How a worm perpetuated wildfires in northern Minnesota

Wildfires are burning through thousands of acres of forest in Northern Minnesota damaging buildings and forcing residents to evacuate their homes.

The yet-to-be-contained Camp House fire, Jenkins Creek fire and Munger Shaw fire have a small accomplice to thank for their continued destruction: spruce budworms, a well-known pest that has terrorized Minnesota forests for at least half a century, killing trees and making them more susceptible to fire.

The fires’ other helper? Humans.

“Spruce budworm’s largest impact, in my opinion, is that it can help perpetuate dense stands of balsam fir on the landscape that are fire prone,” said Mike Reinikainen, a silviculture program consultant with the state’s Department of Natural Resources’ forestry division.

Much of the area was infected by spruce budworms, whose infestations worsened the Greenwood fire near Isabella, Minn. in 2021.

The spruce budworm is a forest caterpillar that feeds on tree leaves until they are able to transform into a moth. Those moths lay around 10 egg masses, which can hold more than one budworm, before dying within a year. The worms are a crucial food source for predators like the Cape May warbler and purple finch, and they often drop balsam fir seedlings that help repopulate forests.

After they hatch, spruce budworms larvae can defoliate mature trees like balsam fir and spruce until they are killed.

Humans’ work to suppress fires may have also exacerbated the budworms’ growth, according to the DNR’s 2024 Forest Health Annual Report. Stopping natural fires allows fir and spruce forests to grow older and denser, which means more food for budworms.

“When there’s a forest fire that’s starting on the ground, say on the grass, the fire can move up into the canopy of the woods via balsam fir, kind of like it‘s climbing up a ladder,” said Sarah Waddle, an educator for the University of Minnesota Extension program. “And there’s a lot more balsam fir in the woods than there would have been pre-settlement of this region.”

Now fires are happening on larger scales than what might have occurred if they were allowed to happen more naturally.

Spruce budworm populations rise and fall in the state every 25-40 years, but their outbreaks can last up to a decade.

Observers have tracked budworm activity every year in Minnesota since 1954, but Waddle said the bugs have been native to Minnesota for “many hundreds, thousands of years.”

Data from the DNR suggests the current outbreak began around 2020.

Spruce budworm outbreaks have been reported in Lake County, Cook County and St. Louis County. The insects infected more than 700,000 acres of forest last year, marking the largest impact recorded since 1961.

“Over 90% of spruce budworm damage in the last 24 years has occurred in these counties,” Reinikainen continued.

Waddle advised people with infected trees to cut them down. Doing so helps the wood decompose and can become shelter for animals.

Plant trees besides the balsam fir to diversify what plants they can go after. And for professional help, contact your local soil and water conservation district or DNR stewardship forester.

Tim Harlow and Jana Hollingsworth of the Minnesota Star Tribune contributed to this story.