Spruce budworm continues its natural infestation in Idaho forests

You might have noticed the reddish brown color of some of southern Idaho’s forested hillsides lately.

The U.S. Forest Service says Western spruce budworm, a native western North America insect, is defoliating Douglas-fir, true firs, spruce, and other conifer species here. The insect populations began to increase in 2004 and has grown to the outbreak stage in many areas.

Aerial surveys in 2011 identified almost 1.2 million acres defoliated in southern Idaho and officials expect about the same level this year. The last outbreak of spruce budworm lasted for over 15 years, with the area annually defoliated ranging then 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.

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.