Longer growing seasons are bolstering some tree-eating insects and taking their toll on Minnesota forests while spruce budworms munched balsam and spruce trees across 664,000 acres in 2023, the most in more than 60 years.

The budworm outbreak, which devastates balsam fir trees, has spread in recent years from St. Louis into Lake and Cook counties, where the insects have left behind miles of red-needled, dead trees that can become catalysts for major wildfires.

Spruce budworms hit 664,825 acres in Northeastern Minnesota in 2023 alone — more than 1,000 square miles, about the size of Rhode Island — up from 488,838 acres in 2022 and the most since 1961.

That was the report Thursday from the Minnesota Department of Natural Resources as state foresters released their annual data that was collected through aerial surveys of 17.3 million acres in 2023 across the state combined with on-the-ground reconnaissance in forests across the Northland.

Meanwhile, the 23-year outbreak of eastern larch beetle has hit 1 million total acres of Minnesota tamarack trees damaged, now equaling about 75% of the state’s total tamarack stand acres.

“Longer growing seasons have allowed these beetles to reproduce more, leading to an increase in beetle populations in tamarack stands,” the report notes.

Oak trees also have been hit hard by insects, especially after being stressed by heat and drought, with oak loss now common across large swaths of the state. The DNR reports that the 2023 growing season was the ninth-driest on record for the state, following an even drier 2021, and the fourth-consecutive year with below-normal growing-season precipitation.

“The most widespread tree health problem (in Minnesota in 2023) was declining oaks, primarily caused by a combination of older age, consecutive years of significant drought, and two opportunistic pests: Armillaria root disease and two-lined chestnut borer,” the report said.

Invasive emerald ash borer was officially confirmed for the first time in Benton, Faribault, Lyon, McLeod, Mille Lacs, Morrison, Cass and Clay counties in 2023, with 46 out of 87 counties now under quarantine for the Asian invader.

There was some good news from foresters, including that tamarack stands seem to be slowly regenerating on their own after larch beetle infestations move through. Total tamarack loss to larch beetles dropped from 303,000 acres in 2022 to 262,000 acres in 2023.

And forest tent caterpillars, the pesky worms that have periodically wreaked havoc across the Northland, remain at historically low levels, with no sign of their usual mass outbreak in sight.

The caterpillars made headlines 20 years ago in what was their largest outbreak ever recorded across the Northland. Usually peaking every 10-15 years, they should have returned by now in huge numbers. But so far that hasn’t happened, with an unprecedented lag since the last big infestation. And no one seems to know why.

In 2023, the native caterpillars, sometimes called armyworms and which eventually turn into buff-colored moths, munched only 13,000 acres across the state, up from 9,000 acres in 2022, but still just a fraction of the more than 7 million acres in each of 2001 and 2002. Since then, there has only been one mild outbreak, 1 million acres in 2013, with only regional pockets of the bugs in recent years.

There were as many as 4 million caterpillars per acre at peak, eating almost every green leaf they could find, especially aspen but also basswood, oak, birch, apple, garden vegetables and more.

The DNR’s full 2023 forest health report can be found at files.dnr.state.mn.us/assistance/backyard/treecare/forest_health/annualreports/2023-annual-report.pdf