Boxelder bugs gather on the sunny south and west walls of buildings in autumn, according to the University of North Dakota's page on the insect. NDSU Extension advises sealing the known openings and replacing damaged screens before cold weather, and vacuuming bugs that get indoors rather than spraying them. This guide explains why the pattern happens and what a household can do about it.
Key takeaways
- Boxelder bugs collect on the sunny south and west sides of buildings, which is why one wall of a Fargo house can carry the whole cluster while the north wall is clean.
- NDSU Extension advises sealing the known openings and replacing damaged screens before cold weather, because the openings are what let the bugs in.
- Vacuuming or sweeping the bugs that have come indoors is the advice; spraying an insecticide inside is not the first step and is not needed for a nuisance problem.
- An older NDSU circular on fall invaders says to judge how serious the problem is before treating it, and the inspection is what makes that judgement.
- The moment to act is the first warm autumn afternoon with a cluster on the wall, not the day the bugs are on the bedroom window.
Why do the bugs choose the sunny side of the house?
The University of North Dakota's page on boxelder bugs says they gather on the sunny south and west sides of buildings. In a Fargo autumn the afternoon sun warms those walls first, and the bugs settle where the surface is warmest, which is why the cluster is so often on one side of the house.
The pattern is one of the most recognisable things about a Fargo autumn. On a mild afternoon the south wall of a house can carry a dark cluster that thickens as the day goes on, while the wall beside it, in shade, has nothing at all. A household that notices this for the first time often assumes the house has a problem everywhere. In most cases the problem is on the sunny side and the openings that let the bugs in are on the same side.
The bugs are not attracted to the house in the way a food source would attract them. They are drawn to the warm, lit surface, and they stay there while the sun is on it. When the sun goes off the wall in the evening, they move on or settle into any gap they can reach. That is why the sunny wall is also where the walk should start: the cluster shows the way in.
The cold changes the picture. As the weather turns, the bugs look for shelter for the winter, and the openings in the siding, the window frames and the doors become the routes they use. A household that closes those openings before the cold arrives has done the most useful thing available, because the bugs then have nowhere sheltered to go.
What do NDSU Extension and the Fargo-Moorhead press advise?
NDSU Extension advises sealing the known openings before cold weather arrives, replacing damaged window and door screens, and caulking the gaps under doors and around frames. If the bugs have come indoors, the advice is to vacuum or sweep them up rather than use an insecticide.
An NDSU Extension entomologist, quoted in a Fargo-Moorhead news report, gave advice that is still the clearest guide for a household. The first step is to seal the openings the household knows about. That means the window and door frames, the threshold under an exterior door, the gaps around pipes and cables, and any vent that has lost its mesh. Each of these is a route in, and closing it is the job that changes the result for the rest of the season.
The second step is the screens. A torn or missing screen is the most visible opening on a sunny window, and it is the one a household can repair with simple tools. The advice is to replace the damaged screen rather than leave it, because a screen that is open is a door the bugs can use all autumn.
The third step is the gaps under doors. A worn sweep or a threshold that does not sit flat on the floor lets a surprising number of bugs through in the afternoon. Caulk, weatherstripping and a new sweep close those gaps, and they are the easiest of the three steps to check afterwards. The fourth step, if bugs are already inside, is to vacuum or sweep them up. The advice is specific on this point: do not reach for an insecticide to deal with a nuisance indoors.
Why judge the problem before treating it?
An older NDSU circular on fall invaders says to judge how serious a problem is before treating it. A few bugs on a south wall is a nuisance that sealing and clean-up solve, while a population inside a wall void needs a different decision.
NDSU Extension's circular on fall pests in homes, first published in 1991, is old enough that its product advice should not be used today. Its general approach still holds: judge how serious the problem is before deciding to treat. That approach separates the household that has a few bugs on a wall from the household that has a population living in a wall void, and it stops a treatment being applied to a problem that sealing would have solved.
A few bugs on a sunny wall in October usually means the openings need closing. The bugs are visible because they are gathering, and they will stop gathering once the sun is off the wall and the cold arrives. A large number of bugs that keep coming indoors from the same spot, or sounds and movement inside a wall, is a different picture. The inspection is the step that tells the two apart.
The inspection is also what gives the household a fair price. A treatment that is not needed is money spent on a product that sealing would have replaced, and a treatment that is needed but is not done leaves the population in place. The decision is best made with the findings written down, which is the reason the price is confirmed on site and not on the phone.
What does the seasonal pattern look like through an autumn?
