The appearance of dozens of black and red insects clustering on a home’s exterior can be alarming to a homeowner. These insects are box elder bugs (Boisea trivittata), and their presence often raises the question of whether they are destructive pests or merely an unsightly nuisance. While their sheer numbers can be unsettling, the box elder bug is overwhelmingly considered a nuisance pest rather than a threat to property or health. The primary issue they pose stems from their seasonal behavior of seeking shelter from the impending cold weather.
Identification and Seasonal Aggregation
Adult box elder bugs have a black body about a half-inch long, highlighted by reddish-orange lines along the edges of their forewings and the thorax behind the head. They also have red eyes and reddish-orange legs. Nymphs, or young bugs, are smaller and appear mostly bright red with black markings that develop as they mature.
Their presence becomes noticeable in late summer and early fall as they begin aggregation, gathering in large groups. These clusters form primarily on sun-exposed, south-facing or west-facing walls of buildings, where they absorb warmth. This behavior is preparation for overwintering (diapause), as the bugs seek protected, dry spaces to survive the cold months. The bugs will crawl into small cracks, crevices, and voids in the structure, making their way inside the home to find shelter.
Assessing Damage to Property and Health
Box elder bugs are not considered a threat to a home’s structure or stored goods because they are not wood-destroying organisms. They do not bore into wood, chew fabric, or feed on pantry items. Their piercing-sucking mouthparts are designed for feeding on plant material, specifically the seeds of female box elder trees, maples, and sometimes ash.
The minor damage they cause is limited to the occasional puckering or discoloration of fruit, like apples or plums, and rarely results in widespread harm to mature trees. The main property issue they present is the potential for staining light-colored surfaces, such as walls, curtains, or upholstery, with their reddish-brown feces. Staining is more likely when large numbers congregate indoors or if they are crushed. Crushing the insects also releases an unpleasant odor, which is part of their natural defense mechanism.
Concerns about health risks are unfounded, as box elder bugs do not transmit human diseases. They also do not possess stingers and are not venomous. While they are not known to bite aggressively, their mouthparts may prick human skin defensively if they are handled roughly. They are considered a harmless nuisance, with the biggest distress coming from their sheer numbers and unsightly presence.
Exclusion and Non-Chemical Management
The most effective strategy for managing box elder bugs is exclusion, which focuses on preventing their entry into the home. Homeowners should inspect the exterior for any small access points the bugs might use to overwinter indoors. Sealing cracks, holes, and crevices around windows, door frames, and the foundation with caulk or weather stripping is recommended.
Repairing damaged window and door screens and ensuring that vents, such as those for the attic or dryer, are properly screened also eliminates common access routes. Once the bugs have entered the home, the preferred method of removal is physical collection, such as vacuuming them up. The vacuum bag should be promptly disposed of or sealed and frozen to prevent the bugs from escaping and releasing their odor.
For outdoor aggregations on the side of a building, a simple non-chemical solution is a direct spray of water mixed with liquid dish soap. The soap breaks down the bugs’ outer layer, causing dehydration and death upon contact. Raking up leaf litter and removing wood piles near the foundation can also reduce potential overwintering sites.