A black bug with red lines is almost certainly a boxelder bug. These flat, oval insects are about half an inch long with a black body and distinctive red stripes running along the thorax and wing edges. They’re one of the most commonly encountered household insects in North America, especially in fall when they gather in large, conspicuous clusters on sun-warmed walls and siding.
How to Identify a Boxelder Bug
Adult boxelder bugs are brownish-black with three lengthwise red stripes on the plate behind their head (the thorax) and red margins along the base of their wings. Flip one over or catch a glimpse when it flies, and you’ll see a bright red abdomen hidden beneath the wings. Adults have dark red eyes and grow to roughly half an inch. The younger bugs, called nymphs, look quite different: they’re mostly bright red, wingless, and smaller, though they darken as they mature.
The red lines are the key detail. They’re thin, clean stripes against a mostly black body, giving boxelder bugs a look that’s hard to confuse once you’ve seen it up close.
Bugs That Look Similar
A few other insects share the same red-and-black color scheme, so it helps to know the differences.
- Milkweed bugs are roughly the same size but flip the color ratio. Where boxelder bugs are mostly black with thin red lines, milkweed bugs are mostly orange-red with large black patches or belt-like bands. They feed on milkweed plants and are typically found in gardens rather than on house walls.
- Red-shouldered bugs have red lines along the sides of the thorax, similar to boxelder bugs, but lack the red lines on the forewings. They’re associated with goldenrain trees and soapberry plants rather than boxelder maples.
- Small milkweed bugs can be distinguished by a noticeable white spot on the exposed portion of the hindwings, a feature boxelder bugs don’t have.
- Two-lined spittlebugs are black with two orange or red lines across the wings and a red abdomen. They’re smaller (about a third of an inch), feed on grasses, and are more common in lawns and turf rather than clustering on buildings.
Why They Swarm Your House in Fall
Boxelder bugs spend summer feeding quietly on the seeds of boxelder maples, and sometimes ash or maple trees. You probably won’t notice them during the warm months. The problems start in autumn, when cooling temperatures drive them to seek sheltered spots to survive winter. Sun-facing exterior walls radiate heat, which attracts the bugs in enormous numbers. They pile into cracks around window frames, door seals, siding gaps, and foundation joints.
Once inside wall voids, attics, or other hidden spaces, they go mostly dormant for winter. But on mild, sunny days the warmth can reactivate them, and that’s when they start appearing indoors, often congregating near windows where sunlight heats the glass. This cycle repeats each spring when they emerge to head back outside.
Are They Harmful?
Boxelder bugs are nuisance pests, not dangerous ones. They don’t sting, don’t transmit disease, and generally don’t bite, though rare defensive bites have been reported. They won’t damage your home’s structure, eat your food, or reproduce indoors.
They do have two annoying habits. Crushing them releases an unpleasant odor, and their droppings can leave reddish-brown stains on light-colored surfaces like curtains, walls, and windowsills. If you have a large indoor population over winter, the staining can become noticeable.
How to Get Rid of Them
The most effective long-term approach is keeping them from getting inside in the first place. Seal or caulk small cracks and holes in siding, window frames, and door frames before fall arrives. Repair broken screens and weatherstripping, and make sure attic and foundation vents are properly screened. These entry points are how hundreds or thousands of bugs make their way into wall cavities each year.
For bugs already inside, a vacuum cleaner is the simplest solution. It removes them without the staining and odor you’d get from crushing them. A broom and dustpan work too. If you spot a cluster on an exterior wall before they’ve moved indoors, hot water between 165°F and 180°F applied directly to the group kills them on contact.
For larger outdoor infestations, a perimeter spray around the foundation can reduce the number that make it inside. Several insecticides are labeled for exterior foundation treatments against household-invading insects. Spraying infested trees in late summer, while the second generation of bugs is still immature and clustered on the branches, is one of the more effective chemical approaches.
Reducing Them Long-Term
Boxelder bugs depend on the seeds of female boxelder trees. If you have a boxelder maple on or near your property that consistently produces large bug populations, removing it (if it’s not a valued tree) can dramatically reduce the problem over time. Only female boxelder trees produce the seeds these bugs feed on, so male trees don’t attract them. If you’re planting boxelder maples, choosing male specimens avoids creating a food source altogether.
Properties without nearby boxelder trees can still see these bugs, since they fly reasonable distances, but the populations tend to be much smaller and more manageable with basic sealing and occasional vacuuming.