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How to identify it
- Adults are about 1/3 inch long with dark, rusty-red and black coloring and a red underside.[1][2]
- Behind the head, the back shows an upside-down black triangle set inside two rusty-red triangles.[1][2]
- Wings cross over the back; near the wing edges there are noticeable white dots mixed with rusty-red and black dots.[1][2]
- Nymphs are wingless and smaller, developing black wing pads and a lighter red abdomen with two black dots in the middle as they grow.[2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Boxelder bug | Slightly larger with distinctive orange-red striping, and it shows up in homes mainly in fall, while elm seed bugs peak in summer.[2][1] |
| Western conifer seed bug | Another smelly seasonal invader, but it shows up in buildings in late summer and autumn, while elm seed bugs arrive as early as mid-June.[2] |
Where it lives
Native to Europe and the Mediterranean; first found in the U.S. in Idaho in 2012 and since detected in Utah, Oregon, Washington, Colorado, New Mexico and Canada.[2][1]
Biology and life cycle
Adults overwinter in and around buildings, woodpiles and the landscape, then move to elms in early spring.[2][1]
Eggs are laid on elm flower cups as the trees bloom, and nymphs feed on elm seeds through five growth stages.[1][2]
There is one generation a year; new adults appear in June and stay active through fall.[1][2]
After elm seeds drop, nymphs and adults gather in large numbers on trunks, the ground and buildings.[1][2]
In very hot weather, adults crowd onto the north or shady sides of buildings; in fall they gather on sunny south sides before moving indoors.[2][1]
Damage and signs
- They feed on elm seeds and leaves with piercing-sucking mouthparts, but damage to the trees is negligible.[1][2]
- They don't damage buildings, but they leave unsightly fecal spots on walls and window frames.[2][1]
- Indoors, they crawl on walls, ceilings, window sills and furniture and may land on people, but they don't reproduce inside.[1][2]
Health risk
Elm seed bugs are not a health threat. Crushed or handled bugs release a foul odor from scent glands.[1][2]
Control
Keep them out
Seal windows and doors tightly with weather stripping, tight screens and door sweeps, paying special attention to the gaps where sliding windows meet.[1][2]
Caulk foundation cracks and gaps in soffits and around plumbing, gas and electrical lines entering the house.[1][2]
Sanitation
Clean up fallen elm seeds and debris around the house; Utah State also suggests removing volunteer elms nearby.[1]
Traps and physical control
Vacuum bugs found indoors rather than crushing them, then empty the vacuum or freeze or toss the bag.[1][2]
Pesticides
An outdoor pyrethroid perimeter treatment around foundations, windows, doors, vents and eaves may reduce entry, but it may not stop all bugs.[1][2]
Spraying or cutting down elm trees is unlikely to help where elms and bugs are both widespread, as on Colorado's Western Slope.[2]
Common questions
- Why do I have elm seed bugs in summer instead of fall?
- Unlike boxelder bugs, elm seed bugs are most active indoors from mid-June through August or September, and heat waves can drive them inside.[1][2]
- Do elm seed bugs bite or breed in the house?
- No. They may land and crawl on people, but they are not a health threat and don't reproduce indoors.[1][2]
Sources
- Utah State University Extension, “Elm Seed Bug.” extension.usu.edu/planthealth/research/elm-seed-bug (accessed 2026-10-06)
- Colorado State University Extension, “Elm Seed Bug: A Nuisance Pest in Colorado Homes (Fact Sheet 5.619).” extension.colostate.edu/resource/elm-seed-bug-a-nuisance-pest-in-colorado-homes/ (accessed 2026-10-06)
- iNaturalist, “Arocatus melanocephalus taxon page (order and family).” www.inaturalist.org/taxa/505749 (accessed 2026-10-06)