SHARON — State foresters and entomologists uttered the same four-letter word when asked what it would take to avoid a repeat of last year’s widespread defoliation of hardwood trees by ravenous caterpillars this spring.

“Rain.”

Late last May, for the second year in a row, spongy moths in their caterpillar stage chewed their way through thousands of acres of forests and hillsides the northern Litchfield County

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

The only thing that may impede a three-peat this spring is precipitation, as wet, moist conditions activate a naturally occurring soil-borne fungus which is lethal to only spongy moth caterpillars, said Christopher Martin, director of forestry for the Connecticut Department of Energy and Environmental Protection (DEEP).

The forester said he was concerned that many trees, especially oaks, may not be able to handle the stress of three consecutive years of defoliation.

“This week does not look good. The caterpillars are hatching,” Martin said in a May 15 interview.

The prior week had been sunny and dry, and the state forester said conditions are similar to the previous year where the soil moisture did not increase in time to activate the maimaiga fungus, a fungal pathogen which kills off the invasive insects.

“We’re not currently in a drought, although we have been getting short, episodic seven to 10-day stretches without a drop of precipitation,” said the state forester. “We need the rain early in spring to late May.”

It did rain Saturday, May 20, but if that is followed by a stretch of warm, dry days it may not be enough to stop another spongy moth feeding frenzy.

“We’re concerned,” said Martin. “Ideally, a half inch per week would ensure continued soil moisture. Anything less than that and the chance of caterpillar survivability increases.”

Gale Ridge, an associate scientist with the Connecticut Agricultural Experiment Station (CAES), said “If we can get rain, and the cool nights slow them down and knock them back,” there is a chance of avoiding a spongy moth outbreak, she said.

Once the caterpillars are infected with the fungus’ pollen-size sized spores, it takes about eight days for them to die.

Ridgeline trees most vulnerable

Spongy moths, previously called gypsy moths, were renamed for their spongy-looking egg masses.

Spongy moth eggs generally hatch between late April and mid-May. The larval, or caterpillar, stage usually lasts about seven weeks. The larvae are the most active during May and June. Adults emerge in late June through the middle of July and can persist into August.

Oaks are the spongy moths’ preferred food choice, although the voracious insects will also attack conifers and deciduous trees.

Most trees grow new leaves in July, although environmentalists said tree mortality is possible after two or three years of moderate to severe defoliation, especially during periods of drought and particularly to older, stressed trees.

The 2021 and 2022 spongy moth infestations, said DEEP’s Martin, already caused significant die-off in forests, hillsides and backyards.

The outlook is grim for White oak, said the state forester.

“They need to push out a new set of leaves and put all their effort into new leaf growth, which really exhausts them, so when they shut down in September, they have not recovered necessary nutrients into their root systems.”

The trees most vulnerable during spongy moth outbreaks are those on ridges and hilltops, where the soil is thinner and there is less moisture.

“They are the ones at the most risk as they lack the energy storage for winter dormancy that trees in the lower-level valleys have due to deeper soil,” said CAES’s Ridge.

“It usually takes three years of severe infestation to knock out a healthy oak, and trees that are marginal could be kicked into dying.”

Egg mass counts for 2021, 2022

Aerial surveillance by CAES of egg mass counts per acre in 2021 and 2022 reveals that, overall, egg masses declined in some Litchfield County towns, and increased in others.

For example, surveys counted 208 egg masses per acre in Kent in 2021, and 528 in 2022. Winchester had zero in 2021, and 48 in 2022.

Towns showing a decline in the past two years include North Canaan, where 3,200 egg masses were counted in 2021 compared to 848 in 2022; Salisbury had 640 egg masses per acre in 2021 and 48 in 2022; and in Sharon 9,600 masses were counted in 2021, vs. 192 in 2022.

The decline in egg masses, said Ridge, is the result of fewer caterpillars making it to reproductive adults.

“This is a direct result of fungal and disease activation rates. The Northwest Corner was a hot spot last year, because clearly the area did not have enough rain during the crucial May fungal inoculation period. It’s all about whether it rains or not.”

There is only one generation of the spongy moth each year, according to a report written by Kirby Stafford III and Victoria Smith of the CAES Department of Entomology.

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, ascent the host trees and begin to feed on new leaves.

Spongy moth egg masses on a Millerton crabapple tree. Photo by Deborah Maier