Invasion of the fir killers: Tiny bugs with big name threaten subalpine fir forests in the Northwest

Entomologists from the Idaho Department of Lands and U.S. Forest Service are keeping tabs on a tiny invasive insect that can hammer subalpine fir forests.

Balsam woolly adelgid, a wingless creature from Europe, was introduced to North America in the early 20th century and, lacking any native predators here, has flourished. The insects were first documented in Idaho in 1983.

The bug, which produces waxy, wool-like threads, has the ability to rearrange the species composition of Northwest forests, according to a news release from the Idaho Department of Lands.

It feeds on true firs, including grand firs, but has an outsized effect on subalpine firs that grow mostly at high elevation but can be found in lower-elevation valleys prone to cold temperatures.

Like many introduced species, balsam woolly adelgid, sometimes referred to by its initials, is relatively benign in its native habitat, where it has evolved alongside its host trees and with other insects that feed on it. But when the insect moved to North America, likely via the importation of nursery stock, the bugs that eat it didn’t come along for the ride.

“There is not a very effective group or guild of predators that feed on this insect,” said Tom Eckberg, an entomologist and forest health program manager for the Idaho Department of Lands at Coeur d’Alene.

It’s found in northern Idaho near Coeur d’Alene and has been documented in the Potlatch River drainage in the Clearwater Basin.

Eckberg said the bugs, which are about 1 millimeter long, can quickly kill any member of the true fir family, which excludes Douglas fir. A U.S. Forest Service pamphlet on BWA states that in some areas “firs are slowly being eliminated from the ecosystem; and adelgid populations continue to spread to previously uninfested areas.”

“They tap into the vascular system of the tree, and they will suck out juices and liquid and also inject toxic saliva which causes abnormal cell growth,” Eckberg said. “It causes the wood to grow abnormally and interferes with transport of nutrients. Over time, it can disfigure the branches.”

The disfigurement causes swelling or bulbous growths on branches and tree trunks known as gouting. The bugs also produce a waxy thread for self protection that can appear like a white fuzz when adults congregate on trees.

“It can kill a tree, sometimes in three years or so,” Eckberg said.

Balsam woolly adelgid are prone to temperatures below 30 degrees, so cold winters can check the spread of the bug. However, deep snow insulates the insects and keeps them from freezing.

There is no effective control of the insect. Chemical treatments can work for trees used in urban landscaping, but widescale application to vast forested areas is not economical.

Entomologists are tracking the insect’s spread to help foresters understand which areas may be at greatest risk for infestation and how forests may change in the future. The surveys can also inform their decisions about future forest management.

The effort is part of a long-term monitoring project that began in 2008, when forest scientists established survey plots. The same areas are visited and surveyed about ever five years.

Subalpine firs are not a commercial timber species. But Eckberg said on some high-elevation ridges and mountains it is the only tree species that grows. The trees are also used in urban landscaping.