Why are stink bugs and boxelder bugs all over my house?

It usually starts on a warm afternoon in late September. One side of the house is suddenly covered in bugs, a few more are on the window screens, and by January there is one walking across the living room ceiling. It looks like an infestation. It is not quite that.

In most Southeast Michigan yards it is one of two insects, and they behave the same way for the same reason. Once you know which one you have and what it is after, the fix gets simple.

Stink bug or boxelder bug?

The two are easy to tell apart once they are side by side. The brown marmorated stink bug is a flat, shield-shaped bug about 17 mm long, a little over 5/8 of an inch, mottled brown, with light bands on its antennae and a black and white pattern along the edge of its body, according to Penn State Extension and the State of Michigan. It is an invasive insect from eastern Asia, first found in Michigan in 2011 and now established across the state.

The boxelder bug is narrower and darker: about half an inch long, black with orange or red markings, and three red stripes just behind the head, per the University of Minnesota Extension. It is a native insect. Its young are small and bright red, and only the adults survive the winter.

Stink bugs: brown, shield-shaped, spread out

Boxelder bugs: black and red, in clusters

Two other fall visitors get mixed up with these. MSU Extension lists the western conifer seed bug, which looks like a stink bug but has an oval body and flattened, leaf-like back legs, and the multicolored Asian lady beetle, an orange beetle that can bite and can stain fabric.

Why they pick your house

None of them want your food or your wood. They are looking for a winter bed. As the days shorten they go into a resting state and stop breeding and feeding. Out in the woods they would spend the winter under loose bark. A house is the same thing, only bigger and warmer.

MSU Extension puts it plainly: they gather on south and west walls, because those get the afternoon sun, and they crawl upward until they find a crack. The spot they want stays between 40 and 50 degrees: warm enough to rest, cool enough that they do not burn through their stored energy. Wall voids, attics and the space behind siding are just right.

That is also why cutting down a boxelder tree rarely helps. MSU Extension notes that where there is one boxelder tree there are dozens, and these bugs can fly several miles.

When they arrive in Southeast Michigan

Stink bugs follow the light, not the thermometer. MSU Extension found they begin to gather on buildings once there are fewer than 12.5 hours of daylight, around the second or third week of September. We checked that against the sunrise and sunset table for Ann Arbor: the day drops below 12.5 hours on September 15.

Boxelder bugs gather on tree trunks and walls through the cycle of warm days and cold nights that follows. MSU Extension says that can start in late September but is most likely in October. So the calendar for a Michigan house looks like this:

What actually keeps them out

Sealing the gaps, before mid-September. Every extension service says the same thing, and MSU Extension says it best:

Caulking, sealing and closing cracks will do more to limit numbers than any pesticide.

Michigan State University Extension

• Look where the sun hits. Check the south and west walls first: gaps around window and door trim, where pipes and cables go in, around vents and the dryer duct, and under the soffit.
• Caulk wood siding, screen the rest. Caulk works on wood. On vinyl it will not seal completely, because vinyl panels are hung loose so they can expand and shrink. Use mesh or screen over vents and larger openings.
• Check the window air conditioner. The gap around a window unit is one of the easiest ways in. Pull it or seal around it before the season.
• Seal the inside too. If the outside cannot be closed up, sealing around outlets, switch plates, light fixtures and ducts at least keeps them in the walls and out of the rooms.
• Knock them off. A strong jet from the hose sends them off the wall; a bucket of soapy water finishes them.

What about spraying the outside of the house? It helps less than people expect. An insecticide on the siding only kills the bugs that touch it, and MSU Extension says a single application is not an effective long-term strategy, because more fly in on the next warm day. It earns its place on a house where hundreds gather every year, and the University of Minnesota says it works best timed to the fall move and combined with sealing.

What to do once they are inside

Do not squish them. A crushed stink bug releases the smell it is named for, and a smashed boxelder bug leaves a red stain. Instead:

• Vacuum them, then empty the canister or bag outside straight away. A shop vacuum handles large numbers.
• Pick up strays with a tissue, or knock them into a cup of soapy water.
• Try a light trap. Stink bugs fly toward light. MSU Extension suggests a lamp shining into a foil pan of water with a little unscented dish soap.

The bug on the ceiling in January is not a new arrival. On a warm winter day the heat of the house wakes up a few that are already in the walls, and they wander out toward the light. They will not lay eggs indoors, so there is no second generation to worry about.

When to call us, and when not to

A dozen bugs on the siding and a few on the windows is normal for a Michigan fall. Vacuum them and seal what you can. You do not need us for that.

It is worth a call when the wall is covered every year, when they keep getting into the living space, or when the gaps are up high where you cannot reach them. Our stink bug control and boxelder bug control visits start with finding how they get in, because that is what decides whether next year is better.

Common questions

About this page

Nick Russler, Owner, Honorable Pest Control

Written by Nick Russler, owner of Honorable Pest Control in Ann Arbor, Michigan. Every fact on this page comes from Michigan State University Extension, the State of Michigan, Penn State Extension or the University of Minnesota Extension, and the day lengths come from timeanddate.com's table for Ann Arbor. All were read on September 26, 2026. If something here is out of date or wrong, tell us and we will fix it.