Spongy moth potentially a threat again next year
This is a flightless female spongy moth, who has laid her eggs on the trunk of an oak and covered them with a felted mat of her own hairs. 
Photo by Willard Wood

Spongy moth potentially a threat again next year

It wasn’t unusual, driving in the Northwest Corner this summer, to see whole hillsides with their trees stripped of foliage, the trunks and branches clearly visible where weeks before there had been an even canopy of leaves.

Over 45,000 acres of forest in Litchfield County were defoliated by spongy moth in 2022, according to a recent tabulation by the Connecticut Agricultural Extension Service (CAES). This followed a 40,000-acre defoliation in 2021 over much of the same area, centering on Sharon and Cornwall. Many of the trees hit hard by spongy moth two years in a row are likely to die.

The hardwoods most severely affected, mainly oaks but also red maple and beech, can generally regrow their leaves once, although they become more susceptible to drought and disease. But the double punch is often fatal.

Jeffrey Ward, who recently retired from CAES, has been following a forest stand on Sharon Mountain where the oaks were defoliated in 2021 and 2022. He noted that 70% of them had recovered less than a quarter of their foliage by late summer.

“From experience and the literature,” said Ward, “this indicates that approximately 70% of oaks in those areas will be dead within a couple of years.”

The culprits of spongy moth defoliation are the caterpillars, which hatch in the spring as tiny, quarter-inch crawlers and climb to the canopy to start feeding on leaves. Over the spring and early summer, they will molt through four or five stages (instars), finally reaching about two to three inches in length.

It’s the voracious appetite of the later instars that we typically notice, both because of the leafless trees above and the obnoxious frass or droppings below.

In late June or early July, the caterpillars will stop feeding and pupate, emerging either as a dark male moth or the flightless white female moth that lays her eggs in tight clusters, by preference on the trunks of oaks, and cover them with a tan-colored felt made from her own hairs. Where infestations are heavy, egg masses can also be found on the trunks of beech and white birch, and even on the sides of houses and in the wheel wells of cars.

Each year, the CAES performs a winter survey of spongy moth egg masses across Connecticut, and when the counts are alarmingly high, as they were last spring in western Litchfield County, the state entomologist, Victoria Smith, will put out a bulletin.

The main check against spongy moth infestation is an introduced fungus, Entomophagia maimagia, now endemic in the Northeast. When spring rains are abundant, the fungus becomes activated and invades the bodies of late-instar spongy moth caterpillars, killing them. These can be seen hanging motionless on the trunks of trees, a grisly but welcome sight that signals the end of an infestation.

Some caterpillars manage to pupate despite the fungus, develop into moths, and lay their eggs for next spring’s crop.

To get a rough sense of the spongy moth presence in your area, locate ten or twelve large oaks and examine the trunks attentively for the tan, felt-like patches of the spongy moth egg masses. If you see none or only one or two on each tree, chances are you will escape a heavy infestation next year.

The CAES ranks anything less than 200 egg masses per acre as a light infestation. If you see four or five egg masses on many trees or, as sometimes happens, clusters of twenty or more, you might think of taking action.

A licensed arborist will have a variety of options for treating a backyard tree or woodlot for spongy moth, and the programs will range from benign to aggressive. The CAES website has a thorough and well-illustrated section on the pest.

If you decide to take matters into your own hands, you can either spray the egg masses with an agricultural oil or insecticidal soap, soaking them completely, or you can scrape them into a container and dispose of them (scraping the eggs onto the ground doesn’t work).

Will it protect your trees? Well, the first-instar caterpillars trail a fine filament behind them as they climb and use it to balloon in the wind, traveling up to 150 yards.

But each egg mass can contain up to 1,000 eggs. And they do make a satisfying crackle when tossed into the fire.

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