Boxelder bugs (Boisea trivittata) are one of the most recognizable fall nuisance pests in Montana. Flat, oval-shaped, and marked with distinctive red and black coloring, they are hard to miss when they congregate by the hundreds or thousands on the sunny sides of homes, tree trunks, and fences in late September and October.
Like cluster flies, boxelder bugs are seeking overwintering sites. They spend the warmer months feeding on the seeds of boxelder, maple, and ash trees — which is why homes near these trees almost always see heavier infestations. As temperatures drop, they migrate toward warm structures and attempt to enter through any available gap in the building envelope.
Once inside, they enter a semi-dormant state and remain hidden in wall voids, attics, and other sheltered spaces until spring warmth activates them again. They are not dangerous — they do not bite, sting, or spread disease — but their sheer numbers are overwhelming, and they leave behind reddish-orange fecal stains on walls, curtains, and upholstery that can be difficult to remove.
Size
½ inch
Color
Black with red/orange edges and three lines on thorax
Active Season
Spring emergence; fall invasion (September–October)
Risk Level
Low (nuisance only)
Habitat
Boxelder/maple trees; wall voids and attics in winter
Colony Size
Sunny, south- and west-facing walls
SIGNS OF INFESTATION
Signs of a Boxelder Bug Problem
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Mass aggregations on sunny walls — Large groups of red-and-black bugs clustered on south- or west-facing exterior walls in September and October.
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Bugs on or near boxelder or maple trees — Heavy insect activity in or around seed-bearing trees on your property in summer.
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Bugs emerging indoors in spring — Slow, lethargic bugs appearing near windows and light fixtures in March and April.
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Red/orange staining — Faint reddish stains on walls, window sills, or light-colored fabrics from fecal matter.
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Bugs in window frames — Boxelder bugs commonly enter through window frame gaps and accumulate on sills.
HEALTH RISKS
Boxelder bugs are not a health threat. They do not bite humans under normal circumstances (though they can pinch if handled), do not spread disease, and are not known to cause allergic reactions. The primary concern is nuisance — the psychological toll of hundreds of insects invading your living space is real, even if the insects themselves are harmless.
PROPERTY DAMAGE
Boxelder bugs do not damage structures, but the red-orange staining from their excrement can permanently discolor fabrics, drapes, and painted walls if not cleaned promptly. When crushed, they release an unpleasant odor. Large numbers of dead bugs in wall voids can attract secondary pests such as carpet beetles and larder beetles.
