“There is an unusual caterpillar killing trees near Republic!”
“What is going on with the trees at Blewett Pass?”
“Brownish caterpillars are everywhere!”
The telephones at many eastern Washington Forestry offices started ringing in June and didn’t stop ringing for weeks. People were noticing defoliation caused by the western spruce budworm. Red, partially eaten foliage, gray tree tops, and caterpillars dropping from conifer trees were common sights across northeastern Washington in 2011.
The western spruce budworm, Choristoneura occidentalis, is a native caterpillar-moth that feeds on the expanding buds and new growth of Douglas-fir, grand fir and occasionally Engelmann spruce and western larch. It periodically reaches “outbreak” levels which can remain high from 3 to more than 20 years!
People notice defoliation in early summer when partially eaten buds continue to expand and the damaged needles start to frizzle and dry out. Fortunately, the “budworm” caterpillars feed only on the newest foliage, and the trees can continue to photosynthesize with their older needles. At least 3 years of heavy defoliation can deplete the trees enough to cause topkill, and four or more years of defoliation can cause tree death. Usually the smaller understory trees are most severely affected by budworm because they have a higher percentage of “new” foliage and larger caterpillars rain down on them as the season progresses.
There has been budworm defoliation somewhere in Washington every year for many decades, which is why the Washington Department of Natural Resources (DNR) and US Forest Service cooperate to monitor budworm. The map with this article shows the draft aerial survey data for 2011. A persistent outbreak in the Mount Adams, Ahtanum and White Pass area has subsided, but activity remains high between the crest of the Cascades and Wenatchee. Budworm populations appear to have declined in western Chelan and Okanogan counties, but defoliated areas have expanded and intensified in central and eastern Okanogan County. Populations are just building in Ferry, Stevens, and Pend Oreille counties and in the Blue Mountains of southeastern Washington.
By examining pheromone traps placed out to catch adult moths in July and August, we can get an idea of the next year’s likely defoliation levels. The most recent trapping data is being compiled now and will be available soon in DNR Forest Health reports (and in a future edition of Forest Stewardship Notes).
There are a number of reasons that budworm outbreaks are becoming more frequent, more persistent, and more damaging. These factors include changes in forest architecture and composition toward more fir, less pine and larch, more tree crowding, more trees weakened by competition, and more canopy layers to catch and sustain falling caterpillars. Although the budworm preferentially feed on the fir trees, this pressure won’t be sufficient to turn a forest back toward pine and larch domination without a significant event such as fire or restoration logging that opens the stand for improved pine and larch germination or planting.
Many birds, small mammals, and insects feed heavily on the budworm, but this is not usually sufficient to stop an outbreak when suitable food (fir buds) remain abundant. Budworm populations decline naturally due to starvation, cool spring weather, atypical frosts in fall or spring, changes in foliage quality over time, and other factors that aren’t completely understood. Landowners’ responses
The defoliation caused by the latest outbreaks may just be the red flag needed to alert more forest landowners to the importance of forest stewardship activities that decrease the vulnerability of their trees to this caterpillar. Landowners who experience budworm activity can take the opportunity to understand the outbreaks and make a response plan.
In the long term, forest management that enhances the proportion of healthy pine and larch, reduces canopy layers, and increases the vigor of fir host trees will shift the balance away from budworm habitat. Some landowners may seek to spray forest insecticides to ensure temporary defoliation relief for their trees. This is an option, but must be planned well in advance. The caterpillars are only vulnerable to insecticides for a few weeks in mid-June and aerial insecticide applications require complex advance permitting and contracting.
In the next few months and years there will be many opportunities to learn more about the western spruce budworm. Start now with the Forest Service’s Leaflet number 53. Another good reference on these defolilators was written several years ago by DNR Silviculturist Louis Halloin, who is now retired.
The people on the phone last summer were not correct when they thought these caterpillars were “unusual” or that affected trees were immediately being killed. But they were on the mark in understanding that this conspicuous defoliation activity is important to heed. The next few years will be a critical time to evaluate your forest’s condition and take action to improve forest health, restore balance, and increase tree vigor.By Karen Ripley, Forest Health Program Manager, DNR, mailto:firstname.lastname@example.org