The box elder bug (Boisea trivittata) is a common, half-inch insect recognizable by its black body and three distinct orange-red stripes on its thorax. Classified as a “true bug,” it possesses piercing-sucking mouthparts and is considered a nuisance pest. They become most noticeable in the fall when they gather in large numbers and attempt to enter buildings. Although they do not sting or transmit disease, they can stain surfaces with excrement and release an unpleasant odor if crushed, prompting people to understand what attracts them.
Essential Plant Hosts (Food & Breeding)
The primary attraction for the box elder bug is the presence of its host plants, which provide the necessary food source for its life cycle. The insect is named for its strong preference for the female, seed-bearing box elder tree (Acer negundo). They use their specialized mouthparts to extract sugary sap and fluids from developing seeds (samaras) and new tender growth.
The female box elder tree is particularly attractive because its seeds are the most nutritious food source for reproduction. Box elder bugs also feed on other maple species, such as silver and red maples, and occasionally on ash trees. These secondary hosts are utilized when populations are high or when preferred box elder seeds are unavailable. The bugs lay their rust-red eggs in crevices on the host tree’s bark, ensuring newly hatched nymphs have immediate access to food.
Seeking Winter Shelter (Structural Attractants)
As the weather cools in late summer and early fall, the box elder bug shifts its focus from feeding to searching for a suitable overwintering site. This seasonal migration is driven by the need to find a warm, dry, and protected location to enter dormancy, known as diapause. This state ensures their survival through the winter, and only fully matured adults seek shelter en masse.
Their attraction to human-built structures centers on vulnerabilities that offer access to insulated, sheltered spaces. The insects probe for any small gap, including cracks in the foundation, poorly sealed window and door casings, and utility line openings. They often gain entry through damaged window screens, gaps beneath door sweeps, or vents leading to attics or wall voids. Once inside, they remain inactive until warmer temperatures prompt them to emerge, sometimes appearing inside the living space during mild winter days.
Environmental Cues That Encourage Clustering
Even upon arriving at a structure, box elder bugs do not disperse evenly; they are attracted to specific environmental cues that encourage large clusters. The insects are strongly drawn to warmth and sunlight, often congregating on exterior walls that receive the longest exposure to the afternoon sun. This preference means that west-facing and south-facing walls are often the primary gathering points.
The bugs bask in these warm microclimates to raise their body temperature before finding an entry point. Darker or heat-absorbing surfaces, such as dark-colored siding or concrete, attract larger numbers because they radiate more warmth.
The mass clustering behavior is reinforced by chemical signals released by the bugs. The presence of a large group signals a suitable location to other passing individuals, leading to a rapid buildup of the population in a localized area.