The deescalating ‘army worm’ threat

The forest tent caterpillar larvae, commonly called the "army worm." PHOTO: University of Minnesota Extension Service

The forest tent caterpillar larvae, commonly called the “army worm.” PHOTO: University of Minnesota Extension Service

Last night I was texting with a colleague who described her evening thusly: “trying to sit outside but all we hear is army worm [poop emoticon] falling from the trees.”

The forest tent caterpillar, also known as the “army worm,” is a June mainstay in the North Woods of Minnesota. Shortly after the forest turns bright green and beautiful, these little blue-gray caterpillars emerge to gobble up leaves, poop a bunch, form a chrysalis and pop out as a fairly indistinct moth.

Now, I know what you’re saying. That’s what caterpillars do. There are millions of caterpillars doing that right now. There’s a beloved children’s story that describes this process fairly succinctly.

That’s true. But there’s a reason the army worm holds such sway over June conversations in Northern Minnesota. Those of us who live up here know that typically every 12-15 years there is what they call an “army worm invasion,” an explosion of forest tent caterpillars so great that certain species of trees, especially aspen, are often stripped bare. Paved roads and trails turn red from the blood of army worms. Riding a bike sounds like popping popcorn as you run over legions of caterpillars.

Hasn’t happened yet. Doesn’t look to be happening this year, either. That would mean the top end of the 12- to 15-year cycle has passed.

From John Myers of the Duluth News Tribune:

Forest insect experts had predicted the critters to follow their trend of reaching peak populations every 10 to 15 years — and the last big invasions occurred in 2001 and 2002 when millions of acres of the Northland were defoliated by billions of the leaf-eating caterpillars.

But this time the peak appears to have been about 1 million acres in 2013. In 2014, that number crashed to just 156,000 acres. In 2015 it was only 200,000 acres “and 70 percent of that was considered trace defoliation, which most people might not even notice,” said Val Cervenka, forest health program coordinator for the Minnesota Department of Natural Resources.

This year’s aerial defoliation surveys haven’t happened yet. But on-the-ground observations hint at only isolated pockets of defoliation. And if you haven’t seen forest tent caterpillars where you are by now, you probably won’t see them this year.

We’ve got forest tent caterpillars out here in Balsam Township right now, but they come in modest numbers. Why? Again, from Myers’s story:

Some entomologists have speculated that parasitic flies — often called friendly flies because they like to land on people, but can’t bite people — increased far sooner than usual during an FTC cycle. The flies hit the caterpillars hard in 2013 and may have kept caterpillar numbers from going higher. Usually the peak friendly fly hatch comes after the peak caterpillar population.

So the cycle was disrupted by flies. Only time will tell when and if this delicate interplay of nature will resume.

Speaking for most in Northern Minnesota, that’s fine. Take your time.


  1. I live by Lake Millelacs and they have demolished my Birch, Fruit trees and are working on. Maple. Isn’t there a spray?

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