In May, an army of spongy moth caterpillars were overtaking trees and showed no signs of slowing down, which could spell danger for tree health. But now many people are noticing a decline in population.

"You're seeing some caterpillar mortality right now, but it doesn't necessarily mean that the the numbers for the outbreak on the decline," said Josh Halman, the forest health program manager at the Vermont Department of Forests, Parks & Recreation.

The caterpillar invasion is slowing down, especially in the Champlain Valley. Margaret Skinner is a research professor in entomology at the University of Vermont. She has seen the recent decline in spongy moth caterpillars because the conditions are perfect for two conditions which are deadly for the creatures.

There are two culprits for the current demise of spongy moth caterpillars, and it's easy to spot when looking at the caterpillars themselves. The shape in which the caterpillars die indicate if they were infected with a fungus or a virus, according to Skinner.

Caterpillars which die in a "v" shape were most likely infected with a nuclear polyhedrosis virus (NPV) and if the caterpillars die in a relatively straight line, attached by the front or hind legs, then they were likely infected with a fungus (Entomophaga maimaiga), Skinner said.

The fungus and virus are subject to environmental conditions, Halman said, which in turn, greatly affects the caterpillar population. "When we had the drought last year, that's when they just aren't very effective and luckily," Halman said. "We we saw increased activity with more rain this year."

Spongy moth caterpillars are seen a lot in the Champlain Valley and Halman agrees that the animals live and thrive in the western counties of the state, for a particular reason.

"The host species spongy moth really prefer is oaks and that's where a lot of our oaks are located," Halman said. However, the Department of Forests, Parks & Recreation has been mapping the damage of the caterpillars via air throughout the state.

"We have had some reports that in certain spots it's more widespread than it was last year and in particular we didn't have any reports last year in the southeast portion of the state, but this year we know that there were some areas that were hit," Halman said.

The damage that spongy moth caterpillars inflict on Vermont trees is more complicated than their population in the spring. There are a number of factors that affect the the removal of leaves by caterpillar), and healthy trees can actually survive a cycle of caterpillar defoliation.

"Usually it takes more than one year of defoliation to actually kill a tree," Skinner said. The caterpillars, unlike some other animals, have one life cycle a year, which limits the damage they are able to inflict on the environment, according to Skinner.

The wet conditions this spring and summer have provided the perfect concoction for the fungus and virus that kill the spongy moth caterpillar. However, next season may bring drier weather, so don't expect this drop in population every summer.

Kate O'Farrell is a reporter for the Burlington Free Press. You can contact her at KOFarrell@freepressmedia.com.