Boxelder Bugs and Cluster Flies on the South Wall of Your Detroit Home

Why do boxelder bugs and cluster flies gather on the south wall?

The south and west walls of a building get the most sun and stay the warmest as days shorten. Insects looking for a place to overwinter follow that warmth, collect on the wall, and crawl upward looking for cracks and crevices to enter. A bright, sunny wall in October can be covered in them.

In late summer and fall, these insects are looking for shelter from the cold. They are drawn to heat, and a sun-warmed wall is the closest thing to it. MSU Extension describes this pattern for boxelder bugs, Asian lady beetles and cluster flies in Michigan: they collect on the south and west sides of buildings and move up the wall to the first gap they find.

That is why a house can seem fine on the shady north side and yet have a crowd on the other face. It also explains why the problem is often worse on a house with light-colored siding, a brick wall that holds heat, or a wall with no shade trees nearby. Where a neighbor's garage or a fence throws shade, the pattern changes. In a Detroit neighborhood of closely spaced houses, the shape of the block can decide which wall gets the sun and therefore the insects.

None of this means the house is dirty or neglected. These insects are not after food or damp. They are looking for a sheltered, protected spot to wait out the winter, and any building with a warm face and a few gaps will do.

What are these insects, and how do you tell them apart?

Boxelder bugs are black with red-orange markings and about half an inch long. Asian lady beetles are orange or red, rounded beetles. Cluster flies are dull gray, slightly larger than houseflies and slow-moving. Stink bugs are shield-shaped. They all overwinter indoors, but each leaves different signs.

Telling them apart matters because the cause and the best response differ. Boxelder bugs are tied to boxelder trees, a kind of maple, and the female trees that bear seeds. Cluster flies are a European native whose larvae parasitize earthworms, so they come from the lawn, not from anything in the house. Lady beetles are usually found where aphids are in the landscape. Other fall invaders, such as stink bugs, may also appear on the same walls.

The table below lists the common ones, with where they come from and what they do in spring. The details come from MSU Extension for the first three; stink bugs are included as other possible fall invaders and are described more loosely.

How do they get into a Detroit house?

Through small gaps: cracks around windows and doors, loose siding, soffit vents, louvers, utility penetrations and gaps where the roof meets the wall. MSU Extension lists those entry points. Detroit's older houses, with settled frames and many generations of paint and repairs, often have more of them.

The insects crawl upward from the warm wall until they find a way in. Window frames are the most common route, especially where caulk has dried and shrunk. Door frames come next. Then siding: a loose board, a lifted corner or a seam where two pieces meet. Soffit and gable vents are also common, because they open directly into the attic.

In a house that has stood for many decades, wood shrinks and paint cracks and caulk fails. A technician will also look at where wires and pipes go through the exterior, at the chimney flashing and at the line where a porch roof meets the house. Any of these can be a route in. Many of the same gaps are what mice use, which is why sealing a basement and sealing the upper walls are two halves of the same job.

Once inside, the insects settle in wall voids, attics and the spaces around window frames. They are mostly inactive in the cold. Some may be seen wandering in a warm room on a sunny winter day.

• Gaps around window frames and between the frame and the siding
• Door frames and thresholds, especially on the south and west faces
• Loose or lifted siding and trim boards
• Soffit vents, gable vents and louvers
• Where utility lines, cables and pipes pass through the exterior wall
• The line where a porch roof or dormer meets the main wall

When should you seal the house, and how?

Seal exterior cracks in late summer or early fall, before the insects start gathering in September and October. Use a quality exterior caulk on cracks around windows, doors and siding, and cover vents with fine screen. Sealing after they are already inside traps some of them in the walls.

MSU Extension advises that sealing exterior cracks with caulk reduces entry. The key is timing. Done in August or early September, it keeps most of them out. Done in November, it may seal a group inside the wall, where they will wake up in spring and look for a way out through the nearest light, often into the room.

For small gaps, use a paintable exterior caulk made for the material, whether wood, brick or siding. For vents and louvers, add a fine insect screen that still lets air through. Replace damaged weatherstripping and door sweeps. If siding is loose, fix it, because a gap behind it is a hidden route.

Also consider the trees. MSU Extension notes that removing nearby female boxelder trees can help, because those trees are the source of the boxelder bugs. That is not always possible, particularly when the tree is a neighbor's, but it is worth knowing when the problem comes back each year.

For larger jobs, such as high soffits, roof lines or a many-sided house, ask a technician. The page on boxelder bug, cluster fly and stink bug control describes what they check and how they treat the outside of the building.

Why do they come back out in spring?

Overwintering insects become active again as the weather warms and they try to leave the building. In Michigan that usually means March into April on sunny days, when boxelder bugs and others crowd windows and sunny walls. Heated attics can wake them earlier, even on a warm day in January.

Many people first notice the insects in spring, not fall, because that is when they wake and move toward light. They crawl across ceilings, collect on windowsills and sometimes drop into rooms. MSU Extension describes the boxelder bug as a real nuisance during spring for exactly this reason. The insects are trying to go back outside, so the best response is to let them, or to remove them gently.

Cluster flies, in particular, can be a surprise in the middle of winter. In a heated attic or a wall near a chimney, a warm spell can wake them, and they appear in a room where no one expected any insects. They are slow and clumsy, and they are not a sign of a new infestation.

Keep the long view. A wall that attracted insects in October will attract them again next October, so the spring is a good time to inspect the exterior, mark the gaps and plan the repairs. The Detroit pest calendar shows where this fits in the year.

What can you do about the insects already inside?

Vacuuming is the usual household approach for those you see, because it removes them without crushing and without a smell. Avoid spraying in wall voids or attics, where dead insects can attract other pests. If numbers are large or the problem recurs each year, a technician can treat the exterior and advise on sealing.

Crushing boxelder bugs and stink bugs can leave stains and an odor, so sweeping or vacuuming them is the usual advice. Empty the vacuum outside. For a few lady beetles or flies near a window, a quick removal is enough.

Do not seal them into walls and do not use a spray inside a wall void or attic yourself. Dead insects left in those spaces can feed other pests, such as carpet beetles. If the numbers are very high, or you are in a multi-unit building where neighbors have the same problem, call a licensed local company through Detroit Pest Control, and ask what they plan to treat and when.

A technician will often treat the outside of the building where the insects gather, and advise on repairs that keep them out. That treatment is usually timed for early fall, before they have moved in. For general ongoing care, see general pest control.