It is black with three red lines on the thorax, a red line along each side, and an oblique red line on each wing. The wings lie flat on the back when at rest. The young nymphs are red and gray. The population of bugs may number into the thousands. Boxelder bugs normally feed on the leaves, flowers, and seed pods of the boxelder tree or silver maple.
Large infestations are usually on the female, or pod-bearing, tree.
While they may feed on male boxelder trees and other trees and plants, they usually do not build up to such large numbers. The adults' search for a place to overwinter brings them into houses where they hide in small cracks and crevices in walls, door and window casings, attics, and around the foundation. During warm days in winter and early spring they come out and scatter through the house. They are primarily a nuisance as they crawl or fly about in the rooms. The adult bugs lay eggs in the spring and the nymphs emerge in a few days.