The invasion should have happened in 2014. Or last year. Definitely by now. But forest tent caterpillars are about as scarce as hen's teeth these days.

In great news for everyone planning June weddings, graduation parties, reunions or camping trips, the cyclical invasion of forest tent caterpillars, sometimes called army worms, appears to have been called off.

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.

"There's really nothing to talk about. It's just not happening in most areas," Cervenka said.

Mike Parisio, the DNR's new regional entomologist in Bemidji, agreed.

"I got a few reports from out west around Detroit Lakes. But when I went over to check it out I hardly found anything," Parisio said. "There have been a few more reports from up around the (Canadian) border. But no emergency situation, that's for sure."

Paul Cigan, forest health specialist for the Wisconsin DNR in Spooner, said the number of egg masses seen on trees has been low. Those eggs were laid last summer by little moths and should have hatched by now into caterpillars.

Those egg surveys "indicate FTC populations will remain low for the year. Defoliation will likely go unnoticed by the public, though, in some locations throughout the region, landowners may observe light defoliation on shade and forest trees," Cigan told the News Tribune.


Squishy, icky mess every 15 years

Forest tent caterpillars last peaked in 2001 and 2002 in Minnesota when they defoliated more than 7.3 million acres of forest each year, the largest outbreaks in recorded history. They rendered trees bare, ruined orchards and wiggled up the sides of homes and across roads and driveways, eating and defecating before they wrapped themselves in cocoons to metamorphose into little brown moths that lay eggs and start the cycle again.

At their peak infestation there can be up to 4 million caterpillars per acre.

There were so many caterpillars in 2001 and 2002 that people canceled vacations and events, and Northland residents spent weeks trying to keep the caterpillars from defoliating their favorite fruit and shade trees and keep them off their houses, decks and patios. Some people reported leaving the Northland, so totally grossed out by the marching army of caterpillars that they couldn't stand it.

As expected, the caterpillar numbers eventually crashed after they overpopulated, eating themselves out of food and starving. They also succumbed to disease and a parasitic fly that emerges on the heels of every forest tent caterpillar outbreak.

By 2006 only 1,900 acres were defoliated statewide, the low point of the most recent cycle.

Then slowly, as they have every decade since people paid attention, the native caterpillars began to expand in number and range across northern Minnesota. By 2012 they defoliated 274,000 acres and in 2013 that number skyrocketed to 1.1 million acres.

But then something happened to reverse the caterpillars' course. In 2014 very few if any caterpillars emerged from their egg masses to munch aspen leaves, their favorite food.

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.