Boxelder Bug

Boisea trivittata — the boxelder bug — is one of the most-encountered nuisance invertebrates in the northern US and southern Canada autumn. Adults are 11-14 mm long with the bright red-and-black warning coloration shared with milkweed bugs and other Müllerian mimics in the warning-color community. The species feeds exclusively on the female boxelder maple tree (Acer negundo) and a few other Acer species, consuming seeds and developing leaves with stylet mouthparts. The species' most consequential biology is the autumn aggregation behavior. As temperatures drop in September-October, adults migrate from feeding sites to overwintering habitat — typically the warmest, most sun-exposed surfaces available. South-facing walls of buildings, tree trunks, fence rails, and rocky outcroppings can host aggregations of thousands to millions of individuals warming up in the afternoon sun. Many of these aggregations migrate INTO buildings through cracks, vents, and gaps around windows, where the bugs spend the winter in attics, wall voids, and basements. They do not feed, breed, or damage materials indoors but the sheer numbers (and the staining liquid they release when crushed) make them a major nuisance pest. The species emerges in spring and disperses back to boxelder hosts. Boxelder bugs are completely harmless to humans, pets, and structural materials.