Glenn Kohler, DNR Forest Entomologist
In August, an outbreak of western blackheaded budworm (Acleris gloverana) was spotted southeast of Mineral on the south edge of the Tahoma State Forest. Blackheaded budworm is a leafrolling caterpillar related to the western spruce budworm. Western hemlock is the preferred host.
The defoliation covers about 8 miles of the Murray Mainline road system along Mineral Creek. The heaviest defoliation seems to be near the bottom of the drainage, with the budworms pupating at lower elevations and feeding at elevations up to 3,000 feet. The budworms were also lightly defoliating Pacific silver fir and Douglas-fir. Hemlock defoliation was primarily in the new foliage, but in a few areas with heavier defoliation they have removed older foliage. In some areas the population is high enough that we saw more than 20 larvae and pupae in a branch beat sample and ground plants were covered with droppings!
Outbreaks of western blackheaded budworm are short-lived due to natural controls like predators, parasites, disease, and cool/wet early summers. Noticeable defoliation usually lasts 2 to 3 years so it’s possible this could expand in area and severity next year. There’s usually very little mortality the first year of noticeable defoliation, but with a 2nd or 3rd year, top-kill and whole tree mortality are a possibility. Hemlock doesn’t tolerate heavy defoliation well and once 75% of the crown has been completely defoliated, they’re likely to die. As long as the budworms are feeding primarily in new foliage, mortality will be limited but mortality and top-kill are likely to be higher in dense, multi-layered stands where trees have less foliage to begin with. Smaller trees can also be heavily defoliated due to ‘ballooning’ caterpillars dropping on them from the overstory. Few long-term impacts are expected for either silver or Doug firs.
There have been a few large western blackheaded budworm outbreaks in western Washington. In 1970 to 1973, approximately 200,000 acres were affected, with an outbreak in 1984 to 1986 peaking at 300,000 acres. There have also been reports western blackheaded budworm defoliation west of Snoqualmie Pass (2010 to 2011) and west of Baker Lake in Whatcom County (2012).
Natural Resources Canada has several publications related to potential control methods and recovery: http://cfs.nrcan.gc.ca/publications/search?query=Western blackheaded budworm&process=Search