Boxelder Bugs in Missouri: Identification and Control

Boxelder bugs are one of central Missouri's most recognizable fall nuisance pests — the flat, red-and-black insects that congregate by the hundreds on sunny walls, tree trunks, and foundation surfaces each autumn. While harmless, they can enter homes in significant numbers and their aggregation behavior draws attention in a way that other pests do not. Here is what you need to know about managing them.

Identifying Boxelder Bugs

Boxelder bugs (Boisea trivittata) are hard to mistake once you have seen them:

• Flat, elongated oval body, approximately 12–14mm in adults
• Black body with distinctive red or orange markings — red lines along the edges of the thorax and the wing margins, with red veining on the wings
• Nymphs are smaller and predominantly red with black wing buds developing as they mature

They do not bite, do not sting, do not transmit disease, and do not damage structures. Their primary defense is an unpleasant odor when disturbed or crushed, and they can leave faint staining on light surfaces from their excrement — an irritation rather than a serious concern.

Why They Aggregate on Your Home in Fall

Like stink bugs, boxelder bugs overwinter as adults. As temperatures drop in fall, they seek sunny, warm surfaces where they can absorb heat — south and west-facing walls, tree trunks, and foundation surfaces receive afternoon sun and warm the bugs for pre-winter aggregation. They then seek cracks and gaps in these warm surfaces to move inside for winter dormancy.

The degree of boxelder bug pressure on a property is directly related to the availability of their host plants nearby. Boxelder bugs feed primarily on the seeds of boxelder trees (Acer negundo) and, to a lesser extent, silver maple and ash trees. Properties with mature female boxelder trees (which produce seeds) will consistently have higher boxelder bug populations than properties without. Male boxelder trees do not produce seeds and do not sustain boxelder bug populations the same way.

The Host Tree Connection

If boxelder bugs are a persistent, severe problem on your property, identifying and potentially removing female boxelder trees near the home is the most effective long-term reduction strategy. This is a significant step, but for properties with very heavy and consistent pressure, removing the host tree produces a more dramatic reduction than any amount of insecticide treatment.

If removal is not practical, reducing boxelder seed production — through professional pruning that removes seed-bearing branches — can reduce the food source that sustains large local populations.

Exclusion and Treatment

The same exclusion steps that reduce stink bug entry apply to boxelder bugs — seal gaps in the building envelope before fall. For the exterior aggregations on sunny walls:

• Direct contact spray with soapy water kills aggregating bugs on exterior surfaces without residual chemical — effective for immediate knockdown of groups on walls or walks, though it does not prevent new bugs from arriving
• Professional residual treatment on south and west-facing walls in mid-September provides lasting knockdown of aggregating bugs before they find entry points
• Removing or reducing boxelder seeds and debris from near the foundation removes the congregation point adjacent to the structure

Managing Boxelder Bugs Already Inside

Boxelder bugs that have entered for winter are dormant in wall voids, attic spaces, and insulated areas. They emerge when warm indoor temperatures or a warm sunny day stimulates activity. Manage interior bugs the same way as stink bugs: capture without crushing, vacuum with immediate outdoor disposal, and reduce interior hiding spots by sealing gaps around light fixtures, electrical outlets, and baseboards. Spring emergence — when overwintered bugs become active and head back outside — is brief and self-resolving.