Many affected trees expected to survive
Vast swaths of the typically green, lush slopes of the North Cascades are brown this year and many trees appear dead. But while some of the smaller trees have perished, many of the older ones will survive.
Washington Department of Natural Resources (DNR) Forest Entomologist Glenn Kohler acknowledged that the scene is “pretty alarming” and, to the average person, the trees look dead.
But “the take-home message is, the trees that you’re looking at that appear dead — for the most part — aren’t actually dead right now, Kohler said. “This is probably a long-term event that could go on for another five years. Unfortunately, there’s no way to predict that how long it might last.”
The trees — primarily Douglas fir and other fir species — are being attacked by the western spruce budworm. The current outbreak is affecting trees on a landscape level, not merely in small, isolated pockets. In some places, the damage isn’t scattered but covers entire hillsides. The budworm outbreak is particularly severe for young, small trees in the understory, many of which have indeed died, Kohler said.
The spruce budworm (named for its original host) doesn’t affect pine or larch. Spruce trees tend to be more resilient and typically recover well, but Douglas and other fir species are badly damaged by the pests, Kohler said.
And, while the trees may recover, they are definitely stressed and don’t have the energy to produce pitch, which helps defend the trees. That makes them susceptible to even more destructive insects, such as bark beetles and fir engravers.
Because the caterpillars of the budworm eat only the new foliage when it emerges in the spring, large trees can recover as older needles create new food that helps the trees survive. The caterpillars don’t kill the buds, so, next May, when the buds open, the trees will look green. Even healthy older needles typically fall off naturally after about five years, Kohler said.
Monitoring
DNR has conducted annual aerial monitoring of forests across the state for almost 80 years. The current outbreak stands out because aerial monitoring by DNR picked up no detectable damage as recently as 2021-23.
Kohler believes that this outbreak is in its third year, but that it wasn’t severe enough to be visible until this year. Still, this outbreak is smaller than the last one, which affected more than half a million acres across the state.
The last significant outbreak of spruce budworm, which included Okanogan County, was in 2011-12 and collapsed in 2016-17. It also involved a lot of damage from bark beetles, Kohler said. Last year, some 75,000 acres in western Okanogan and Whatcom counties were infected by the budworm. This year, the infestation has spread to areas including Stevens Pass.
Budworms drop from the upper canopy on spider-web-like threads in July and eat the trees in the understory. The impact of an infestation tends to look worst in August, after the budworm has devoured all of the fresh needle growth, coinciding with the increasing concern over expanses of brown trees. The outbreak is most visible heading west from Rainy Pass.
About 1/8 inch long when they hatch, small spruce budworm caterpillars can live inside a fir needle and feed on it. The smooth, hairless larvae, with white spots on their abdomens, grow to 1 1/2 inches before they make cocoons, “They’re really hard to miss, especially when you’re walking through the woods and they’re dropping on you and going inside of your shirt — which is always very pleasant,” Kohler said.
Budworms are susceptible to natural diseases and many are eaten by birds. And, since the worms concentrate on the more nutritious larger needles, an outbreak usually collapses once the caterpillars run out of high-quality food. A hard freeze in the spring can also kill the worms.
In the 1970s and ’80s, there were millions of acres of spruce budworm outbreaks in Washington, which the state attacked by spraying pesticides. Although caterpillars died after eating the needles, some of the pesticides were harmful to other wildlife. Bark beetles are more difficult to control because they require treatment of individual trees.
Today, entomologists have better tools, such as biological controls using bacteria that don’t kill beneficial insects, Kohler said.
The best long-term tool for controlling the budworm are overall forest-health treatments, including understory thinning and prescribed burning, and areas that have been treated typically have fewer budworms, Kohler said. Much of the area now affected by the budworms is national forest land, and DNR and the U.S. Forest Service work together on forest health through approaches like the 20-Year Forest Health Strategic Plan for Eastern Washington.
Links to drought
Spruce budworm outbreaks typically follow an extended period of drought because, as a drought persists, nutrients become concentrated in fewer and fewer needles and the caterpillars get more bang for their buck, Kohler said. Moreover, insect attacks during drought are more likely to cause tree mortality.
Washington has declared a drought in at last part of the state in six of the past 10 years. The snowpack in the mountains has been especially low the past few years, according to the state Department of Ecology.
Although drought appears to make trees more susceptible to some pests, the underlying cause is a decline in overall forest health and diversity. A century of fire suppression has led to unnatural stand conditions with too many firs, particularly in Okanogan County, Kohler said. Because fires are a natural part of the ecosystem, small ground fires would typically have burned saplings, helping the mature trees survive, Kohler said.
Although dry needles and trees that die after being attacked by bark beetles are more susceptible to fire, scientists debate whether the budworm increases vulnerability to wildfire, Kohler said.
Other pests
While the budworm is a concerning pest, some insects are worse. The Douglas fir tussock moth eats old and new foliage and can kill a tree within a year, Kohler said. But because damage from the tussock moth is so extensive, the outbreaks usually collapse on their own in a couple of years when the moths run out of food.
There are several types of bark beetles, which primarily attack Ponderosa pines. These beetles generally follow a drought, and infestations have been growing in the past decade. More than 120,000 acres across the state have been affected by western and mountain pine beetles in the past couple of years, Kohler said.
Douglas fir beetles and engravers are less connected to drought conditions. In fact, the Douglas fir beetle, which needs a windstorm to spread, is currently below average in the state. The mountain pine beetle can wipe out an entire stand of lodgepole pines.
If the current bark beetle infestation persists, Kohler expects to see more trees die by 2027, although larger trees will most likely survive.
