The boxelder bug, Boisea trivittata, is one of four insects that come to the walls of American houses every fall for the same reason and get confused with each other constantly. It is the narrow black one with the red lines.
You will also see the species written as Boisea trivittatus; University of Kentucky Extension uses trivittata and Wisconsin Horticulture uses trivittatus. It is the same insect, and the variation is in the literature rather than in the animal.
Identification
Two extension descriptions, both quoted rather than blended, because they emphasize different things:
- University of Kentucky Extension: black bodies with red lines running down the center and sides of the pronotum — the plate covering the thorax. Adults grow up to half an inch. Nymphs appear bright red. Both nymphs and adults have dark red eyes.
- Wisconsin Horticulture: gray and red adults about half an inch long, with a single red line on each side, a diagonal red line on each wing, and three red lines on the thorax. Nymphs are bright red with darker heads and look like the adults but without developed wings.
The practical shorthand: narrow, flat, black, thin red lines, half an inch. The shape is the fastest discriminator, because the insect people most often confuse it with is a completely different silhouette.
What It Is Not
- Not a stink bug. The brown marmorated stink bug is a broad brown shield, almost as wide as it is long. A boxelder bug is a narrow black oval. Nothing about them looks alike once you have seen both.
- Not a milkweed bug. University of Kentucky Extension flags this as the real lookalike: the large milkweed bug, Oncopeltus fasciatus, is roughly the same size and shares the colors, but the pattern differs. Boxelder bugs are mostly black with thin red stripes; milkweed bugs are mostly orangish-red separated by large patches or belt-like stripes of black. Read the ratio of black to red and you have it.
- Not a lady beetle or a cluster fly. Those are round and spotted and a slow dark fly respectively.
The Tree Connection
This is the part that makes boxelder bugs make sense.
University of Kentucky Extension explains that boxelder bugs are found most often on boxelder trees, Acer negundo, which act as their primary host, and that they rely on the tree’s seeds as a food source — which is why they stick to female fruit-producing trees and are rarely found on male trees.
Boxelder is dioecious: individual trees are male or female, and only the female trees carry the winged seeds. So two apparently identical boxelders in the same yard can produce completely different outcomes, and a neighbor with no problem at all may simply have a male tree.
Wisconsin Horticulture adds that during the summer they are found primarily on female boxelder trees where they feed by sucking plant sap from leaves and developing seeds, and that they will also feed on the fallen seed on the ground beneath. University of Kentucky Extension notes they are sometimes found at the base of the tree eating seeds from fallen fruit, which makes up most of their diet.
Other hosts exist but are secondary. Kentucky reports sightings on silver maples, sycamore maples, apple, pear and plum. Wisconsin notes ash, maple and occasionally strawberries and grasses, but states they normally cannot complete their development on these other plants.
The Year in a Boxelder Bug
Wisconsin Horticulture describes the cycle:
- Spring: adults emerge from overwintering sites and females deposit small red eggs on host plants. Nymphs hatch in 10 to 14 days. University of Kentucky Extension gives 11 to 19 days for eggs laid on leaves and branches.
- Mid-summer: nymphs mature into adults, and those adults lay eggs for a second generation.
- Fall: the adults from that second generation leave the trees to find somewhere to spend the winter. They stay active late into the fall and congregate in sunny spots on the south sides of buildings, trees and rocks on warm afternoons.
Wisconsin Horticulture also observes that they tend to be most abundant after summers with a very warm May and a dry July, while noting that numbers vary a lot from place to place regardless of the weather. That second half of the sentence is the honest part and worth keeping.
What They Do and Do Not Do
- They do not bite or sting. University of Kentucky Extension states this directly for humans and animals, and adds there is no evidence they transmit diseases.
- They do not damage the house or its contents. Wisconsin Horticulture states they do not damage food or other items in the home.
- They do not feed or reproduce indoors. Same source. As with the stink bugs, the indoor population is finite.
- They can stain. Wisconsin Horticulture notes boxelder bugs may stain curtains and walls. This is the one genuine indoor cost, and it is an argument for removing them intact rather than crushing them.
- They rarely damage the trees. Wisconsin Horticulture states they rarely cause significant damage to any plants.
Wisconsin Horticulture also explains the behavior that puzzles people in January: as long as the bugs remain cold within walls or attics they are inactive, but if they are warmed by the furnace or the sun they become active and may crawl into the rooms. A warm spell in midwinter is not a new invasion.
Keeping Them Out
Wisconsin Horticulture is explicit that the best means of preventing problems with boxelder bugs is to prevent entry into the home: caulk all openings around windows and doors and in walls, siding or foundations.
