The elm seed bug (Arocatus melanocephalus) has limited distribution in Oregon, Washington, and Idaho. It has also been found in Utah. The elm seed bug is native to southcentral Europe. While we do not know how it arrived, there have been interceptions of the elm seed bug at eastern seaports on imported tile from Italy. Like most introduced insects, numbers are initially low, and the pest is rarely seen until numbers build over several years then it becomes epidemic.
Pest description and damage The elm seed bug is a small (0.33 inch) true bug with rusty red markings on the thorax, wings, and legs, an orange underbelly, and alternating dark and white marks along the edge of the abdomen. The nymphs are brighter and reddish with a black head, similar to the related boxelder bug. Adults and nymphs thrive on the abundant seeds of elms and other trees such as oak or linden. Since they feed on seeds in the trees, they are not a major landscape pest. However, they cause concern when adults and nymphs drop onto decks, congregate on building siding, and then enter homes in large numbers. This occurs in July and August as they seek relief from the heat, in the autumn when they are seeking winter shelter, and again in spring when they leave their sheltered overwintering sites. Both adults and nymphs emit an unpleasant odor.
Pest biology and life cycle The adults emerge from overwintering sites in spring, mate, and lay eggs. While there is only one generation per year, the adults have an extended egg-laying period and both young and adults are present throughout the summer and fall. Eggs hatch and young nymphs go through a series of molts. After each molt, their wing buds become more visible until the final molt when they have fully developed adult wings.
Pest monitoring Watch for adults on house siding, on seeds in the trees, on the ground, or on the siding of buildings during hot periods and in fall.
Management—cultural and physical control
Pest-proof your home including caulking of all cracks and crevices around siding, windows, doorways, faucets, electrical fixtures, etc. Remove adults and nymphs with a shop vac (remember that the vacuum can become odorous from the bugs) when they are congregating. If the problem persists, consider removing elms from your home landscape. Rake or vacuum elm seeds in the fall from siding, walkways, and areas where seed bugs congregate, such as firewood. Inspect boxes, pots, and firewood logs before bringing them inside.
Management—chemical control
See “True bug” in Table 1:
Chemical Control of Landscape Pests
For further information:
Ellis, J. 2013. Elm seed bug, Arocatus melanocephalus: an exotic invasive pest new to the U.S. Idaho Department of Agriculture. https://www.landcan.org/pdfs/Spring_2013_ESB_Fact_Sheet_update.pdf
Stokes, B. S., E. Bechinski, & P. Castrovillo. 2016. Managing Elm Seed Bugs around Your Home. University of Idaho Extension CIS 1223. https://www.uidaho.edu/-/media/UIdaho-Responsive/Files/Extension/publications/cis/cis1223.pdf