Western conifer seed bugs are invading

By Jeff Mitton

One of the joys of living in a heated house is that you get so many uninvited visitors: house centipedes, black widows, jumping spiders, buzzing flies, miller moths.

On this day, some sort of leaf-footed bug greeted me as I stepped out of the shower.

Although I recognized, by odd structures on its hind legs, that it was one of the leaf-footed bugs, I did not know which one — I needed an expert.

So I put it in a jar and I took it to Deane Bowers, who is my colleague in ecology and evolutionary biology but also curator of entomology at the University of Colorado's Museum of Natural History. In a few moments she proclaimed, with authority, that it was a western conifer seed bug, Leptoglossus occidentalis. 

The life cycle of seed bugs takes a year in the forests west of Boulder, but in warmer climates in Mexico, they can race through three generations per year.

Attempts at eviction frequently provoke the bugs to release a mixture of six chemicals, principally hexyl acetate and hexanal — not a smell anyone wants in their home.In the Front Range, they overwinter as adults, but appear in May or early June to feed on first-year cones of the local conifers. They lay eggs on the conifers in about 10 days, and after the eggs hatch, the early nymphs feed on needles and the softest parts of cone scales.

Older, larger nymphs use their piercing mouthparts to feed on developing seeds. In late summer, nymphs change into adults that feed on ripening seeds until fall and then seek overwintering sites under the bark of pines or in dead Douglas-firs.

Western conifer seed bugs are the ultimate generalists, for in their native range in the forests of western North America, they feed on about 40 species of conifers. Most of their hosts are pines, but they also consume seeds of Douglas-firs and several species of spruces and firs.

In the U.S. and Canada, many people consider the seed bugs to be a nuisance —they don't appreciate the way the bugs are able to find their way into homes and office buildings.

Attempts at eviction frequently provoke the bugs to release a mixture of six chemicals, principally hexyl acetate and hexanal — not a smell anyone wants in their home.

Some people have complained about dozens in their home, and a warehouse facility in British Columbia reported an infestation of 2,000 bugs. But on the bright side, they carry no diseases, they do not bite or sting, and they leave in spring.

What worries me is that they have been reported to drill holes in polyethylene tubes used in plumbing and radiant heating systems.

Historically, western conifer seed bugs were native to pine, spruce and fir forests in western North America. But they have demonstrated that they are efficient hitchhikers, traveling in trucks to distant destinations. They were reported in Pennsylvania in 1992, and, since that time, they have spread south to the Gulf Coast and north to Nova Scotia.

Seed bugs hitched a ride with a shipment of lumber to northern Italy in 1999 and then were accidentally introduced to several other localities. They had reached Spain in 2003, and, by 2007, they were established in Slovenia, Croatia, Hungary, Switzerland, Austria, Germany, France, the Czech Republic and England.

Most recently they have been reported in Turkey, Russia and the Ukraine. On the other side of the world, they have become established in Korea and Japan.

In northern Italy, the stone pine, Pinus pinea, takes three years to develop seeds, which are harvested and sold as pine nuts. The seed bugs have taken a liking to stone pine cones and seeds and have diminished the harvests by as much as 95 percent.

Farmers hammered by this localized economic disaster demanded that something be done to combat the stinking invader. Forest biologists are considering management strategies, and they currently favor introduction of one of its egg parasitoids, Gryon pennsylvanicum, from North America. This wasp lays its eggs in the eggs of seed bugs, killing them.

Biologists are currently conducting experiments to determine if the egg parasitoid will attack any of the native European insects. Caution is warranted, for some management introductions have gone awry.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.This column originally appeared in the Boulder Camera.

March 2, 2016