One doesn’t have to walk very far into the Sharon Town Forest before noticing that the once-lush hemlock trees don’t have their needles anymore.
And there are caterpillars hanging from silk threads, dangling off of nearly every branch and dropping onto the forest floor, including on unsuspecting hikers.
Swatches of bare hemlocks, one of the most common trees in the area, stretch for thousands of acres in the Monadnock region, specifically parts of Sharon, Peterborough and Rindge.
This heavy defoliation is caused by an insect larvae: the hemlock looper caterpillar.
“Hemlock looper is a native insect, a native moth, found throughout North America. They are pretty common where we have hemlock,” University of New Hampshire cooperative extension forester Steve Roberge said. But this year their numbers have exploded. Roberge explained, “They can be under the radar for a long time, and then there’s an outbreak.”
This is what the Sharon Town Forest and parts of the Fremont Conservation area started experiencing last year; the outbreak has continued into 2022.
It has been decades since the last time the species made its mark on the region.
“We haven’t seen an outbreak of hemlock looper since the ’80s,” Roberge said.
The hemlock looper is an inchworm. Its body creates a loop as it moves, hence the name. Roberge explained that they are sloppy eaters. They will take a bite out of a needle and then move along to the next one. The remaining part of the needle will die and fall off, so the caterpillar is able to cause a lot of destruction rapidly.
On Friday afternoon Roberge and other cooperative extension foresters, local experts and most of the Sharon Conservation Commission walked into the woods, the trailhead on McCoy Road in Sharon, to have a look at the damage, talk about hemlock loopers, and discuss possible options for the forest.
“Hemlock produce a really dense canopy and provide a lot of shade,” Bill Davidson said. He’s with the New Hampshire Division of Forest an Lands in the forest health program.
The hemlock loopers have wiped out the hemlock in the understory. A few of the tallest trees still have needles on their tops, but it’s unclear how many trees will survive such heavy defoliation.
And the Monadnock region is in a drought. Roberge explained that this will put extra stress on the trees, making it harder for them to survive this.
“It’s an additional stress climate change is putting on all of our trees,” Roberge said. “Hemlock is very susceptible to drought ... Droughts can lead to more insect outbreaks.”
Roberge said that these outbreaks are usually ended by natural forces balancing out the ecosystem. The hemlock loopers may have eaten all the hemlock needles in the area or fungus can spread and that can kill the caterpillars. Fungus, however, grows best in damp conditions.
And there are other insects that target hemlocks as well. When both the elongated hemlock scale and hemlock woolly adelgid are on one tree, it “causes pretty quick mortality,” Roberge said. Both exist in New Hampshire.
Davidson pointed out a woolly adelgid on the bottom of a needle in a patch of forest where there were still some Hemlock with foliage. It looked like a tiny white orb.
“When trees get stressed out their immune systems are weakened,” Davidson said. He explained that this leaves them at higher risk for secondary pathogens and other outside forces.
Davidson said the hemlock loopers are getting ready to pupate. They will create a cocoon in the leaf litter on the forest floor and emerge as moths in August and September.
Roberge emphasized that there will likely be a lot of moths in the defoliated forest during that time.
And if there are not? Davidson said that might be a good thing. It might mean the outbreak is ending. The moths mate and lay eggs for next year so if there are fewer moths there will be fewer caterpillars the following season.
The areas of forest that have been impacted by this outbreak “will definitely be different,” Roberge said. “For folks who have experienced looper, there has already been a big change.”
But this will also change the composition of the forest in the future. “I think there will be some hemlock that comes back but we’ll see a lot of dead trees,” Roberge said, “With trees dying comes the opportunity for other trees to grow.”
The cooperative extension foresters are available to visit woodlots and advise communities and landowners on forest health related issues. They can be found at extension.unh.edu/countyforesters.