lay eggs indoors, and do not eat fabrics or get into stored food, according to MSU Extension. Both insects are mainly a nuisance once they are indoors.
The practical point is that boxelder bugs are unpleasant company rather than a food or fabric threat. They gather on windowsills and in corners, and a vacuum is the usual way to clear them from inside. MSU's advice is to vacuum the ones already inside rather than rely on sprays, because an insect that is sprayed in a wall space may still be out of reach. The boxelder bug control page covers how entries are checked.
Cluster flies have a different life cycle. Part of it is spent as a parasite of earthworms, an unusual detail that MSU includes when it explains their habits. Their presence indoors comes from the wall voids, which is why a fly that appears in a living room usually came in through a gap in the building rather than from a nest in the room. Finding that gap is the job that matters most.
Why does spraying indoors disappoint?
Insecticides applied indoors work poorly against these insects because they hide in spaces that cannot all be treated. Sealing the entry points does more than any indoor spray.
MSU Extension says to expect insecticides to work poorly indoors, because the hidden harborage cannot all be treated. Cluster flies and boxelder bugs settle in wall voids and attics, where a spray cannot reach them in a useful amount. A treatment that looks effective on the surfaces of a room may leave the insects inside the wall untouched, and the next warm afternoon brings more of them out.
This is why the approach MSU recommends starts outside. Caulking exterior cracks and holes keeps insects from getting in, and it is a job a homeowner can do with basic materials. Vacuuming removes the insects that have already arrived. The combination works better than relying on indoor sprays, which can give the impression of control while the entries stay open for the next group.
What entry points should be sealed first?
Start with the cracks insects can use, including gaps around windows, door trim, siding joints and wherever wiring or pipes enter the building. Cluster flies can fit through a crack about the thickness of a credit card.
MSU Extension says cluster flies can squeeze through a crack roughly the thickness of a credit card. That size matters because many gaps that look too small to matter are big enough for these insects. Walk the outside of the building, especially the south and west walls, and look for cracks around window frames, door trim, siding joints and the places where wiring or pipes come through.
Boxelder bugs also enter through small openings in autumn. MSU's advice includes removing nearby female boxelder trees where possible, as one way to reduce the number of insects that come toward a home. Removing a tree is a larger decision than sealing a crack, so it is worth talking with a tree professional before deciding. Sealing the building is the step most homeowners can complete on their own before the first cold nights.
What does the timing of repairs look like?
The big invasion months are September and October, so repairs should be finished before then rather than after the insects have already moved in.
MSU Extension says the big invasion months are September and October. Repairs finished in those months may arrive too late for insects already moving toward the building, so the work is most effective when it is done in late summer. Setting a reminder in August is a practical way to make sure the inspection and sealing happen on time, before the weather changes.
Timing also matters for cluster flies, which are usually noticed in winter. A repair finished in the autumn keeps them from settling into wall voids, which means fewer emerging on warm winter afternoons. Once they are inside a wall, they are much harder to deal with, so the best time to act is before the cold, not after the first sunny winter day brings them to the window.
How should I clean up the insects that are already inside?
A vacuum is the simplest tool for insects already inside. Vacuum the windowsills, corners and curtains, and empty the vacuum outside so the insects do not come back in.
Vacuum the insects from windowsills, corners and curtains, and do it regularly during the autumn, since insects keep arriving while the weather stays mild. Empty the vacuum outside or into a sealed bag, so the insects do not come back out into the room. A quick pass once or twice a week through the rooms that face the sun is usually enough to keep up with the arrivals.
Vacuuming is a short job that removes the insects but does not seal the building. Pair it with the sealing work outside, because a cleaned-up room fills again if the entries stay open. Brushing the insects into a container is an option when a vacuum is not available. Either way, the cleanup is a short-term measure, and the real progress comes from closing the gaps.
What does MSU say about the cluster fly life cycle?
MSU Extension describes cluster flies as overwintering in wall voids and emerging when the building warms in winter sun. Their indoor appearances are tied to the structure of the building.
Cluster flies spend part of the year outdoors, and the adults find their way into building walls in autumn. They overwinter there, and the warmth of a sunny wall or a heated room draws them out during the cold season. Because they are small and move slowly, they are most often seen gathering near windows, where the light is strongest.
The insects that appear in winter are usually the ones that entered the wall the previous autumn. That is why autumn sealing matters more than any winter response. A homeowner who waits until the flies appear will see the insects but will not stop their arrival, since the entry points stay open for next year unless they are closed before the cold.
When should I bring in a technician?
Bring in a technician when insects keep appearing after sealing, when the building has many entry points, or when the walls are hard to access. The technician confirms the price on site.
A technician is useful when the insects keep appearing after the obvious cracks have been sealed. That often means the entries are hidden in places that are hard to see from the ground, such as the corners where siding meets the foundation or spaces around an attic vent. A technician can check the wall voids and attic spaces and explain the entries in plain terms, so the next steps are clear.
Describe the building, the rooms where the insects gather, the window and wall orientation and the months when they appear. That information helps the crew plan the visit. A visit the same day can happen when a technician is free, but no arrival time is promised. The technician confirms the price on site before any work begins, and sealing is often quoted separately from treatment.
Frequently asked questions
Are boxelder bugs dangerous to people or property?
MSU Extension says boxelder bugs do not bite and do not eat fabrics or stored food. They are mostly a nuisance because they gather in large numbers on windows and walls in autumn. Their presence does not by itself mean the building is damaged. Sealing cracks and vacuuming the insects that have come inside handles most of the problem for a typical home.
Do cluster flies breed inside my house?
MSU Extension describes cluster flies as overwintering in wall voids rather than living in the rooms of a home. The flies that appear indoors usually came in through cracks the previous autumn. Part of their life cycle is spent as a parasite of earthworms, which happens outside the building. Sealing the entries is the way to keep new flies from arriving.
Does insecticide help at all?
MSU Extension says to expect insecticides to work poorly indoors, because the hidden harborage cannot all be treated. Sealing the outside of a building and vacuuming indoors give more lasting results than indoor sprays. Anyone choosing a product should read the label carefully and follow the directions. A technician can explain which options fit the building.
Why do they come out on warm winter days?
Cluster flies are usually noticed in winter, when sunlight warms the south or west side of a building. The warmth draws them out of the wall voids where they overwintered. A few flies on a sunny window in January are a sign of entry points rather than a new infestation. Sealing the autumn entries reduces how many appear the following winter.
How soon before September should sealing be finished?
MSU Extension says the big invasion months are September and October, so repairs should be finished before then. Late summer is the practical window for most homeowners. Work done in October still helps, but the insects may already be inside. Keep a short list of the cracks you find so next year's work starts from a record.
Should I remove the boxelder trees near my home?
MSU recommends removing nearby female boxelder trees where possible, as one way to reduce the insects that come toward a building. Removing a tree is a larger decision than sealing a crack, so it is worth discussing with a tree professional first. Sealing the building is the step most homeowners can complete on their own before the cold arrives.
Sources
- Michigan State University Extension
- Michigan State University Extension
- Michigan State University Extension
This guide is general information for Montcalm County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.