Local News

Native caterpillar causes worst damage since 1961 to spruce, fir trees

KAXE

Published January 16, 2025 at 4:09 PM CST

Eastern spruce budworm is a native caterpillar and is the most destructive pest of spruce-fir forests in eastern North America.

The spruce budworm has impacted more than 2,000 square miles since 2021, mostly in St. Louis, Lake and Cook counties, although a small population in Itasca County persists.

An annual report on forest health in the state shows damage caused by spruce budworm in northeastern Minnesota was the worst in more than 60 years.

The Minnesota Department of Natural Resources noted that overall, the return of above-average precipitation during the growing season benefitted many of the state’s forests. This includes a 70% drop in damage in oak trees caused by the two-lined chestnut borer and a slowing of oak decline.

While the rain was generally beneficial, it did promote fungal leaf diseases on deciduous trees and shrubs across the state. Particularly impacted were poplars, which displayed foliage marred with black or brown splotches, with many mostly defoliated before autumn. The good news is that these diseases look much worse than they are, and trees should leaf out fine this spring.

A map produced by the Minnesota Department of Natural Resources shows defoliation caused by the spruce budworm in 2024. The agency says it is the worst damage noted since 1961.

But the spruce budworm, on the other hand, has impacted more than 2,000 square miles since 2021, mostly in St. Louis, Lake and Cook counties. Some small populations outside the Arrowhead, such as in Itasca County, also seem to be persisting.

The DNR stated that the resulting fir and spruce death have caused major concerns with wildfire potential and the inability to efficiently manage the forest.

The agency received funds last year from the Lessard-Sams Outdoor Heritage Council to diversify and regenerate budworm-impacted land. But according to the agency, a number of factors and the sheer extent of the outbreak are making dealing with spruce budworm a big challenge. These factors include limited staffing, limited market demand, not enough loggers and contractors and diverse land ownership patterns.

Typical feeding damage from spruce budworm on fir in late June. Note the caterpillar at the base of new shoot.

Outbreaks of spruce budworm occurred in the past and now may be exacerbated by 20th and 21st century fire suppression. Fire suppression allows forests to grow denser and older, benefiting spruce budworm.

Outbreaks in a specific area typically last six to 10 years, which is about the amount of time balsam fir and white spruce can withstand feeding. After outbreaks subside, budworm will cycle back about 30-60 years later.