Two details worth knowing. They enter through small gaps around windows, doors and pipes and through small cracks in the foundation or siding, and they often gather in large congregations on walls or foundations while seeking entry — so the mass on the wall is a map of where to seal. And they are attracted to buildings with a lot of southern exposure regardless of the color of the building, which is a useful correction to the widespread belief that painting the house differently would help.
For the ones already inside, Wisconsin Horticulture recommends sucking up the occasional bug with a vacuum cleaner, and states that insecticide sprays are generally not recommended for boxelder bugs that have already made their way indoors.
For clusters on the outside of the building, the same source notes they can be sprayed with registered insecticides, insecticidal soap or hot soapy water, and that sprays may need repeating as more bugs arrive, especially when using soapy water. We make no claim here about how well any of those perform; that is the source’s statement of what is used, not ours about what works.
The Tree Question
People ask whether to take the tree down, and the honest answer is that it helps less than the tree’s central role would suggest.
Wisconsin Horticulture states that removing boxelder trees, or spraying heavily infested trees with a registered insecticide in mid-summer, may help reduce boxelder bug numbers — but that the adults are good fliers and can move a considerable distance from host trees, so this will not eliminate boxelder bug problems. Your neighbor’s female boxelder two lots over is still in range.
The same source gives two more useful suggestions: remove accumulations of boxelder, ash, elm and maple seeds, since the bugs can feed and reproduce on these as well as on the trees; and choose species other than boxelder when planting shade trees. The second is the only one that solves the problem permanently, and only for whoever owns the yard in twenty years.
Questions
What does a boxelder bug look like?
University of Kentucky Extension describes adults as having black bodies with red lines running down the center and sides of the pronotum, the plate covering the thorax, and says adults can grow up to half an inch long. Wisconsin Horticulture describes the adults as gray and red, about half an inch, with a single red line on each side, a diagonal red line on each wing, and three red lines on the thorax. Nymphs are bright red with darker heads and have no developed wings. Both nymphs and adults have dark red eyes.
Do boxelder bugs bite?
No. University of Kentucky Extension states that boxelder bugs do not bite or sting humans or animals, and that there is no evidence they transmit diseases. Wisconsin Horticulture states they will not bite humans or pets.
Why are they called boxelder bugs?
Because the boxelder tree, Acer negundo, is their primary host. University of Kentucky Extension explains that they rely on the tree’s seeds as a food source, which is why they stick to female fruit-producing trees and are rarely found on male trees. That single fact explains most of what happens in a yard with a boxelder problem.
Do boxelder bugs only live on boxelder trees?
Mostly, but not exclusively. University of Kentucky Extension reports they have also been seen on silver maples, sycamore maples, apple, pear and plum trees. Wisconsin Horticulture notes they can be found on ash, maple and occasionally on strawberries, grasses and various other plants, but normally cannot complete their development on these other plants. Feeding somewhere and breeding there are different things.
Do boxelder bugs damage trees or houses?
Wisconsin Horticulture states they rarely cause significant damage to any plants, and that although they do not feed or reproduce inside the home, boxelder bugs may stain curtains and walls. They do not damage food or other items in the home. University of Kentucky Extension adds that they do not have the potential to become a serious indoor pest because they rely on fruit-producing trees to reproduce.
Should I cut down my boxelder tree?
It will help less than you would expect. Wisconsin Horticulture notes that removing boxelder trees or spraying heavily infested trees with a registered insecticide in mid-summer may help reduce numbers, but that the adults are good fliers and can move a considerable distance from host trees, so this will not eliminate the problem. It also suggests removing accumulations of boxelder, ash, elm and maple seeds, and choosing species other than boxelder when planting new shade trees.
How do I keep boxelder bugs out of the house?
Wisconsin Horticulture states that the best means of preventing problems is to prevent entry: caulk all openings around windows and doors and in walls, siding or foundations. They enter through small gaps around windows, doors and pipes and through small cracks in the foundation or siding, and they are attracted to buildings with a lot of southern exposure regardless of color.
What do I do about the ones already inside?
Wisconsin Horticulture says the best method of control once they are inside is to suck up the occasional bug with a vacuum cleaner, and that insecticide sprays are generally not recommended for boxelder bugs that have already made their way indoors. For clusters on the outside walls, it notes they can be sprayed with registered insecticides, insecticidal soap or hot soapy water, and that sprays may need to be repeated as more bugs arrive, especially when using soapy water.
Sources
- University of Kentucky Entomology (Blevins, Larson and DeVries). Boxelder Bugs (ENTFACT-444). Accessed August 21, 2026. https://entomology.ca.uky.edu/ef444
- Wisconsin Horticulture, University of Wisconsin–Madison (Susan Mahr). Boxelder Bug, Boisea trivittatus. Accessed August 21, 2026. https://hort.extension.wisc.edu/articles/boxelder-bug-boisea-trivittatus/