Boxelder Bugs and Asian Lady Beetles on Your Minnesota Home

Why do boxelder bugs and lady beetles gather on sunny walls?

They are looking for a sheltered place to pass the winter and they are drawn to warm surfaces. University of Minnesota Extension says boxelder adults leave host trees in fall and favor buildings with southern exposure, and Asian lady beetles overwinter on the sunny sides of tall buildings. A bright October afternoon is when you see them.

The attraction is warmth and a vertical surface. A south-facing or west-facing wall in the afternoon sun is warm enough for insects to move about even when the air has turned cold. They land, walk upward and look for cracks. Light-colored siding, white trim and brick that holds heat are all popular.

The boxelder bug is named for its host: the boxelder tree, a kind of maple. The adults spend the summer feeding on seeds and leaves, and when the days shorten they leave the trees. Houses near a mature boxelder are the usual targets, but the insects fly, and a house with no tree on the lot can still be covered.

The Asian lady beetle was introduced as a garden predator and now behaves in autumn much like the boxelder bug. Tall buildings with a sunny side are favored, and in the Twin Cities that includes apartment blocks, church steeples and large houses.

What are the big three home invaders in Minnesota?

Extension lists boxelder bugs, Asian lady beetles and brown marmorated stink bugs. All three move into buildings in fall, all three are mainly nuisances, and none breeds indoors. The brown marmorated stink bug is the newest on the list. The table compares what each does and what to expect.

The table summarizes the points that Extension makes about the three, with the practical advice that follows. Where we have no source for a detail, such as the exact weeks of arrival, we leave it out.

Why do they come back on warm days in winter and again in spring?

The insects that found a gap do not leave; they sit inside walls, attics and under shingles. A warm spell wakes some, which wander into rooms and die within days without food. In spring, those that survive try to leave toward windows and light, so you see them again at the thaw.

A heated house is a poor hiding place, because a heated wall wakes insects that should be dormant. On a mild afternoon in January you may find a few on a window. They are not new arrivals; they came in months earlier. They crawl toward light, find no food and die.

In late winter and early spring the pattern repeats on a larger scale. As the days lengthen and the sun warms the south wall, the insects in the wall voids try to get out. If the gap they used leads into your living space, they appear indoors instead.

Because they overwinter in places you cannot reach, there is no good way to treat them in January. Even where a treatment could reach them, it does not change the real fix, which is to stop the next group from entering. See boxelder bug and Asian lady beetle control for what a technician does in autumn, and cluster fly and stink bug control for related insects that overwinter in attics.

What should you seal, and when?

Seal in late summer and early autumn, before the insects gather. Check the gaps around windows and doors, under siding edges and trim, at soffits and fascia, around utility entries, and at vents and chimney flashing. Repair torn screens and add door sweeps. Do it on a dry day with weather-resistant caulk and mesh.

Timing matters. Once the insects are crowding the wall, sealing a gap may trap some inside the wall, where they will wake later. The best time is before they gather, which in many years means September. If you are late, seal anyway, because it still keeps out the next wave and the ones that arrive in spring.

A tall house is hard to reach. Do not climb a ladder on a steep roof to do it yourself. A technician can do roofline work safely and will tell you what can be closed from the ground. Large houses with several roof lines, such as those near Lake Minnetonka, take longer because there are more edges.

• Windows and doors: replace worn weatherstripping and add a door sweep to exterior doors
• Siding and trim: caulk the joints and the gaps behind trim boards where the wall is sunny
• Soffits and fascia: check for loose panels and gaps where the roof meets the wall
• Utility entries: seal around pipes, cables and meter bases
• Vents and louvers: add fine screen to attic, crawl-space and gable vents
• Chimney flashing and roof edges: ask a technician if the roofline is too high to reach safely

What should you do about the ones that are already inside?

Vacuum them up and empty the bag or canister outside. Crushing boxelder bugs and lady beetles can stain surfaces and some give off an odor. Spraying indoors does little for insects that are in the wall, and they die within days anyway without food. Focus the effort on sealing so the next group stays out.

A vacuum with a hose attachment works well for windowsills and ceilings. Put a stocking over the end of the hose or use a bag so that the insects collect without being drawn through the fan. Empty it promptly, because lady beetles and stink bugs can release an odor when disturbed.

Do not use insecticide foggers in the house. They are not a fix for insects that overwintered in wall voids, and they can leave residue on surfaces. If numbers are high, a technician may treat the outside wall surface or recommend a different approach, and will explain the plan before applying anything.

If you see dozens at a time in the same place, look for the gap. A line of insects at the top of a window frame or at the edge of an attic hatch points straight to the route. Mark it and give the location to the technician.

Do these insects damage the house, and how do they differ from cluster flies?

They do not damage the structure. Boxelder bugs and Asian lady beetles do not feed or breed indoors, according to University of Minnesota Extension, so the harm is stains, odor and annoyance. Cluster flies are a different insect that also overwinters in attics. Telling them apart helps decide the response.

Extension describes boxelder bugs as native to Minnesota, feeding on boxelder, maple and ash trees, and the Asian lady beetle as the most common lady beetle in the state and a fall and spring nuisance. Cluster flies are a different insect that overwinters in attics and appears on warm winter afternoons around upper-floor windows, so a fly in a sunny room is not one of these two.

If what you have is stink bugs, the brown marmorated stink bug is the one on Extension's list. The same sealing steps apply. If you have flies in an upstairs room, see our cluster fly and stink bug control page.

What does a professional treatment add to sealing?

Sealing is the lasting step and you can do much of it yourself. A technician adds reach, since rooflines, soffits and high walls are hard to seal safely, and judgment about where insects are entering. University of Minnesota Extension says a perimeter treatment by a licensed professional may help where problems recur.

Think of the work in three layers. The first is exclusion: the gaps are closed from the ground up to the eaves. The second is a targeted exterior treatment on the sunny walls in autumn, if one is chosen. The third is cleanup indoors with a vacuum. Only the first layer is permanent.

If you hire help, ask what will be sealed, where the treatment will go and when to expect a follow-up. A technician should not promise that the insects will stop appearing. In a hot, dry summer followed by a sunny October, even a well-sealed house may see some on the wall, and the test is whether they get indoors.

For homeowners in Minneapolis, Saint Paul and the suburbs, the best calendar is simple: seal in September, check the south wall in October, vacuum through the winter and look again at the thaw. Our monthly pest calendar places this in the rest of the year, and general pest control covers a seasonal plan built around it.