Seasonal Invaders
They don't bite, don't breed indoors and don't damage anything — and they're still the number one fall complaint here. The problem is that by the time you see them, the useful window has closed.
7 min read
Some October afternoon you'll walk around the south side of the house and find a few hundred black-and-orange insects sitting on the siding, doing nothing in particular. That's it. That's the whole event. Boxelder bugs in the Tri-Cities are the most-complained-about fall pest in the region and simultaneously the most harmless, which is a strange combination that makes them genuinely frustrating to deal with.
Boxelder bugs gather on warm, sunlit walls in fall looking for somewhere to spend the winter, then squeeze into gaps around windows, siding, soffits and vents. They don't bite, sting, breed indoors or damage your house. Killing the ones you can see accomplishes nothing, because more arrive the next warm afternoon. The only two things that work are sealing the gaps and treating the exterior before they mass — which in the Tri-Cities means late August through mid-September, not October.
Adults are about half an inch long, flat, black, with three red-orange lines running down the back of the thorax and a red-orange edge outlining the wings. Underneath they're bright red. The nymphs are smaller and almost entirely red, which is why people often think they're seeing two different insects on the same wall.
They feed on the seeds of boxelder, maple and ash trees. Boxelder is a scrappy, fast-growing maple that turns up along the Columbia and Yakima rivers, in older neighborhoods, in alleys, and in every unmaintained fence line in Kennewick and Pasco. It seeds heavily and lives where nobody planted it. If you have a big population and you don't have a boxelder tree, one of your neighbors does, or there's a stand along the nearest greenbelt.
They're looking for heat. In fall, a south- or west-facing wall in full sun runs many degrees warmer than the air, and the insects use that warmth to stay active while they look for an overwintering site. That's why the swarm is almost never on the north side of the house, and why it shows up in the afternoon and thins out by evening.
Three things make a house a magnet here:
This is why one house on a street gets covered and the house next door barely notices. It usually comes down to sun exposure and one tree.
Worth being clear, because the internet is not:
So the honest framing is that this is a nuisance problem, not a damage problem. That doesn't make it a small problem — a few thousand insects on the front of your house is a real thing to live with — but it does change what a sensible response looks like.
This is the part that frustrates people most, so it's worth explaining properly.
When you spray the visible cluster with a contact product, you kill that cluster. The insects on the wall at that moment are a fraction of the population, and they're mobile. The next warm afternoon, more arrive from the tree, the fence line, the neighbor's wall. You've made a dent in a group that refills itself within a day or two.
Soapy water works the same way — it kills on contact and leaves nothing behind. It's genuinely useful for knocking down a cluster you want gone right now, and it's completely useless as a strategy.
What changes the outcome is a residual treatment applied to the surfaces they land on and enter through, before the mass gathering starts. Then the wall itself is doing the work every day for weeks instead of you doing it every afternoon.
Here's the calendar for the Tri-Cities:
| Late August – mid September | Seal gaps. Treat the exterior. This is the window. |
| Late September – October | They mass on sunny walls and move in. Treatment still helps, sealing is now partly too late. |
| November – February | They're inactive in wall voids and attics. Nothing outside changes anything. |
| Warm winter days | A few wake up and wander into living space. Vacuum them. |
| March – April | They leave, over several weeks, often through your rooms rather than back outside. |
Most people call in late October, when the wall is covered. That's understandable and it's also the point where the cheap, high-leverage part of the job has already passed. If you're reading this in September, you're early, and early is worth a lot here.
Boxelder bugs are flat and they only need a gap about the thickness of a credit card. Work the sunny walls first — you get most of the benefit from a fraction of the house.
You will not seal a house perfectly, and you don't need to. Closing the top ten gaps on two sunny walls changes the scale of the problem substantially.
People ask whether they should cut down the boxelder. It's a real lever — removing a nearby female (seed-bearing) tree meaningfully reduces the local population over a couple of seasons. It's also a big, expensive, permanent decision, and if the tree is on someone else's property it isn't yours to make.
Our honest advice: don't remove a tree you like over a nuisance insect. Seal and treat first. If you were already going to take out a scrubby volunteer boxelder along the fence, then yes, that will help.
A perimeter application in early fall puts a residual barrier on the surfaces boxelder bugs land on and enter through — the sunny walls, the foundation line, around windows and doors, eaves and soffits. Insects that contact it don't establish, and the barrier keeps working through the weeks when pressure peaks, rather than for the fifteen minutes a contact spray lasts.
It also handles everything else arriving in the same window, which is the part people underestimate. The same treatment covers stink bugs, spiders, crickets and earwigs pushing at the wall line at exactly the same time of year. Boxelder bugs are just the ones visible enough that you notice.
If your south wall gets covered every fall, the fix is a treatment in early September and an hour with a caulk gun — not a bottle of spray in late October. Call (509) 222-9828 or get a free quote. First visit $149 instead of $249, plans from $59/month, free re-service between visits if anything comes back.
Related reading: Pest-Proof Your Kennewick Home: 9 Simple Fall Fixes and 7 Common Pests in Kennewick, WA.
No. They don't bite, they don't damage the structure, and they don't reproduce indoors. They stain surfaces with their excrement and they smell if crushed. It's a nuisance problem.
They're gathering on a warm sunny surface in fall to find an overwintering site. It's almost always the south or west wall, and it usually means there's a boxelder, maple or ash tree within a couple hundred yards.
Contact sprays kill the ones you hit and nothing else — more arrive the next warm day. What works is a residual exterior treatment applied before they mass, plus sealing the gaps they enter through.
Late August through mid-September. By late October, when most people call, the gathering has already started and the sealing window has largely closed.
Not usually. Removing a nearby seed-bearing tree does reduce the population over a couple of seasons, but sealing and an early-fall exterior treatment solve the problem for most homes without touching the tree.
They came in last fall and are overwintering in a wall void or attic. A warm day wakes a few and they wander into living space. Vacuum them — there is no colony breeding in there.
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