Winter Moths Are Coming: The Threat of Defoliation

The winter season isn't the only thing stripping leaves off trees and leaving them bare. Cold months also bring with them a veritable swarm of winter months, insects that could lay eggs and potentially cause another spring of defoliated and dying trees.

Heather Faubert of the Plant Protection Clinic at the University of Rhode Island, after monitoring the population of the insects each year, warned that the invasive adult moths would appear from the ground during Thanksgiving and complete their lifespan just before January 2017. "Their caterpillars defoliated 27,000 acres in Rhode Island in the spring of 2015, but even though we had winter moths everywhere last year and I saw a zillion eggs, they caused almost zero defoliation."

Last year's statewide defoliation in New England began after winter moth caterpillars had long become inactive. This defoliation was caused primarily by gypsy moths and, in some communities, forest tent caterpillars. Winter moth eggs typically hatch during a warm spell in April, but last year they began hatching during a warm period in late March. Two weeks later, in early April, temperatures dropped well below freezing and probably killed many of the caterpillars.

"I went looking for dead caterpillars but didn't find many," she recounted. "Maybe the caterpillars hatched too far ahead of the foliage development, so they didn't have anything to eat. I'm not sure what really happened, but it definitely had something to do with our screwy weather."

Faubert warned that the combination of several years of defoliation in a row and the extended drought conditions could result in more trees dying in the coming year.

One strategy Faubert is using to control winter moth populations is the release of a tiny parasitic fly that lays its eggs on tree leaves. When the winter moth caterpillar consumes the eggs while eating the leaves, the eggs hatch inside the caterpillar and the fly larva eat it from the inside out. This method has succeeded in controlling winter moth populations in Wellesley, Massachusetts, and it appears to be on its way to having favorable results in Seekonk as well.

Faubert released the flies in seven locations in Rhode Island between 2011 and 2015, and she hopes to soon see signs that it is beginning to work.

"It's still too early to tell, but we hope the flies will get our moth population down to manageable levels," she concluded.