The name and large swarming tendencies of Elm Seed Bugs suggest it is dangerous, but the harmless insect is really just a seasonal nuisance.
First sighted in 2009 in counties in Idaho, the Elm Seed Bug is native to Europe and how it came to North America is unknown. It is a relative of Boxelder Bugs and Stink Bugs, and like them, produces an odor when crushed or smashed. What makes them so troublesome is their swarming nature. Hundreds of them congregate and have a tendency to find their way indoors. They do not harm people, or even the tree they are named after, but the infestation inside a home is naturally disturbing. Because adults do not die in winter, they tend to look for warmer places to ride out the cold weather. They also tend to look for cooler environments in the hot summer, so ridding a house or building of them may be a biannual chore.
Government agencies in Idaho and Utah suggest vacuuming (using a Shop-vac or other brand of workshop vacuum) any swarms that get indoors and establishing routine insecticide treatments around the perimeter of an infiltrated house or building. Treating window wells and door frames also helps deter Elm Seed Bugs from using them as entry points. Larval Elm Seed Bugs feed on the seeds of elm trees during the spring and early summer. They are therefore likely to be found in neighborhoods or regions where elms are present. Preventing an invasion is easier than removing one, so raking up and destroying fallen elm seeds, and inspecting firewood before bringing it indoors are recommended.
Our thanks to Fred and Jan T. in Idaho for their assistance in identifying this insect.
Scientific Name: Arocatus melanocephalus
Size (Adult; Length): 8mm to 13mm (0.31in to 0.51in)
Colors: brown, tan
Descriptors: stinky, smelly, swarm, horde, infestation, invasion, cluster, flying, triangle, tree pest
Note: An insect's reach is not limited by lines drawn on a map and therefore species may appear in areas, regions and/or states beyond those listed above as they are driven by environmental factors (such as climate change), available food supplies and mating patterns. Grayed-out selections indicate that the subject in question has not been reported in that particular territory. U.S. states and Canadian provinces / territories are clickable to their respective bug listings.