A decennial pest species called the forest tent caterpillar is midway through an outbreak in Vermont this year, and egg masses have just begun to hatch, according to state officials.
The caterpillar is native to Vermont and every 10 years or so an outbreak of the species will defoliate large areas of deciduous trees. The last big outbreak in Vermont began in 2004 and continued for about three years, stripping foliage from more than 300,000 acres of forest.
Maple sugar makers are among those keeping a wary eye out for the bugs as summer fast approaches.
“Everything looks good right now — the leaves look fantastic,” said Peter Purinton, owner of Purinton Maple in Huntington. But, as is the case in any agricultural operation, he said, “when you think things look good, look out, because something’s after you.”
Purinton said he’s been hit more than once by forest tent caterpillar outbreaks since he began his sugarbush in the late 1970s. The most recent infestation before this one was around 2010.
“They just eat the leaves and leave the trees with no leaves,” he said. “The first of July, it’s like it’s January.”
Maples continue to produce sap, Purinton said, but the extra energy the trees require to respond to the stress of defoliation causes the sap to contain less sugar, making the sap less sweet.
Purinton said he’s found a few of the caterpillars, but a survey of their egg masses last year left him convinced that his sugarbush is unlikely to suffer from an outbreak this year, he said.
Chris White, co-owner of Mount Mansfield Maple Products, said he hasn’t seen any caterpillars lately, but said signs of infestation are visible on hillsides around the state.
“I know it’s advancing — a lot of places are getting hit pretty hard,” he said. “We fortunately haven’t seen it in our sugarbush.”
The caterpillars raise maple mortality rates by a meaningful amount, but most trees that succumb to the pest are probably stressed by other factors, said Mark Isselhardt, a maple specialist at the University of Vermont Extension Program.
Normal mortality rates in maple stands are between 1 to 1.5 percent annually, Isselhardt said; an outbreak of forest tent caterpillars can drive that number up to 2 to 3 percent, he said.
Many Vermont maple producers elect not to spray for the caterpillars, choosing instead to let nature run its course, Isselhardt said.
Some producers believe defoliation caused by forest tent caterpillars affects the sweetness of a tree’s sap, Isselhardt said, but a far more pronounced effect on flavor and production results from the weather during the final four to six weeks of the season, when maples require a temperature swing to stimulate sap flow.
Although the caterpillars prefer sugar maples over many other species, “they eat just about anything,” Isselhardt said.
The current flourishing of forest tent caterpillars began in 2016, when the bugs chewed their way through 30,000 acres of forest. Last year they ate another 60,000 acres, according to the Agency of Natural Resources.
The caterpillars typically don’t kill off the trees they infest, and while they often infest sugar maples, the trees can remain viable nevertheless, said Josh Halman, a forest health specialist with the ANR’s Department of Forests, Parks and Recreation.
Researchers from the department last winter surveyed more than 70 sugarbushes in an attempt to predict what areas will be hardest hit this year.
Thirteen of those sugarbushes will receive treatment in upcoming weeks with a biological pesticide called bacillus thuringiensis. Those treatment locations are in Essex, Franklin, Lamoille, Orleans and Windham counties, Halman said.
The pesticide will be sprayed from planes contracted by landowners, he said, and landowners who will be receiving the spray are supposed to notify neighbors. Municipalities are also meant to alert residents before spraying commences, Halman said.
Spraying began this week because, in many locations, egg masses are hatching and caterpillars have been observed feeding on foliage, Halman said. This year’s caterpillar populations have not yet been seen to be widespread, he said.
The pesticide is actually a bacteria that spurs the growth of toxic crystals inside the gut of affected moth and caterpillar species. The creatures must eat the bacterial spores in order for the pesticide to operate, Halman said.
The bacillus thuringiensis bacteria is essentially harmless to humans, but some people who suffer allergies may be sensitive to direct contact with the pesticide, according to the Department of Forests, Parks and Recreation.
The pesticide is approved for use on organic farming operations, Halman said.
Correction: The photo originally used with this story was an Eastern tent caterpillar, not a forest tent caterpillar.
