Fungus causes spongy moth decline

Northwest Corner residents have become all too familiar over the past two years with spongy moth caterpillars chewing their way through forests and backyards, devouring any bit of green and leaving a mess of frass in their path.

But this year, despite patchy defoliation in some areas, the invasive insects seem almost non-existent, according to state environmental officials.

The fuzzy caterpillars have almost entirely been killed off by a naturally occurring soil-borne fungus in the soil which is activated by moisture and is lethal only to spongy moths in their caterpillar stage.

“We are seeing lots of dead caterpillars, lots of spent egg masses and very little defoliation. Good news all around,” said Dr. Victoria Lynn Smith, state entomologist with the Connecticut Agricultural Experiment Station (CAES) in New Haven.

Christopher Martin, director of forestry for the Connecticut Department of Energy and Environmental Protection (DEEP), said while there may be “a few remnant caterpillars lurking about,” he believes that the 2021-2022 infestation in Northwest Connecticut has ended, based on reports from state field foresters.

Martin noted that “far less defoliation” has been reported this year compared to the past two years.

“There are reports of some defoliation, but sporadic,” he said. “The damp spring activated the fungus so early caterpillar instars did not survive. And dead caterpillars mean less moths which lead to far less egg masses which would hatch caterpillars in spring of 2024.”

Dr. Gale Ridge, associate scientist with the CAES, also noted last week that the spongy moth mortality sets the stage for what could be a much-needed break for stressed-out trees next year.

Caterpillars hatch from buff-colored egg masses in late April to early May, which may contain 100 to hundreds of eggs and may be laid in several layers. A few days after hatching, the quarter-inch long, buff-to-black colored caterpillars, or larvae, ascend the host trees and begin to feed on new leaves.

In an interview earlier this spring, Ridge had said that a combination of rain and cool nights would “slow them down and knock them back.” As it turned out, that is exactly what happened.

DEEP’s Martin noted that the Connecticut Agricultural Experiment Station expects to have detailed reports of acreage defoliated by the spongy moths later this summer/early fall from aerial surveys.

In May of 2022, for the second year in a row, spongy moths in their caterpillar stage devoured thousands of acres of forests and hillsides in northern Litchfield County.

By June of that year, the landscape, particularly in hard-hit Sharon and Cornwall, was eerily barren.

According to the CAES, while the outbreaks over the past several years were severe, they don’t match the estimated 800,000 to 1.5 million acres impacted in Connecticut in the 1970s and 1980s.

“Unfortunately,” DEEP’s Martin noted, “much damage has been done and on the hilltops of Litchfield County there are areas of significant oak canopy mortality.”

CAES’s Smith said the state will begin assessing the extent of damage caused by the voracious insects within the next few weeks.

“We are predicting some defoliation, but probably not as severe as in previous years. Many trees have already died due to the combined effects of defoliation and drought.”