DULUTH, Minn. -- They came by the millions, eating every green leaf in their path and then pooping on everything below.

Forest tent caterpillars covered houses and garages, ruined picnics and camping trips, defecated into parked boats and onto decks and patios, ruined orchards and gardens and forced outdoor weddings indoors. Then they hatched into moths that swarmed by the thousands near any outdoor light.

The caterpillars made headlines 20 years ago in what was their largest outbreak ever recorded across portions of northern Minnesota. Usually peaking every 10-16 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.

Last year saw the lowest total defoliation in 22 years by forest tent caterpillars, just 1,300 acres statewide, and 2020 doesn’t look to be much worse.

“We’re not hearing much about it again this year. There are some anecdotal reports (of defoliation) east of Mille Lacs, in Aitkin County, and up by Biwabik. … But it’s not going to be a big year,’’ said Eric Otto, forest health specialist for the Minnesota Department of Natural Resources based in Grand Rapids.

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How bad did it get during the last major outbreak? There were as many as 4 million caterpillars per acre at peak, eating most every green leaf they could find, especially aspen, but also basswood, oak, birch, apple, garden vegetables and more. (They generally don’t eat evergreen needles.)

In Minnesota, forest tent caterpillar outbreaks peaked in 1891, 1898, 1912, 1922, 1937, 1952, 1969, 1978, 1990 and 2001 — the last two outbreaks noticeably larger than any others. In the record outbreak in 2001, more than 7.5 million acres of hardwoods were defoliated. In 2002, they nearly matched that at 7.3 million acres — the second-highest ever.

Having all their leaves eaten doesn’t usually kill the tree. Most trees sprout new leaves within a few weeks. But already stressed trees may be more vulnerable to disease and drought and about 10% of impacted trees perish. Forest experts say multi-year outbreaks can actually reduce forest production and change the makeup of the trees in the woods.

Forest tent caterpillars are a native species, so nature has some built-in defenses. Eventually, after a few high population years, starvation, diseases and especially a parasitic fly — called the friendly fly because they can’t bite people yet swarm around and bother people — caused forest tent caterpillar numbers to crash.

By 2005, they returned to background levels over just a fraction of their peak range. Then we braced for the next outbreak. Their numbers grew slowly until 2013, when about 1 million acres were defoliated in Minnesota in what appears to be the most recent peak, yet the lowest peak on record.

"And what there was that year was pretty light defoliation, nothing like the early 2000s," Otto said, noting only 35,000 acres in 2013 had serious defoliation.

This summer, DNR staff will not be allowed to conduct aerial surveys due to COVID-19 restrictions. But Otto said they are using satellite images to compare this year to others and keep track of defoliation.

Assuming 2013 was really the last peak outbreak, that should mean the next outbreak should peak anywhere from 2023 to 2029, and we can expect a gradual increase in forest tent caterpillars each year until then. So far that’s not happening, Otto noted.

Why the insects peaked at just 1 million acres during the current cycle is still unknown. One guess is that their biggest predator, the friendly flies, may have hatched earlier in the caterpillar cycle than usual and may have trimmed caterpillar numbers enough to prevent any big increase.

The last widespread outbreak of forest tent caterpillars took place in 2001. (File / Forum Communications)
The last widespread outbreak of forest tent caterpillars took place in 2001. (File / Forum Communications)

Some people have speculated that the polar vortex winter in 2014 may have froze the caterpillars in their eggs. But experts note that we’ve had far colder winters during which the caterpillars survived just fine to come back in huge numbers the same spring.

"There really hasn’t been much research into why this is happening now," Otto said.

Wisconsin also had its last big outbreak in 2002 and, like Minnesota, hasn’t seen major defoliation by forest tent caterpillars since. In 2019, surveys showed very low populations of the insect sustaining what has been an unusually long 16-year trend, said Paul Cigan, forest health specialist for the Wisconsin Department of Natural Resources, in his 2019 report.

Cigan suggested that cooler- and wetter-than-average spring weather may reduce caterpillar populations by enhancing the activity of fungus that kills them.

An army of forest tent caterpillars swarm the yards of residents in Lakewood Township near Duluth in May 2000. (News Tribune file photo)
An army of forest tent caterpillars swarm the yards of residents in Lakewood Township near Duluth in May 2000. (News Tribune file photo)

If caterpillars were abundant in your area you’d know it by now. You’d see denuded trees and caterpillars moving to new areas. Full-grown caterpillars are over an inch long with hairy and mostly velvety-black with blue stripes on their sides. A characteristic pattern of a yellow keyhole-shaped marking runs along the tops of their backs.

In May, after the aspen buds sprout into leaves, tiny caterpillars emerge from their winter egg masses and start to eat, active from mid-May to the end of June. Once fully grown, they spin cocoons and metamorphose into buff or tan-colored moths in late June and July. Those moths congregate in the tens of thousands (especially near bright lights) as they propagate the next generation, laying eggs on trees and then dying quickly.

Northlanders who experience an outbreak can spray caterpillars off trees or buildings with a hose or knock them into a bucket of soapy water to kill them. During past peaks, some people used caterpillar traps on trees, with petroleum jelly or oil-covered aluminum foil wrapped around the trunk. That only works, however, if the tree doesn't overlap with the canopies of other, unprotected trees.

Forest tent caterpillars are an entirely different species than army worms, which tend to frequent agricultural areas causing crop damage.