Note: As of late 2023, elm seed bug (ESB) has not yet been found in Wisconsin, but could show up in our area in the near future.
The elm seed bug (Arocatus melanocephalus | Hemiptera: Lygaeidae) is an invasive insect species native to parts of Europe. It was first detected in North America in 2012 in western Idaho and is now established in western parts of the continental US and Canada. In the eastern US, we haven’t seen much of this insect yet. It was technically spotted in the general Detroit area in Michigan in 2015 and reports continue to pop up in southeastern Michigan. This last summer, I helped confirm the presence of the elm seed bug (and the Asiatic garden beetle!) in the Twin Cities area for the Minnesota Department of Agriculture. Given the proximity to the Wisconsin state line, the elm seed bug could make an appearance in our state in the not-too-distant future.
Overall, elm seed bugs have habits similar to boxelder bugs and birch catkin bugs in that they feed on trees outdoors before invading structures later in the year. True to its common name, the elm seed bug is associated with elm trees and is especially fond of feeding on the seeds. They have little overall impact on the health of the trees. Later in the year, adult elm seed bugs seek out sheltered overwintering spots. In natural settings, they would typically overwinter beneath loose bark of trees or similar locations. However, they can readily invade homes and other structures in large numbers. Indoors, elm seed bugs are harmless to humans, but can be a general nuisance. Like brown marmorated stink bugs, elm seed bugs can also produce an unpleasant odor when crushed. Because ESBs are a nuisance invader like boxelder bugs and multicolored Asian lady beetles, management is going to be very similar and should focus mostly preventing these insects from getting indoors, e.g., physical exclusion and exterior crack and crevice treatments.
Since we have not yet seen elm seed bugs in Wisconsin, having this insect on our radar and being able to recognize it are the most important things at the moment. Elm seed bugs are similar in body shape to boxelder bugs, but are slightly smaller at roughly ¼ – ⅓ inch long. The adults are a dark brownish-black color with rusty-colored patches behind the head. There is a blackish, triangular structure (scutellum) on the middle of the back which sits within a rusty-colored, square-shaped patch. The edge of the abdomen is marked with a series of small, pale spots and the underside of the abdomen is a rusty, reddish color as well.
Wisconsin residents believing that they’ve found elm seed bugs are encouraged to collect a sample and contact me at the UW Insect Diagnostic Lab to definitively confirm the identity of the specimens: insectlab.russell.wisc.edu.