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Shenandoah National Park launches aerial attack on damaging moth

Spongy moths can strip a tree of its leaves and ultimately kill it.
National Park Service
Spongy moths can strip a tree of its leaves and ultimately kill it.

A lone helicopter will be flying low across the area around Big Meadows in Shenandoah National Park this week – the pilots hoping to defend oaks and other trees that suffered in previous years. Evan Childress is chief of natural and cultural resources at the park.

“In 2022 we had about 10,000 acres of trees that were defoliated," he recalls, "and in ’23 about 17,000.”

The culprits were the offspring of what used to be called gypsy moths before bug experts began using a new name – spongy moths.

“They have a raised clump of eggs that overwinters on the trees that’s slightly spongy," Childress explains.

After a time in those beige blobs, hungry black caterpillars emerge.

“They eat little holes in the leaves, and they can actually strip the leaves entirely off a tree.”

Spongy moths were brought to this country in the 1860’s by an entrepreneur who thought they could be used to produce the raw material for silk. Instead, they escaped their Massachusetts home and have been spreading across the country ever since.

But Childress says a pesticide called BT can be deployed to limit the damage.

“It’s among the most common pesticides used in the United States, and it’s used in certified organic agriculture. It is specific to moths and butterflies that are actively feeding at the time of treatment. It only lasts for a few days. When it’s out in nature it degrades quickly.”

And timing is everything. The pesticide must be sprayed when the caterpillars are young. The older they get, the more finicky they are.

“As caterpillars get larger they start to be a little bit more discerning about what they eat and can avoid the areas of treatment,” Childress says.

Fortunately, he adds, there are not many other species of caterpillar feeding at this time, so Childress says there should be limited collateral damage as a result of this aerial attack.

“There is one population of moth in the area that we had some concerns about, and so we are not treating that area. We are trying to avoid anything that’s sensitive.”

This is the first time since 2008 that the park has felt it necessary to spray. A naturally occurring fungus had kept the spongy moth population in check until dry weather sidelined that fungus.

“We were not getting significant defoliation in 2020 or 2021, and then in 2022 we saw these larger areas. In ’23 they grew even more, and so that triggered the decision to treat this year in 2024 to make sure those populations would be knocked back.”

He says the spray being used is not hazardous to humans or to wildlife, but this week visitors are being advised, and signs around the 3,000-acre treatment zone warn people to use trails and campgrounds in other parts of the park.

This report, provided by Virginia Public Radio, was made possible with support from the Virginia Education Association.

Sandy Hausman is Radio IQ's Charlottesville Bureau Chief