The elm seed bug hasn’t lived in La Plata County for long, but it didn’t struggle to gain a foothold in the region. It is harmless to humans, but it is an invasive species that preys on the seeds of another: the Siberian elm tree.
The elm seed bug, or Arocatus melanocephalus, is considered a nuisance pest, according to the Colorado State University Extension Office. An adult elm seed bug can squeeze its 0.8-centimeter body through cracks and crevices to enter a home or office, which the insects do in droves from mid-June through September to escape the heat and cold. They also produce a bitter almond scent from glands on their abdomens.
The bugs are red and black with upside-down triangular patterns on their backs. They are mobile insects with wings.
“People hate them,” said Payden Bell, owner of Rocky Mountain Bird and Pest Solutions based in Bayfield.
He said he’s seen examples where elm seed bugs haven’t just sneaked through a customer’s window – they’ve shown up in people’s food and drinks as well. Pesticide treatments with permethrin can be applied around doors, windows and housing exteriors, but the treatments must be consistent to keep the elm seed bugs at bay.
Without removing the Siberian elm tree population, elm seed bugs will never be completely culled, he said. People can try to seal the bugs out of their homes, but if windows or doors are being left cracked open in the summer, the bugs will get in eventually.
“They’re extremely persistent and seem to breed pretty quickly, or at least in a lot of numbers,” he said.
Bell said elm seed bugs are similar to boxelder bugs, which are no strangers to La Plata County. Boxelders can typically be found in greater numbers later into the fall and treatment is mostly the same for both species.
“The best thing to do with pests, one thing I can’t stress enough, is it’s never really a one-and-done treatment,” he said. “They are pests, they are going to come back. So having scheduled maintenance is key to reducing the population numbers.”
Bell said Rocky Mountain Bird and Pest Solutions operates in Bayfield, Durango, Ignacio, Pagosa Springs, Silverton and as far as Hinsdale County.
Darrin Parmenter, La Plata County CSU Extension director, suggested a bucket of soapy water and a vacuum if one prefers to not use pesticides in their home or office.
Simply suck the bugs up with the vacuum, dump them out of the vacuum container into the soapy water and let them soak. They will eventually die, he said.
The best solution to a surplus of elm seed bugs, though, may not be the most practical, he said. But if one is desperate enough, removing a Siberian elm near one’s home will likely get rid of the pests. But that comes with downsides such as expenses and the loss of shade.
Elm seed bugs are native to Europe and the Mediterranean and were first documented stateside in Idaho in 2012. In 2017, the species was discovered in Colorado in Mesa, Delta and Montrose counties. By 2019 or 2020, the bugs had made their way to La Plata County in what Parmenter called an “impressive” migration.
“Watching this elm seed bug move over in less than a couple years, that’s pretty impressive,” he said. “Typical migratory patterns would probably dictate that would be a pretty tough migration south.”
Parmenter said he isn’t sure how the elm seed bug reached La Plata County. But Siberian elm trees that produce the seeds the bug eats are a weedy species that is cut for firewood and shipped around. If the bug caught a ride on chopped Siberian elm wood, that would explain how it reached La Plata County so quickly.
Matt Besecker, arborist for the city of Durango, said elm seed bugs remind him of the emerald ash borer that is native to northeastern Asia and was brought overseas to Michigan.
“They were actually embedded inside these pallets that came from Asia,” he said. “Those insects didn’t ‘know’ that they were going to be able to survive in that area. But they found those ash trees right away. It’s kind of the same thing that’s going on with these elm seed bugs.”
He said invasive bugs and beetles will fly around until they can find “something to chew on.”
Parmenter also noted a resemblance between the emerald ash borer and the elm seed bug.
“There is huge fear of this insect (emerald ash borer), as there should be, because it definitely decimates ash trees,” he said.
The emerald ash borer has been recorded in the Denver Metro area, Boulder and Larimer County, but it hasn’t reached La Plata County. And while the emerald ash borer poses a serious threat to ash trees, the elm seed bug doesn’t appear to do any real damage to Siberian elms – to Parmenter’s disappointment.
“To be honest, probably a lot of us wish it would (kill elm trees),” he said.
The Siberian elm tree is an aggressive and hardy invasive species that guzzles water and produces billions of seeds, Besecker said.
Parmenter added that elm seeds are themselves a nuisance with a high germination rate that leads to seedlings “everywhere.”
“They are also a very weak wooded tree because they are so fast-growing. So in terms of a good landscape tree they are definitely not that,” he said. “A heavy storm, snow on leaves, a heavy snow in fall or spring just creates all sorts of havoc in terms of broken limbs and branches.”
Siberian elm trees invade sewer pipes and systems, are adaptive to wet and dry environments and are drought-tolerant, Parmenter said. They are too aggressive to be ideal trees for urban environments.
“If they find water, they can utilize it and grow faster. I think in terms of water consumption, my thought is that they are probably pretty efficient,” he said.
Besecker said he doesn’t know exactly when they were brought to Colorado but assumes it was around the 1920s.
“I think that people were afraid of Dutch elm disease that affected American elm, which is native,” he said. “So then I think these Siberian elms were brought in to maybe replace the trees that they thought were going to die.”
Besecker said he supposes the Siberian elm trees and the aptly named elm seed bugs co-evolved.