Elm Seed Bug

What it does

Elm seed bugs feed on developing elm seeds through spring and early summer, then move to buildings as it heats up, entering through gaps around windows, doors, eaves, and vents. They cluster on and inside walls, especially sunny sides, and may become active indoors on warm winter days. Crushed or disturbed, they give off a bitter, almond-like odor.

Where it lives

The elm seed bug lives wherever elm trees grow, especially Siberian elm, in yards, parks, and streets. First detected in Idaho in 2012, it has spread through much of the interior West and Pacific Northwest, including Utah, Oregon, Washington, Colorado, and New Mexico. It overwinters as an adult in sheltered spots, including inside homes.

How it breeds

There is generally one generation a year. Adults overwinter, then mate in spring and lay eggs near elm seeds. Nymphs, reddish and dark, feed on the seeds from late spring into summer and mature into adults over the season. The new adults are the ones that push into homes in the summer heat seeking shelter.

What it eats

Elm seed bugs feed mainly on the seeds of elm trees, particularly Siberian elm, using piercing-sucking mouthparts. They cause no meaningful damage to the trees and do not feed on structures, fabric, or stored food. Indoors they are simply seeking shelter, not a food source.

Identification

Adults are about a third of an inch, dark brownish-black with rusty-red patches and a dark triangular scutellum set in a rust-colored, squared patch on the back. They resemble boxelder bugs but are smaller and lack the bold red lines. Mass appearance at windows in summer and the bitter odor when crushed help confirm them.

Signs of an infestation

• Dark, rusty-marked bugs clustering on and around windows in summer
• Bugs entering through gaps at doors, eaves, and vents
• A bitter, almond-like smell when they are crushed
• Live bugs indoors on warm days, even in winter
• Large numbers on sunny exterior walls

Prevention tips

• Seal cracks and gaps around windows, doors, eaves, pipes, and vents
• Install or repair door sweeps and window screens
• Vacuum bugs indoors and empty the canister, avoiding crushing them
• Reduce Siberian elm near the home where feasible
• Apply an exterior perimeter insecticide to entry points in early summer if numbers are high

Professional solutions

Elm seed bugs are a nuisance invader, not a damaging pest, so exclusion and vacuuming are the mainstays. A pest control professional can apply an exterior perimeter treatment around entry points and gathering areas in early summer to reduce how many get inside. Indoor spraying is generally discouraged; sealing gaps gives longer-lasting relief.