Adult boxelder bugs are 1/2 inch long and dark brown to black with distinctive red markings on their backs. Nymphs look similar to the adults, but lack of wings. Boxelder bugs feed on a variety of trees, preferring boxelder. They sometimes enter structures in the fall, looking for a place to overwinter. Boxelder bugs don not bite or sting; but they can become a nuisance, especially when they move inside in large numbers.