Aerial surveys in 2011 identified almost 1.2 million acres defoliated in southern Idaho. Indications are the amount of defoliation in 2012 will be similar. The last outbreak of spruce budworm lasted for over 15 years, with considerable fluctuation in mortality each year. The area annually defoliated then ranged from 200,000 to 600,000 acres.
Normally, spruce budworm does not kill trees, but may cause them to have reduced growth and dead tops. Smaller trees experience more mortality because larvae drop from the large trees and collect and feed on smaller trees. The more defoliation a tree experiences, the higher the likelihood of mortality. Large Douglas-fir trees that have been heavily defoliated for multiple, consecutive years may be more prone to attack by Douglas-fir bark beetle, resulting in mortality.
Control options are somewhat limited over large areas. Keeping stands of Douglas-fir and true firs growing vigorously is the best way to reduce the effects of budworm. The only way to reduce defoliation is by killing the budworm larvae. Several insecticides are available for treating individual trees or small sized tree stands, but they are most effective when larvae are small and have just begun to feed, typically in early summer. Private landowners wishing to treat individual trees with budworm infestations should contact their local Idaho Department of Lands office for additional information.
The larvae commonly descend from the needles on silk threads and land on smaller, understory trees. In August the adults begin to fly and lay eggs. An abundance of these small, reddish-brown mottled moths will soon appear in many areas of southern Idaho.
Budworm populations are usually highest and have the most significant effect in certain types of forests. Dense forests that are on warm and dry sites, with poorer vigor are the most likely to be affected by spruce budworm. More details on western spruce budworm can be found on the internet at http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5357260.pdf