When caterpillars eat our trees

What Impact is All This Caterpillar Poop Having on Forest Floors?

By TODD McLEISH/ecoRI News contributor

Denuded trees in Exeter seen from the air (Patch)

The massive defoliation of trees in southern New England by winter moth and gypsy moth caterpillars this spring and summer has totally changed the look of the regional landscape. And while scientists say it’s unlikely that many trees will die as a result of one year of defoliation, it raises the question of how it will affect other species of wildlife.

University of Rhode Island ornithologist Peter Paton noted that several varieties of songbirds are likely benefiting from the huge number of caterpillars swarming the area. Black-billed and yellow-billed cuckoos, two species that are known to eat large numbers of caterpillars, including the prickly gypsy moth caterpillars that many other birds avoid, are likely to thrive this year. 

Yellow-billed cuckoo (© Joe Fischer in Birdnotes)

He said birdwatchers in the region have noticed an unusually large number of very active cuckoos since the birds arrived from their wintering grounds in South America in May. As a result, these birds will probably have a very successful nesting season.

“Last week I was watching some robins 40 feet up in a tree foraging, which is a pretty unusual place to find them eating,” Paton said. “So I’m guessing they were probably feasting on caterpillars, too.”

Paton also observed blue jays and hairy woodpeckers tearing apart some of the abundant caterpillar cocoons, another unusual behavior brought about by the caterpillar infestation.

“And if the defoliation ends up killing trees,” he said, “that could eventually have a positive impact on woodpeckers,” which consume insects that live in dead trees and which drill nesting cavities in dead trees.

As for other possible impacts on wildlife, he speculated that the absence of leaves on many trees will enable sunshine to filter down to the forest floor and other areas that are typically shady, which may provide additional sunny areas for turtles and snakes to nest and sun themselves.

On the other hand, fewer shady areas may make it more difficult for wood frogs and salamanders living in the forest to remain cool and moist, according to David Gregg, executive director of the Rhode Island Natural History Survey.

“Ferns and other forest floor plants are also more likely to have a negative experience of this phenomenon than a positive one,” he said.

Natural History Survey botanist Hope Leeson said there will be both winners and losers on the forest floor, depending on the needs of the species living there. “If there is an understory of trees and shrubs, they’ll be happy to have the sun.”

Photo by Will Collette

Leeson noted greenbrier as one plant that will thrive with the additional sunlight penetrating to the forest floor, and it will provide benefits to other species that may be at risk.

“At the moment, any small mammal living in the forest doesn’t have any cover,” she said. “Deer have eaten all the tree seedlings and shrub seedlings, so there isn’t anything for the mice and chipmunks to hide under. Once the canopy was removed by the caterpillars, it made it easy for the hawks and owls to see the small mammals pretty well.”

The increased growth of greenbrier, she said, will provide the small mammals with new places to hide.

Both Leeson and Gregg also noted that some unwanted invasive species may also thrive this year, thanks to the defoliation. 

Amur cork trees, for instance, an Asian species, have invaded forests throughout the Mid-Atlantic states and are now found in small numbers in coastal forests of Rhode Island as well. They grow very slowly in the shade, with some 25-year-old trees no more than 8 feet tall with trunks only an inch or two in diameter.

“They just wait it out in a shady situation,” Leeson said. “They just eek out an existence and wait for the moment when there’s light, and that’s when they put on a lot of growth.”

This year could be the year they will shoot skyward.

According to Rick Enser, retired biologist for the Rhode Island Department of Environmental Management, if the region experiences another gypsy moth infestation next year, tree mortality will likely increase, which could create gaps in the forest for new species to move into.

Winged Euonymus, a.k.a. Burning Bush.
National Parks Service photo

“Unfortunately, invasives are highly adept at dispersal, although small infestations within forest openings should theoretically be reduced or eliminated as the canopy returns,” he said.

Enser noted winged euonymus, also known as burning bush, as one invasive species that can spread quickly to new areas and survive when the forest canopy returns. It’s a shrub that has become a primary concern at URI’s W. Alton Jones Campus in West Greenwich.

Since it has been more than 30 years since Rhode Island has experienced such a severe defoliation, many of the environmental effects are uncertain and unstudied, leaving some scientists with more questions than answers.

“I was wondering about the nutrient balance,” Gregg said. “Normally oak leaves breakdown in a certain way at a certain time, but this year they've been consumed by caterpillars and turned into manure and sprinkled all over the forest floor. So is that good for the plants? What’s the nutrient analysis of gypsy moth poop?”

Rhode Island resident and author Todd McLeish runs a wildlife blog.