In 2012, aerial surveys identified almost 700,000 acres defoliated in southern Idaho. Since the current outbreak began, 2.2 million acres have been impacted by budworm. Impacts from defoliation can be worse in years when the weather is warm and dry, such as this one.
Normally, budworm does not kill mature trees, but may cause reduced growth and dead tops. Smaller trees experience more mortality than larger trees because they are less tolerant of defoliation. The more defoliation a tree experiences in one year, the higher the likelihood of mortality. Large trees that have been heavily defoliated for consecutive years may be prone to attack by bark beetles. The previous large scale outbreak lasted over 15 years, with considerable fluctuation in defoliation each year, averaging annually from 200,000 to 600,000 acres.
Control options are somewhat limited over large areas. The best way to reduce the effects of budworm is to have vigorous growing Douglas-fir and true firs in stands with pines and other tree species. Dense tree stands are impacted more severely than open stands. Budworm populations are usually highest and most impactive on warm and dry sites, overly dense sites, and trees with poor vigor.
For backyard trees, defoliation can be reduced by killing the budworm larvae. Several chemical and biological insecticides are available for treating individual trees or small sized tree stands. These treatments are most effective when larvae are small and have just begun to feed, typically early summer. Recent field assessments indicate that direct treatments should begin now and continue over the next several weeks in central and southern Idaho. Private landowners wishing to treat individual trees with budworm infestations should contact their local Idaho Department of Lands office for additional information.
Later this summer larger larvae will descend from the bigger trees on silk threads, landing on smaller, understory trees. In August, adult orange to tan mottled colored moths will fly and lay eggs. Budworm outbreaks generally crash when their natural enemy populations increase or harsh winter temperatures or spring frosts occur. More details on western spruce budworm can be found at http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5357260.pdf