Elm seed bugs: Utah's mid-summer house invader

How to tell them from boxelder bugs

Both are red and black and both gather on sunny walls, which is why people mix them up. Elm seed bugs are smaller, about a third of an inch, with a red and black pattern along the wing edges and a red underside.

• Timing: elm seed bugs peak in mid-summer; boxelder bugs in fall
• Smell: elm seed bugs give off a bitter-almond odor when crushed
• Food: elm seeds for elm seed bugs; maple and box elder seeds for boxelder bugs

Why they come inside in July

Elm seed bugs move indoors during the hottest part of summer, looking for shade and cooler spaces, then again as they look for somewhere to spend the winter. Homes near Siberian elms, where seeds pile up in spring, get the most.

Elm seed bugs were first confirmed in Utah in July 2014 from Salt Lake County and are now widely distributed along the Wasatch Front and in Cache County, reported as far as Tooele County. USU Extension says they emerge in March, lay eggs in early May and invade homes from late May through late September, and that unlike boxelder bugs their activity peaks in mid-summer. The same source calls pest-proofing buildings the best control.

What doesn't work

Because they're so mobile, spraying bugs you can see only helps a little. Crushing them stains light walls and fabric and releases the smell. Cutting down one elm rarely helps when the neighborhood is full of them.

What does work

Pest-proofing the house is the most effective control.

• Tight window screens and door sweeps with no gaps
• Seal cracks around windows, siding, vents and eaves
• A residual treatment along the foundation, sills, doors and eave undersides
• Vacuum bugs indoors, or knock them into soapy water outside

When to act

The best time is late spring, before the invasion starts in late May. By July you're dealing with bugs that are already on the walls and in the gaps.

Sources

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