The pattern below follows the sources this guide relies on. The early autumn is when the sunny walls first show a cluster, the mid-autumn is when the openings should be closed, and the approach of the cold is the deadline for sealing. Spring and summer details are not in the sources and are not given here.
The autumn is a sequence, and the steps are easiest to follow when they are set out in order. The first warm afternoons of the season are when a cluster is most likely to appear on the sunny walls. The sealing should be done before the cold, so that the openings are closed while the bugs are still outside and looking for a way in. A household that has the list of openings ready when the first cluster appears is working to the sequence rather than reacting to it.
The table below sets out what to do at each point. It uses only the steps the sources support, and it marks the parts of the year the research did not cover rather than filling them with guesses.
| When | What a household notices | What the advice says to do |
|---|---|---|
| First warm afternoons of autumn | A dark cluster on the sunny south or west wall | Walk the sunny walls and find the openings nearest the cluster |
| Early autumn, before cold | Bugs at a window or door on a warm afternoon | Replace damaged screens and seal the known openings |
| Before the first hard cold | Gaps still open at frames, thresholds and penetrations | Caulk the gaps and close them; repair or list anything larger |
| Bugs already indoors | A few insects on a windowsill or floor | Vacuum or sweep them up; do not spray indoors |
| Spring and summer | Not covered by the sources used here | Not stated in this guide |
What should a household do this autumn?
Walk the sunny walls, list the openings, replace the damaged screens, caulk the gaps and vacuum what gets inside. Call for an inspection when the signs are inside, when they keep coming back after sealing, or when the list is too long to close alone.
Start with a walk of the outside, on the sunny side first. Note each cluster, its height on the wall and the openings nearest it. Look at each window frame, each door and threshold, each screen, each vent and each pipe that comes through the wall. A notebook and a pencil are enough, and a photograph of each opening helps the next visit.
Next, repair the screens and seal the gaps you can reach safely from the ground or a stable ladder. Do not work on a ladder alone or in wind, and keep the ladder on firm ground. The caulk goes into the seam where the trim meets the siding, and a gap larger than a finger is listed rather than filled with caulk alone.
Then clean up. Vacuum the bugs off the windowsills and floors, empty the canister outdoors into a sealed bag and repeat the clean-up on the warm afternoons when more appear. Keep a note of the rooms and times, because the note shows whether the sealing is holding.
Call for an inspection if the bugs come inside in numbers after the sealing is done, if they appear from a wall or a ceiling rather than a window, or if the list of openings is too long to close in a weekend. The technician will walk the house, explain what was found and set out the options and the price before any work begins.
What do households get wrong about boxelder bugs?
The common mistakes are spraying indoors, treating only the wall that shows the cluster and assuming the problem is over once the bugs stop gathering. Each one leaves the openings open for the next autumn.
The most common mistake is spraying an insecticide on the windowsills and in the rooms where the household spends its evenings. The advice is against it, because it leaves residue where people and children spend time and it does not close the opening the bugs are using. A vacuum does the same job for the bugs that are inside, without the residue.
The second mistake is treating only the wall that has the cluster. The bugs gather where the sun is, but the openings can be on any side of the house, and a shaded wall can have a gap that lets the bugs in as easily. A walk of the whole house is the only way to be sure the openings on every side are found.
The third mistake is assuming the problem is over when the bugs stop gathering. They stop gathering when the sun goes off the wall or the weather turns, not because the openings are closed. The same openings will be used next year unless they are sealed now, and a household that keeps a record of the openings can check them each autumn.
Frequently asked questions
Do boxelder bugs bite or damage the house?
This guide does not describe the bugs as a health hazard or a structural problem, because the sources it relies on are about where they gather and how to keep them out. The practical problem is the number of bugs that come indoors, and the advice is about closing the routes in and cleaning up what gets inside.
Why do the bugs keep coming back every year?
They gather on the sunny walls each autumn, and any opening that was not closed stays open for the next season. A house that is sealed and recorded gives the bugs fewer routes in each year, and a household that keeps the list can see the openings that still need work.
Should I spray the outside walls?
The advice puts sealing and clean-up first and treats a treatment as a decision that follows the inspection. A technician can explain whether a treatment is needed for a given population and where it would go, and the price is confirmed before any work starts.
Is sealing enough on its own?
For many houses, sealing the known openings and cleaning up what is inside is enough. Where the inspection finds a population in a wall void or a problem that sealing does not reach, a treatment may also be part of the plan.
This guide is general information for Cass County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.