Stink Bugs and Boxelder Bugs: Your Fall Houseguests

The calls change in late September. "There are dozens of brown bugs on the garage wall, and now they are in the bedroom." Or: "Little black and red insects are covering the whole south side of the house." Both are fall houseguests. Stink bugs in the house and boxelder bugs on the siding are after the same thing: a dry crack to sleep in until spring. They do not bite, they do not breed indoors and they do not damage anything. There are just a lot of them, and one of the two smells.

How do I tell a stink bug from a boxelder bug?

The brown marmorated stink bug (Halyomorpha halys) came from Asia and settled in southern Quebec during the 2010s. It is 12 to 17 mm, shield-shaped, mottled brown, with small pale bands on the antennae. Those bands separate it from our native stink bugs, which also wander in some falls, but in much smaller numbers. It is a slow, clumsy flier that smells strongly when crushed.

The boxelder bug (Boisea trivittata) is native. About 12 mm, slender, black with three red lines on the back and red edges on the wings. It feeds on the seeds of the Manitoba maple (box elder, or érable à Giguère in Quebec French), and a female tree in your yard or the neighbour's guarantees you will have them. On sunny September afternoons they pile up by the hundreds on the south or west wall, wherever the siding is warm.

Are these bed bugs?

No, and that is the first thing we sort out on the phone. Bed bugs are small (4 to 6 mm), flat, reddish brown, wingless, and they live near the bed, not on exterior walls. Neither a stink bug nor a boxelder bug bites anyone. If you are waking up with bites or finding black specks on the mattress, that is a different problem: read our post on the signs of bed bugs and give us a call.

Why are stink bugs in the house every September?

Both species spend the winter as dormant adults, under loose bark or in a rock crevice. A house is a cliff full of warm cracks. Once nights drop below about 10 degrees, they leave their tree and head for the warmest, brightest surface in sight: the south wall, the pale siding, the window frame in the sun.

From there they look for a gap. Behind the siding, under flashing, around an air conditioner, through a torn screen, under the garage door, into the soffit. Most end up inside the walls or the attic, and you will not see them again until spring.

They do not reproduce in the house. What comes in during September is all you will ever have, and the number can only go down. A mild day in January wakes a few, and they come out through light fixtures and outlets. That is normal, and they are not new arrivals.

The real nuisance is the numbers, the spots on the curtains and the smell. The stink bug earned its name: crushed, or even just handled roughly, it releases aldehydes that smell like rotten cilantro, and the smell stays on your fingers. The boxelder bug is milder, but crushed on a pale wall it leaves an orange-red stain that does not come out easily. Rule one: do not crush them.

How do I get rid of stink bugs and boxelder bugs?

In most houses, four moves take care of it.

1. Vacuum, then straight to the outdoor bin. The single most effective tool. A nylon stocking slipped into the hose and held with an elastic catches the bugs before they reach the canister, so the smell does not live in your vacuum for months. Drop the bag or the stocking in the outside bin right away.
2. Soapy water outside. Clusters on the south wall can be sprayed with water and dish soap. It kills on contact, with no pesticide and no residue on the siding. Repeat on warm afternoons for two or three weeks.
3. Seal. This is the only move that counts for next year. Caulk around windows, doors, wire and pipe penetrations, the dryer vent and the air conditioner. Fix the screens and put mesh over soffit vents. Do it in August, before they arrive: sealing in October traps the ones already in the walls and pushes more of them indoors.
4. Do not spray insecticide indoors. Hardware store aerosols kill the ones you can see, not the ones in the walls, and they will keep arriving for a month. A vacuum does the job better, with no product in the kids' bedroom.

For boxelder bugs the tree is still the main factor: a female Manitoba maple within 20 metres guarantees a return every fall.

Already at hundreds a day on the house and losing ground? Extermination BETA serves Brossard, Saint-Hubert, Longueuil and the whole South Shore out to Chambly. The first inspection is free, we respond the same day and the first service is 15 percent off. Call 438-253-3000 or use the contact page.

When is it worth calling an exterminator for stink bugs?

The honest answer: for most people who call us about stink bugs in the house or boxelder bugs on the wall, it is not. Twenty a week for three weeks is a vacuum job, not a treatment, and we say so on the phone. There are situations where it does make a difference.

• Hundreds every day. When the wall is black with insects every afternoon and dozens get inside daily, the problem has outgrown the vacuum.
• An exterior perimeter treatment, next year. The useful window is late August or early September, before they arrive: a residual product on the siding and at the entry points. Applied in October, once they are already in the walls, it does close to nothing. A residential service with us starts with that assessment.
• Sealing work. When the entry points are numerous or high up, walking the house with someone who knows where insects get in beats caulking at random.
• You are not sure what it is. A brown insect in the bedroom, and you have not slept well since. We come identify it, the inspection costs nothing, and most of the time we leave telling you your bed is fine.

Our technicians are certified and hold the Quebec CD pesticide permit. And if fall is also bringing spiders into the house, it is the same phenomenon: more insects looking for shelter, more predators following them.

Frequently asked questions

Are stink bugs dangerous to people or pets?

No. They do not bite, carry disease or contain anything toxic. If the smell irritates your skin, wash your hands and it is done.

Will they lay eggs in my walls over the winter?

No. Both species lay eggs outdoors, on leaves, in spring and summer. What is in your walls in November is asleep, and it will head back outside in April or May.

Why do they come back to the same wall every year?

Because it is the warmest, brightest wall, and because the source (the tree, the field, the woodlot) does not move. The stink bug also releases an aggregation pheromone that pulls others to a good site. Seal that wall first.

The short version

Stink bugs in the house and boxelder bugs on the south wall are fall knocking on the door. Vacuum, soapy water, no crushing, no indoor spraying, and caulking in August: for most homes on the South Shore, that is the whole plan.

If you are instead looking at hundreds a day, Extermination BETA is based in Brossard and serves Longueuil, Saint-Hubert, Saint-Lambert, Greenfield Park, La Prairie, Chambly, Montreal and Greater Montreal. Call 438-253-3000 or reach us through the contact page. We will tell you straight whether it is worth it, and we are used to saying no.