Boxelder bugs are common insects known for gathering in large groups. These “true bugs” are identified by their black bodies with distinctive reddish-orange markings and are about half an inch long as adults. Unlike social insects such as bees or ants, boxelder bugs do not construct nests. Instead, they aggregate in specific locations, a behavior driven by their feeding habits and a seasonal need for shelter.
Outdoor Aggregation Sites
During warmer months, boxelder bugs congregate outdoors, especially around certain trees. They are largely associated with boxelder trees, particularly the female, seed-bearing varieties, as well as maple and ash trees. These insects use their piercing-sucking mouthparts to feed on sap from leaves, tender twigs, and developing seeds of these host plants. While their feeding rarely causes serious damage to the trees, these locations are important for their development and reproduction.
Boxelder bugs lay their red eggs in the spring within bark crevices or on the leaves of these host trees. Nymphs, which are bright red and wingless, emerge within days and continue to feed on the trees throughout the summer until they mature into adults. Large numbers of these bugs can often be seen clustering on the bark, leaves, or around the base of these trees, especially on surfaces warmed by the sun. This clustering behavior helps them absorb heat, aided by aggregation pheromones they release.
Seasonal Movement Indoors
As temperatures drop in late summer and early fall, boxelder bugs migrate from their outdoor feeding grounds. This movement is driven by their need to find warm, sheltered locations to survive the colder months. They are sensitive to temperature changes and seek refuge from the impending cold, making human dwellings attractive overwintering sites.
Boxelder bugs are attracted to buildings with southern or western exposure, as these areas receive more sunlight and retain warmth. They gather in large numbers on the sunny exterior walls of homes, absorbing heat before attempting to gain entry. This mass gathering precedes their indoor presence, as they search for insulated cracks and crevices to hide in for the winter. While they do not seek food indoors, their primary motivation for entering homes is to find a protected environment where they can remain inactive until spring.
Preferred Indoor Overwintering Locations
Once inside a structure, boxelder bugs seek out secluded, protected areas to overwinter. These insects do not reproduce or feed indoors, but they remain largely inactive in these hidden spots. They can often be found in wall voids, attics, and crawl spaces, which provide insulation from cold outdoor temperatures.
Boxelder bugs commonly gain entry into homes through small openings in the building’s exterior. They exploit vulnerabilities such as unsealed areas around window and door frames, utility pipes, and foundations. Tears in screens or unsealed vents also serve as entry points. Once inside, they may also congregate around electrical outlets, switch plates, and other fixtures that emit warmth. During mild, sunny days in winter, boxelder bugs may become mobile and emerge from these hiding spots, moving towards windows or other sunny areas within the living space.