Boxelder bug, Boisea trivattata
The boxelder bug is a common household pest in Colorado. They can feed on a variety of plants, and boxelder seed pods are the preferred food source. These insects are often found sunbathing on warm, south-facing structures in large numbers near female boxelder trees.
Adult boxelder bugs are about 1.3 cm (0.5 inch) in length. They have black bodies with a single red line on each side, a diagonal red line on each wing, and three red lines on the thorax. Boxelder bugs are fluid-feeders and have piercing-sucking mouthparts. Nymphs are noticeably smaller and lack fully formed wings. Young nymphs also have a bright red abdomen, which differentiates them from adults. The eggs of boxelder bugs are small, red, and ovular eggs.
Two insects in Colorado are likely to be misidentified as boxelder bugs: the small milkweed bug (Lygaeus kalmii) and Jadera haematoloma. Unlike the boxelder bug, the small milkweed bug has a notable white spot on the exposed portion of the hindwings. Jadera haematoloma has similar red lines along the sides of the thorax but lacks red lines on the forewings.
- Boxelder bugs are long black insects with characteristic red lines on the forewings and thorax. In Colorado, two common insects that resemble boxelder bugs are the small milkweed bug and Jadera haematoloma.
- Boxelder bugs are considered a nuisance pest since they rarely bite and do not cause damage to homes.
- It is easiest to prevent boxelder bug infestations by sealing up cracks in doors and windows before late summer. Individuals that do migrate indoors can be vacuumed up.
Adult boxelder bug. Note the diagonal red lines running along the edge of the forewings and the three red lines on the thorax. Boxelder bugs are a common household nuisance during winter and can often be seen aggregating on south-facing structures late in the summer. This insect is most likely to be problematic in areas with female boxelder trees. Effective management relies on sealing potential entries to indoor structures and removing vegetation on the south and west sides of structures. Image credit: Joseph Berger, Bugwood.org
Early boxelder bug nymph. Note the small developing wings and red abdomen. Image Credit: Jim Baker, North Carolina State University, Bugwood.org.
Adult of the small milkweed bug. Note the patch of white coloration on the back, which is not present on boxelder bugs. Image credit: Bruce Watt, University of Maine, Bugwood.org
Adult of Jadera haematoloma. Note the absent red lines on the forewing, which distinguishes this species from boxelder bugs. Image credit: Jim Baker, North Carolina State University, Bugwood.org
Life history and habits
Boxelder bugs undergo incomplete metamorphosis and have three life stages: egg, nymph, and adult. Two generations are produced each year. As temperatures begin to drop in the fall, adult boxelder bugs of the second generation will often seek overwintering shelter. In the spring, boxelder bugs will emerge after overwintering and begin laying eggs in bark or on leaves of the host plant. Upon hatching several days later, the emergent nymphs can be found on vegetation near boxelder trees until seeds develop, at which point nymphs begin feeding.
In August and September, adult boxelder bugs will begin to migrate indoors for overwintering shelter. They tend to prefer dry, tight, sheltered areas, and can often be found in window panes, attics, and doors. They can also be found in spaces outdoors, such as ditches and under debris. On warm winter days, they may briefly emerge, only to return to their overwintering site at the end of the day.
Boxelder bugs are a nuisance pest since they rarely bite and do not damage structures in the house. However, they can stain fabric and may also emit an unpleasant odor when crushed. The best way to prevent boxelder bugs from entering the house is to seal up possible entryways before they search for overwintering sites. During the summer, regularly inspect sites where boxelder bugs aggregate and seal any possible entryways. This may include caulking up windows or other possible openings in the structure, screening off vents, and sealing off small holes near plumbing or gas structures. In addition, removing plant debris such as fallen leaves from the sides of south- and west-facing structures can help reduce the density of boxelder bugs. When found indoors, it is recommended to use a vacuum to remove the insects.
In extreme instances, host trees, including boxelders and various maple species, can be cut down to remove the bugs from the local ecosystem, though this is often impractical and not typically advised.
Chemical management is not advised since boxelder bugs are nuisance pests and there are effective cultural practices that can be used to prevent and manage their infestations.
Utah State University Cooperative Extension. (2010). Boxelder Bug. Utah State University – Extension. Available https://extension.usu.edu/pests/schoolipm/structural-pest-id-guide/boxelder-bug
University of Missouri Extension. (2022). Boxelder Bug. University of Missouri – Extension. Available https://extension.missouri.edu/publications/g7360