A new nuisance pest has been added to our list in western Colorado, the elm seed bug. Native to Europe and the Mediterranean region, it was first detected in the U.S. in 2012. We picked it up in Mesa, Delta and Montrose counties in 2017.
Because its host plant, Siberian elm, is so widespread, this insect is destined to be a worse pest than even the boxelder bug. The boxelder bug's host plant is the boxelder tree. There are far fewer boxelder trees than the ubiquitous Siberian elm. And of course this bug feeds primarily on elm seeds although there are reports of finding immatures of these bugs on lindens and oak.
Unlike boxelder bugs, which appear in homes in the fall, elm seed bugs most often appear in homes from mid-June through September and may become active in homes during the winter if temperatures are favorable outside. Adults overwinter in structures, wood piles, and other protected sites where they avoid the winter elements. When warmer temperatures in the spring arrive, they emerge to feed on developing elm seeds.
Like boxelder bugs, elm seed bugs are a nuisance because of their propensity to enter houses. And to add insult to injury they can produce an unpleasant odor like bitter almonds when crushed.
It is the winged adult stage that usually enters houses. They are about 1/3rd inch long with dark, rusty-red and black coloration. Their underside is red. Next to the edges of the wings are noticeable white dots interspersed with rusty-red and black dots. A boxelder bug adult is longer than an elm seed bug. See comparative pictures of the elm seed bug and the boxelder bug.
Immature elm seed bugs develop on seeds outdoors, lack wings and are smaller than the adults. Eggs are laid when elm trees begin to flower with eggs deposited through June. There is only one generation a year but that single generation can be quite numerous and overwhelming for homeowners under attack. While large congregations of adult and immature elm seed bugs are often found on structures and plants, they cause no harm.
They can congregate in large numbers in soffits, around windows and under vinyl siding. When temperatures exceed 100° F. during the summer, adults may congregate on the northern side of buildings or in shaded areas attempting to escape the heat. They may also be seen in the fall congregating on the southern side of structures seeking radiant heat from the sun.
During the fall when the insects start to seek overwintering habitats they will often start invading homes. They may enter homes via cracks and crevices, but most commonly enter via windows and doorways that have poor seals and no door sweeps. Upon entering the home they may be seen around windows sills, walls, ceilings, furniture and behind wall hangings. Excluding these bugs from the home can be done by sealing windows and doors with weather stripping while insuring screens and door sweeps are in good condition. Ensure the caulking around windows and door frames are in good condition with no cracks. Gaps in soffits, gas, electrical and plumbing lines leading into the house should be sealed with caulk.
Once they enter the home they can be vacuumed up and disposed of outside. A perimeter barrier of insecticide around window and door entryways may be the best way to deter bugs from entering into the home. Insecticide products containing one of the registered pyrethroids is recommended for outdoor spraying. But spraying or cutting down elm trees to reduce elm seed bug populations is ineffective because the elm trees they feed on are everywhere and the bugs are in high populations and highly mobile in our area.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.