Why boxelder bugs cover south walls every September

What boxelder bugs are

Boxelder bugs are half-inch black insects with red markings along the edges of their wings. They don't bite people, don't breed indoors and don't damage the house. Their only real harm is numbers, and the stains they leave when crushed.

Outdoors they feed on seeds and plant juices, which is why homes near certain trees see so many more of them.

Why the south wall

In late summer, boxelder bugs start looking for somewhere to spend the winter. South and west walls soak up the afternoon sun, and the bugs cluster on them to stay warm.

Every gap around window trim, siding joints, soffits and attic vents on those walls becomes a doorway into the wall voids and attic where they'll sleep through the cold.

USU Extension describes boxelder bugs gathering in large numbers on south-facing, sun-heated walls in August and September before pushing indoors to overwinter. In Utah they feed on maple, ash, stone fruit, apple, grape and grass, and they're known for going after ripe peaches, which is why neighborhoods with mature maples and fruit trees see the heaviest waves. A single tree on the block can supply bugs to every house around it.

Why they turn up inside in winter and spring

Bugs that got into the walls don't stay put. On warm winter days and in early spring they wake up and wander toward the light, which puts them on sunny windows and in light fixtures long after the fall invasion.

That's the moment most people notice the problem, and it's also the hardest time to fix it.

What works

Timing is everything. The best results come from treating the sunny walls and sealing gaps before the bugs move inside.

• Book a wall treatment for mid to late August
• Repair torn window screens and weather stripping on sunny sides
• Seal gaps around trim, siding joints and attic vents
• Vacuum bugs indoors and empty the vacuum outside

What doesn't

Cutting down a host tree rarely helps, because boxelder bugs fly in from nearby trees. Crushing them leaves stains on light walls and fabric, and indoor foggers don't reach the wall voids where they're hiding.

Sources

• extension.usu.edu
• extension.usu.edu