On a warm afternoon in late September, the south side of a Star home can be covered with black-and-red bugs. Boxelder bugs gather on sunny siding in fall, looking for a protected place to spend the winter, and some of them find their way into wall voids, attics and living spaces. They don’t bite, breed indoors or damage the house, but a large group can be a real nuisance.
Here’s how to recognize boxelder bugs, why Star homes see so many of them and what keeps them on the outside of the house.
What Are Boxelder Bugs?
Boxelder bugs (Boisea trivittata) are about half an inch long, black with red or orange lines along the back and edges of the wings. Young bugs (nymphs) are smaller and bright red. They feed mainly on the seeds of female boxelder trees, and also on maple and ash.
Through summer they stay in the trees. As days shorten in late summer and fall, adults move to warm, sunny surfaces, especially south- and west-facing walls, and look for cracks to crawl into for the winter.
Why Star Sees So Many
- Host trees. Boxelder and maple are common in yards, along ditches and in older plantings around Star and the rest of the Treasure Valley.
- Sunny walls. Light-colored siding with afternoon sun warms up and attracts the bugs in large numbers.
- Open lots and new construction. Homes at the edge of fields and newer subdivisions can draw bugs from a wide area. Boxelder bugs travel a long way, so removing one tree rarely solves the problem.
- Gaps in the house. Small openings around windows, siding, soffits and utility lines give them a way into wall voids and attics.
Signs of Boxelder Bugs
Outside: clusters on siding, window frames, fences and tree trunks in fall and on warm spring days. Their droppings can leave small reddish-orange spots on light surfaces and curtains.
Inside: slow-moving bugs near windows, especially on sunny winter days when the walls warm up; dead bugs on sills and in light fixtures; a mild odor if they’re crushed. The bugs you see indoors in January usually got in during the fall.
Keep Them Out: Sealing Comes First
Exclusion is most effective in late summer, before the bugs start gathering. Walk the sunny sides of the house and look for openings:
- Caulk gaps around window and door frames, siding joints and where pipes, wires and vents go through the wall.
- Repair torn window screens and add door sweeps to exterior doors, including the garage service door.
- Check that attic and soffit vents have intact screens.
- Seal gaps around fascia boards and where the roofline meets the siding.
Our post on bug spray for doors covers what helps at entry points and what doesn’t.
Outdoor Steps That Help
- Soapy water. A spray bottle of water with a little dish soap kills boxelder bugs on contact. It’s useful for clusters on siding or a fence, though it has no lasting effect.
- Clean up seeds. Rake up fallen boxelder seeds and leaf litter near the foundation.
- Thin the clusters. Rinsing groups off the siding with a hose moves them along but doesn’t keep them away for long.
Removing female boxelder trees close to the house can reduce numbers a little, but bugs from neighboring trees will still come. Sealing the house does more.
If They’re Already Inside
- Vacuum them up and empty the bag or canister outside. Avoid crushing them on fabric or light walls, which can leave stains.
- Don’t spray inside wall voids. Dead bugs left in walls can attract other insects.
- Note which windows and rooms they keep showing up in. That points to the gaps to seal next summer.
The University of Minnesota Extension page on boxelder bugs covers their life cycle and why indoor spraying isn’t recommended.
How Professional Treatment Works
A technician looks at where the bugs are gathering, which walls get the most sun and where they’re getting in. Treatment focuses on the outside of the house: a residual application along the foundation, around windows and doors, eaves and other entry points, timed for late summer or early fall before the bugs move in. Combined with sealing, that keeps far more of them outside. Bugs already in the walls will keep appearing on warm days until spring, so interior work is limited to vacuuming and spot treatment where it makes sense.
Boxelder bugs are one of several fall invaders around the valley. Our guide to fall invaders covers stink bugs, elm seed bugs and the others, and the winter pest guide for Idaho homes covers what to watch for once the weather turns cold.
Common Questions
Do boxelder bugs bite?
They rarely bite and don’t spread disease. They’re a nuisance pest, mainly because of their numbers, the spots they leave and the smell when crushed.
Will one treatment take care of them?
One well-timed exterior treatment helps for that season, but new bugs arrive every fall. Seasonal service paired with sealing works better over the long term.
Why do they show up inside in the middle of winter?
They’re coming out of wall voids and attics where they’ve been sheltering since fall. A sunny day warms the walls and they wander toward the light.
Help With Boxelder Bugs in Star
Fall invaders such as boxelder bugs are part of our regular residential pest control plans, along with spiders, ants, wasps and hornets, mice and rats. We offer quarterly, bi-annual and one-time service. Quarterly service plan customers get free return visits. No contracts; cancel with 30 days’ notice. Rodent service is included at no extra cost with quarterly service plans; traps and materials are billed separately. We also work with commercial properties, from offices to storefronts.
ProGuard is based in Eagle and serves homes and businesses in Star, Boise, Meridian, Nampa, Caldwell, Middleton, Emmett and Garden City. Call (208) 352-2062 or contact us to schedule an inspection.