Five facts: Elm seed bugs and their relatives

Red and black seed bug, one of many seed bugs found in western US. (Image: Whitney Cranshaw, Colorado State University, Bugwood.org)

Article by, Nicole Sanchez,
Horticulture, OSU Extension Service, Klamath County

Warmer weather brings emergence of all the insects that have overwintered in our gardens, yards, and homes. Insects can overwinter in a variety of ways, all dependent on the species and available resources: insects may spend the winter as eggs, pupae, or adults. Overwintering in the larval stage is less common.

Recent submissions to the Klamath Extension office have included several samples of Elm seed bugs: smaller, duller relatives of the Boxelder bugs so commonly submitted in fall. Like Boxelder bugs, Elm seed bugs cause alarm due to their numbers and appearance in and around the home. Also like Boxelder bugs, Elm seed bugs have an unpleasant odor when alarmed or crushed.

It’s easy to understand that numerous, smelly bugs showing up on the side of the house, and sometimes inside the house, would cause concern, but it’s usually not necessary to use pesticides for Elm seed bug.  Unlike termites or roaches, Elm seed bugs pose no real threat to structures or the people inside them. A closer look at the life cycle and habits of the Elm seed bugs and their relatives reveals how this can be.

Diet of seeds. As their name implies, Elm seed bugs live entirely off the seeds of elms. They are attracted to Siberian elm, a common landscape tree in the Klamath Basin. Elms produce large numbers of seeds- a mature tree can be host to thousands of Elm seed bugs. Elm seed bugs are members of the Lygaeidae family, a group of “true bugs” with piercing sucking mouthparts that pierce developing seeds and extract juices, much like a straw. Lygaeidae is a large insect family with numerous members named for their food plant associations: Elm seed bug, Milkweed bug, Pine seed bug. Other Lygaeids feed on multiple kinds of plant seeds, and just a few members, such as the Big-eyed bug, are predators.

No harm to tree. Elm seed bug feeds only on the seeds, no damage is done to the tree itself. Adults lay eggs in the flowers: upon hatch the insects develop along with the seeds. In spring, tiny Elm seed bugs are already in the seed clusters, feeding, but are not usually noticed until the seed clusters fall to the ground. By this time, the bugs are larger and more mobile. They are often seen sunning on rock walls or pavement in mid-summer. It could be argued that these bugs provide a service: their feeding ensures that thousands of tiny elm seedlings don’t spring up in our yards.

End of the road. Adults overwinter in a state of diapause, the insect equivalent of hibernation. Upon spring emergence, they mate and lay eggs in elm tree flower clusters. Thus their life cycle ends with perpetuation of the species. If treatment is desired- again, they cause no real damage to the tree- there is a better way than applying pesticide sprays into the canopy. Timing would need to coincide with the new generation hatching: overwintered adults were going to die anyway, and most pesticides don’t penetrate eggs well. It is difficult to attain good coverage in large, established trees, and best practices with pesticides are to not spray trees in flower- even in wind pollinated trees such as elm and birch. 

Vacuuming – most effective. A better way to disrupt the Elm seed bug cycle is to dispose of them before they have a chance to mate and lay eggs in elm trees, where their offspring are difficult to manage. For this, the vacuum is the best defense. Like other insects that overwinter in and around homes, Elm seed bugs hide in woodpiles, in the space between siding and the home exterior, in rock wall crevices, and under mulch around the perimeter of the home. Vacuum individuals found in the home, apply the shop vac to clusters of the insects found outdoors around the house. This disrupts the life cycle by leaving fewer adults to perpetuate the colony.          

Perimeter defense.  Caulking, sealing around windows and doors, raking elm leaves away from the house, and inspecting firewood are all non- pesticide measures that can be taken to reduce the nuisance created by Elm seed bug. Some pyrethroid pesticides, listed for outdoor use, could be applied around doors, siding seams, and windows as well. More information on Elm seed bug is available in the Pacific Northwest pest management guide:  https://pnwhandbooks.org/insect/emerging-insect-pests/previous-eradication-programs/elm-seed-bug-2009.

Elm seed bugs can be annoying, but don’t cause any real damage to trees or homes. It’s up to the individual to determine whether treatment is warranted. Targeted treatments to entry points into the home, along with vacuuming, seem to be more effective than pesticides applied into the tree canopy.