Years-long spongy moth epidemic fades

MILLBROOK — The spongy moth epidemic of eastern Dutchess and Columbia Counties and northwestern Connecticut appears to be receding after more than three years of infestation, based on forecasting work and early-summer land surveys undertaken at Millbrook’s Cary Institute.

The spongy moth epidemic has marked widespread defoliation, affecting trees particularly in the area of the New York–Connecticut border.

The current epidemic appeared in Dutchess and Columbia Counties and in Connecticut’s northwest corner in 2021 as a result of various regional ecological factors, combined with the spongy moths’ ability for rapid proliferation.

“The white-footed mouse, which preys on spongy moth [egg masses], is usually enough to keep the population under control,” said Clive Jones, a terrestrial ecologist at the Cary Institute. “If the mouse population collapses, as it did, then the spongy moth population can escape.” Populations of parasitic wasps and flies, which bring the population down to normal levels, can take a few years to grow enough to curb the spongy moth. During this time, when the moths have few significant predators, their population grows unchecked and they rapidly defoliate trees.

This year’s wet spring contributed to the counter-epidemic of both a virus and a fungus antagonistic to spongy moth larvae.

The Cary Institute, in its 2025 forecast of the spongy moth’s prevalence, estimated the outbreak would abate during the summer, leading to a negligible population of spongy moths in previously-affected areas. “We see the population collapse due to the fungus and the virus and due to the other natural enemies’ population growing because of the moth’s rise,” Jones said.

Given the passage of an appropriate period of time for natural mechanisms to control the moth population, “we knew at Cary that there was a very low probability of much defoliation by the spongy moth [in previously-affected areas] this year,” Jones said.

Jones and his colleagues at the Cary Institute compiled their forecast of the danger the spongy moth would pose to trees this summer through the collection of data at the Cary Institute’s own 1,924-acre arboretum, the observation of satellite imagery and through samples taken around the Hudson Valley and into Connecticut’s northwest corner.

Defoliation rarely kills trees directly, even if it occurs for years in a row. But defoliation in conjunction with other harmful factors does kill trees. “Many of the trees out there are fine… but quite a lot have succumbed due to the drought in 2022 and then defoliation in ‘23 and ‘24,” Jones said.

“Spongy moth flare-ups are not cyclical in the sense that they are regularly spaced,” Jones said. “You can fit a periodicity to spongy moth outbreaks which shows that they occur about every 10 years, but it’s no more than a very rough guideline and can be completely wrong.” Before the 2021 epidemic, the last spongy moth infestation occurred in Millbrook nearly

30 years prior, limiting the statistical measures which can be taken to forecast a future spongy moth epidemic.