As one plague rages, another subsides — Tent Caterpillars

A cluster of Forest Tent Caterpillars. Photo by David A. Hofmann (Flickr).

An outbreak of forest tent caterpillars in 2016 that threatened Vermont’s sugar maple trees has mostly subsided, researchers and maple sugar producers say.

Forest tent caterpillars are light brown in color, with pale blue lines along each side and white spots on their backs in the shape of footprints. They lay their eggs on twigs and branches in barrel-like shapes. The eggs hatch around the same time that the leaves unfurl in spring, when the trees are particularly vulnerable. These hungry little caterpillars can completely strip a tree of all its leaves.

“We haven’t had any reports of forest tent caterpillar damage this year,” said Josh Halman, a researcher with the Vermont department of Forests, Parks and Recreation.

Halman said the Caterpillars by themselves don’t create a large threat to the trees, even a tree that has been completely defoliated can grow its leaves back and recover. But when combined with already harsh conditions, such as drought, they can become particularly destructive.

“This particular outbreak caused a little bit of tree mortality in spots, and that is in large part due to the fact that we had so much dry weather at the same time we had this outbreak occurring. When we do have other stressors in the environment for the trees the impact of the insect can be a little greater than they would be otherwise,” Halman said.

When the outbreak was in full swing in the spring of 2018, state researchers teamed up with the Civil Air Patrol to get an aerial view of how much damage the caterpillars had done. They try to fly every summer to keep tabs on the forests’ health, but this year’s pandemic complicated that.

 “This year was completely different with COVID so we weren’t able to get up in the plane as we were normally able to. But we were able to access some standing fire towers and hike some mountains to look out across the landscape and map any damage we could see, but there was none caused by Forest Tent Caterpillars,”  Halman said. 

From these flights, along with their other research, they noticed a few things that can help slow the damage caused by the caterpillars. 

“The areas that were most heavily hit by Forest Tent Caterpillars were very sugar maple dominated forests with not a lot of diversity, keeping that diversity of species makes it harder for the population of the insects to build to a level that can be devastating,” Halman said.

According to Halman this means that maple syrup syrup farms are particularly susceptible to the caterpillars, as their forests are mostly made up of the sugar maples that produce the sweet sap used to make maple syrup. When an infestation takes hold in a predominantly maple forest the abundance of food makes it easier for the caterpillars to spread. 

Halman said that maple producers should not tap excessively and follow state and professional recommendations for size of trees to tap and number of taps per tree.

At the Couching Lion Maple Farm in Huntington the caterpillars have always been a factor, but thankfully haven’t caused many serious issues according to co-owner Chaska Richardson.

“We definitely see them every year, but we've never experienced so many that it seems like it decreases the sap yield,” Richardson said. “It doesn't look like whole trees are defoliated.”

Richardson attributes their success with the caterpillars to the variety of trees in their sugar bush.

“Our sugar bush is certified by the Bird Friendly Maple Project through the Audubon, and a big part of that certification is having a diverse forest. So, that might be something that is really helping us right now. We have, you know, predominantly sugar maples but we manage for diversity and we don't take out every tree that's not a sugar maple, and we also leave standing dead trees and a lot of woody debris,” Richardson said.

The 2016 to 2018 outbreak isn’t the first time these caterpillars have caused an issue, and it won’t be the last, Halman said.

“This is a native pest, we're used to it occurring in Vermont and on the landscape, so these outbreaks do happen,” Halman said.