Boxelder bugs are one of the many pests that come in on a potted plant, or through a crack, an open door, a hole in a screen, or a piece of firewood. Boxelder bug nymphs and adults feed on trees, but many only notice them when they congregate on houses in the fall.
Adults are ½ inch (`13mm) long, narrow black bugs with red markings. The bottom side of the abdomen (body) is red with some black markings. Nymphs (immature life stage) are much brighter and redder with black markings. Boxelder bugs are ‘true bugs’, often confused with squash bugs and milkweed bugs so look for the three red lines on the pronotum (between the head and the body, on the topside of the bug).
Feeding may cause damage to fruit, but these bugs do little damage to ornamental plants. Like stink bugs and leaf-footed bugs, they can be annoying winter guests inside buildings. Boxelder bug excrement can cause spotting on fabrics and painted walls, and the bugs emit a strong scent when crushed. Most die indoors.
Somewhere near your home, boxelder bugs are finding food— preferably a female box elder tree, but occasionally the male tree. Other choices are maple and ash trees, grapes, or fruit from apple and Prunus spp., (cherry, plum, peach, etc). In spring, overwintering adult females lay yellow eggs in crevices of box elder bark. Eggs turn red before the nymphs hatch. Nymphs develop quickly into adults and there is a second generation that summer, so there are often both adults and nymphs found in large migrating groups.
In early autumn, adults getting ready for winter gather to find suitable shelter, become a nuisance on your house or by finding a way inside gaps in walls and foundations. On warm winter days they may again become active inside the building.
Because they are mostly a nuisance pest, exclusion is your best bet. Prevent insects from entering gaps in siding, window and door sills, and in soffits or the foundation (it is important to note that screening or mesh fibers may be needed for these openings to prevent insect entry but allow air or water to move through. Sealing soffits or weep holes in ways that prevent airflow can lead to moisture problems). Repair tears in screens. Outdoors, eliminate hiding places and debris, especially around foundations and consider removing female box elder trees (Acer negundo). If a large group has gathered on the side of your house, use a strong stream of water from your hose to wash them off. Use of an insecticide is generally ineffective. Once boxelder bugs are indoors, vacuum or hand-collect.
Boxelder Bug - Cornell University Insect Diagnostic Laboratory