As Vermont navigates its second year amid a spongy moth outbreak, scientists face an uncertain future on what the coming years will look like.
The outbreak began last summer and can last for two to five years, according to Josh Halman, the Forest Health Program Manager for the Vermont Department of Forests, Parks and Recreation. Outbreaks typically occur every six to 10 years.
The caterpillar form of the spongy moth, formerly known as the gypsy moth, strips away huge swaths of leaves — known as defoliation — from several species of trees. They prefer to munch on oak trees, but will dine on a range of other hardwoods, such as maple, apple and birch, and will even dabble in softwood trees, such as spruce and hemlock.
According to Judy Rosovsky, the State Entomologist for Vermont, spongy moth defoliation is not likely to kill a tree, but the process adds another stressor to the tree's stored energy. Many trees regrow their leaves in the same season, especially hardwood species. Softwood species are more at risk of mortality as a result of defoliation.
In 2021, the Department of Forests mapped just under 51,000 acres of defoliation across the state. The Champlain Valley, along with the western side of the state, was most affected, but the defoliation spanned from Franklin to Rutland counties.
The Department of Forests has not determined the overall defoliated acreage this year, but Halman reported that defoliation is on the rise from last year, with new spots in the southeast corner of the state.
“That was kind of to be expected with what we found with our egg surveys, which are our predictive tool that we use in the fall and winter to look at how many eggs were laid in different parts of the state,” said Halman.
The state uses aerial mapping to determine the total acreage and Halman says they should have this year’s data by next month.
But even though this year’s defoliation total is proving higher than last year’s, Halman said more caterpillars died this year, which he referred to as “good news.”
The crawlers’ higher death rate can be attributed to the return of their “natural enemies,” said Rosovsky.
The lack of drought conditions is the most notable return. According to the Department of Forests, when an environment has more moisture, the spongy moth’s natural predator, the fungus Entomophaga maimaiga, can grow more effectively to maintain the caterpillar population.
Vermont has faced a dry season every year since 2016, said Rosovsky. This spring, the state had a much more wet spring than in previous years, which helped the fungus control the number of caterpillars.
Rosovsky identifies climate change as a major indicator to the future of spongy moths. “If we continue to get a lot of dry weather, we're going to see a reduction in the fungal population,” she said.
But as more areas become affected by the spongy moth, Vermonters often look for at-home solutions to stop the mass defoliation.
Vermont Invasives — an online resource developed by the University of Vermont Extension; the Department of Forests, Parks and Recreation; the Vermont Department of Environmental Conservation; and the Vermont Chapter of the Nature Conservancy — recommends trapping the caterpillars with burlap during the defoliation stage and scraping the egg masses off of trees in the fall. It also advises putting the caterpillars and eggs into a container of soapy water or alcohol to kill them.
Aerial spraying and chemical controls are also options for those with large infested areas.
Currently, the state does take active control measures but offers education and outreach for those impacted by the spongy moths.
Rosovsky also highlighted that spongy moths can easily travel to new places through human-assisted movement and start infestations in previously untouched areas. Spongy moths lay their eggs across varying surfaces, including cars, bikes, outdoor furniture and yard tools.
“When people pack up and move they can inadvertently bring a couple of egg masses with them and start a whole new infestation,” Rosovsky said in an email.
While the spongy moth outbreak has its own characteristics and distinctions, many scientists make the comparison between them and the forest tent caterpillar.
In 2018, the Department of Forests, Parks and Recreation mapped just over 71,000 acres of defoliated forest in Vermont due to the forest tent caterpillar.
Additionally, forest tent caterpillars prefer to munch on sugar maple trees, posing a potential problem for the maple syrup industry. Spongy moths, if hungry enough, will also dine on sugar maples.
According to Mark Isselhardt, a maple specialist at UVM Extension, spongy moths could have a negative impact on sap production when combined with other stressors and factors, such as drought and weather conditions during the sugaring period.
Depending on when that theoretical tree loses its leaves, he said, the new leaves “are almost always stunted and less productive in terms of producing new sugar compared to the original settle leaves,” Isselhardt said. “They're just not as efficient at converting sun to sugar.”
Even though spongy moths aren’t as likely to defoliate a tree to the extreme as a forest tent caterpillar, Isselhardt says they’re still something to watch out for in the sugarbush.
“Anything that impacts the forest, even if it isn't their crop trees, we are concerned about,” he said, referring to the crop trees of maple sugaring.
But for scientists, the answer to what the spongy moth outbreak will look like in the coming years is still undetermined. This year’s egg mass count conducted in the fall will be a good indicator of what to expect, said Halman.
“Based on those numbers, we're able to predict whether or not the outbreak has peaked or if it's in decline or if it's still increasing,” he said. “It's pretty hard to say right now in July what this is going to look like in the years to come or even next year.”
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