When full grown, the boxelder bug is about 1/2 inch long and one-third as wide. Adults are mostly black and have three red lines on the pronotum of the thorax (one down the middle and on each margin) and several fine red lines on each wing. The wings lie flat on the bug’s back when it is at rest. The abdomen is red. The young nymphs are bright red and when approaching adulthood, become marked with black and begin to develop black wing pads. Eggs are yellow when first laid but become red as nymphs develop inside.
Boxelder bugs are true bugs (Order: Hemiptera) in the family Rhopalidae. They are sometimes confused with other true bugs including squash bugs (Anasa spp., family Coreidae); the bordered plant bug (Largus cinctus, family Largidae); small milkweed bug (Lygaeus kalmii, family Lygaeidae); leaffooted bugs (Leptoglossus spp., family Coreidae); and the red-shoulder bug (Jadera haematoloma, family Rhopalidae). The three lengthwise red lines on the pronotum distinguish boxelder bugs from these other species.
In spring, the overwintered female bugs lay eggs in the cracks and crevices of the box elder tree’s bark, and the nymphs hatch in a few days. The nymphs grow larger and develop into adults during the summer. They then mate and lay eggs that hatch into the nymphs of a second generation. Adults and nymphs periodically migrate in large groups, often covering tree trunks, the ground, fences, and the sides of houses.
Most of the nymphs of the second generation grow to full size in August and September. In fall, the adults seek overwintering places in dry, sheltered hollow tree trunks; under rocks, boards and other debris; in cracks and crevices in walls; in door and window casings; around building foundations; and inside houses. They become a nuisance outdoors on patios or indoors when they invade in significant numbers. On warm days during winter and early spring, boxelder bugs sometimes appear on light painted surfaces outdoors on the south and west sides of the house, resting in the sun.
The bugs do little damage to ornamental trees. They may occasionally cause puckering or distortion of fruit in commercial orchards such as pear or kiwi, but this is rarely a significant problem. They do not injure people or pets; but when they come indoors, they can be annoying and may spot curtains, furnishings, and clothing with their excrement. When crushed, they give off an offensive odor. They do not breed indoors. If trapped in basements or houses, they will eventually die.
Boxelder bugs do not cause significant damage to landscape plants, and management need only be directed at keeping them out of homes, where they may become a nuisance. If boxelder bugs frequently invade homes, seal up entry points such as cracks and screen windows and doors.
Sanitation practices such as vacuuming can be used to reduce population numbers, but vigilance may be required during fall migration. Outdoors, eliminate hiding places and debris and consider removing female box elder trees. Insecticide use is rarely justified.
Since the box elder tree is the main source of food for the boxelder bug, removing the trees, especially the female or pod-bearing trees, is the most effective way of controlling the pest. Be careful not to plant or allow the establishment of new box elder trees in your yard.
However, elimination of trees on your property won’t always completely resolve the problem. Winged adults can fly for distances of several blocks, so boxelder bugs may also migrate in from a neighbor’s tree.
If removal of trees is not an option, clear fallen seeds from beneath and near trees. Use a broom or shop vacuum on hard surfaces such as patios and driveways. A shop vacuum will also remove most seeds from grassy areas.
Repair torn screens and close up places where the bugs can enter the house, such as cracks around doors and windows and attic or basement vents. Use caulk, weatherstripping, fine-mesh screen, steel wool, or expandable foam as appropriate. Boxelder bugs that enter the home may be controlled by hand-collecting or vacuuming. Repeat as needed.
Eliminate hiding places such as piles of rocks, boards, leaves, and general debris close to houses. Boxelder bugs hide during the day or overwinter in these sites. Rake leaves and remove weeds and grass from a 6- to 10-feet wide strip around the foundation, particularly on the south and west sides of the house. Keep box elder tree seeds swept up. A weed and debris-free area tends to reduce the congregation of bugs near the foundation.
Boxelder bugs are susceptible to drowning, so wash boxelder bugs off walls or tree trunks with a forceful stream of water.
Insecticide sprays are generally not recommended for boxelder bug management. They are often no more effective than vacuuming and hosing, and repeated applications may be required. Insecticidal soap applied in a forceful spray of water may reduce populations on tree trunks. Pyrethroid insecticides are available for treating foundation walls around the perimeter of buildings. If required, these applications are best done by a professional. Special care must be taken to avoid runoff of pesticides from walls and foundations into storm drains, because they lead directly into creeks and rivers. Do not use sprays for boxelder bugs inside the house.