Forest tent caterpillars are here, but worst of the outbreak may not be this year
Admittedly, Kathy Carlson of Duluth doesn’t know very much about forest tent caterpillars. But she does know she never wants to see one again.
Consider her wish (partially) granted.
The Minnesota Department of Natural Resources has been reporting a growing number of the caterpillars — known for munching on leaves and invading backyards — over the past several years, causing some experts to predict an imminent outbreak. But the department said Tuesday that hatching this spring has been sporadic, and that numbers might actually be declining in some areas.
“They’re horrible, they’re slimy, they’re everywhere,” Carlson said about the caterpillars, which most recently invaded Duluth during a record outbreak in the early 2000s. In 2001, the caterpillars ravaged nearly 8 million acres of forest in Minnesota.
The caterpillars themselves are between two and four inches long, brownish in appearance with blue and yellow designs, and often are seen hanging around silk “tents” they build in tree branches.
They prefer feasting on the green leaves of deciduous trees — mainly aspen, birch, oak and basswood — but sometimes snack on shrubs and garden vegetables when food is scarce.
Carlson said her East Duluth neighbors have wrapped tree trunks with aluminum foil coated in grease to deter the caterpillars from reaching leaves.
“They just come, and you hope there are lots of birds or something to eat them,” she said.Outbreaks typically occur every 10 to 15 years, and persist for two to three years. They can affect massive swaths of land in North America, from coast to coast and from forests in Canada to swamps in Louisiana.
During outbreaks, the caterpillars become so overpopulated that there isn’t enough food to support them. Up to 95 percent can die of starvation.
Since 2007, the number of caterpillars in the northern half of Minnesota has been growing, according to the DNR. Aerial photos taken by the department show that the area defoliated by the caterpillars has recently quadrupled, from 250,000 acres in 2012 to more than 1 million acres in 2013.
The upswing is reminiscent of one that occurred from 1998 to 1999, which led to the early 2000s outbreak. But a similar invasion doesn’t appear likely this time, at least not yet.
Mike Albers, DNR forest health specialist, said a sampling of five Northeastern Minnesota counties last July showed that some cocoons were being attacked by a native flesh fly called the “friendly fly.”
“Those high rates of parasitism, up to 90 percent in some areas, were much higher than expected at this point in the outbreak,” Albers said in a statement. “It’s too soon to know if it signals a downturn in the caterpillar population cycle, or was a fluke in the small sample size.”
But don’t thank the recent winter — Duluth’s second-coldest and third-snowiest on record — for helping keep the caterpillars at bay. While snow and cold can delay hatching in the spring, caterpillar eggs are able to survive storms and temperatures as low as minus 40 degrees.
The caterpillars typically hatch in May and begin robbing trees of their leaves almost immediately. They eat their way through the caterpillar life cycle, eventually spinning cocoons and undergoing metamorphosis sometime in July. They emerge two weeks later as small tan-colored moths. The moths lay eggs, renewing the cycle.
Barbara Stark and her family moved to Duluth in 2000, just as the forest tent caterpillars — sometimes called army worms, which are a separate species — were crawling into town.
Stark said she remembers taking a trip to Enger Tower, where the landscape was thick with caterpillars.
“It made it unpleasant to be in the woods or the yard because they drop down on their little silk strands, and you can hear them munching on the leaves,” she said. After the leaves of their favorite trees were gone, Stark said the caterpillars began eating leaves from a patch of apple trees in her backyard.
“Our kids were little then,” she said, “So they had lots of fun collecting them.”
Kay Jankofsky’s kids were a little older than Stark’s during the last outbreak. Jankofsky, also of East Duluth, said the caterpillars intervened one day when her family was having a picnic at Brighton Beach.
“They were just falling off the trees, onto the table and into the barbecue,” Jankofsky said. “We had to cover the food.”
The caterpillars also are known to make travel a hassle, migrating across roads in search of something to eat. When stepped on, run over or in any way disturbed, they often emit a greenish-black fluid.
“I remember going up Wallace (Avenue), and the street was just crawling with them,” Jankofsky said. “The streets actually got slippery because there were so many.”
While Duluth residents are largely welcoming news that another caterpillar outbreak might be more manageable, or at least a year away, Jankofsky said there is one thing she might miss about the little guys.
“They’re actually very beautiful,” she said.