Booming caterpillar population munches through forests

Forest tent caterpillars. Photo courtesy of Vermont Agency of Natural Resources

Swaths of leafless, gray-brown trees are discoloring the normally verdant forests in some parts of Vermont this summer.

The defoliation is caused by an outbreak of forest tent caterpillars, which occur about every 10 years. The caterpillars, which hatch in late June and early July, eat an astonishing amount of foliage as they grow.

“When the population builds to these outbreak levels, these caterpillars eat through quite a bit of the forest,” said Josh Halman, a forest health specialist with the Vermont Department of Forests, Parks and Recreation.

Forest tent caterpillars prefer the leaves from hardwood species, especially sugar maple.

The current outbreak began last summer when hordes of the hungry caterpillars chewed their way through 25,000 acres. Halman said last summer’s defoliation was isolated to northeastern and north-central Vermont but that his department has heard reports this summer from farther west and south.

If it follows previous trends, this outbreak should be over in a year or two, but not before increasing in scale.

A maple tap. Stockxchng image
Mark Isselhardt, a maple specialist at the University of Vermont’s Proctor Maple Research Center, said the state’s most recent tent caterpillar outbreak lasted from 2004 to 2007. At its peak in 2006, the outbreak caused defoliation in 310,000 acres of forest, mostly in the southern half of the state.

We probably won’t see quite that many acres affected this year, Isselhardt said, but a dramatic increase over last year is likely.

Jared Nunery, the state’s forester for Orleans County, said it’s difficult to assess the financial costs of defoliation. Potential costs would come primarily from tree death, he said, but even then the calculation depends on how the lost trees are being used.

“For a sugarmaker, if you have significant die-off within your sugarbush, each one of those trees that dies is a tap that’s lost which has an annual production of revenue,” Nunery said. “If you’re producing timber, you’re looking at lost growth.”

Halman said the increased tree mortality in areas experiencing defoliation is not usually calamitous. The trees that succumb are mostly those that are already under other stresses such as poor soil nutrition or drought conditions. Many consecutive years of defoliation can also contribute, but that rarely happens.

“The forest will bounce back from this,” Halman said.

Halman and Isselhardt both emphasized that the forest tent caterpillar is a native species, not an invasive pest, and that the ecosystem has evolved a response to the occasional spikes in its population.

A fly species known colloquially as the “friendly fly” preys on the caterpillar, and its population swells in response to outbreaks after a slight delay. The forest tent caterpillar population is also kept in check by a virus and fungus, the latter of which Halman said will likely flourish because of this summer’s rain.

Another benefit of the recent wet weather is that it will help the defoliated trees put out another round of leaves before winter.

“The leaves might be a little smaller, they might be a little discolored, but the trees will get another flush of foliage,” Halman said.

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