UMass Amherst Entomologists Report Major Progress in Reducing Populations of Winter Moth in New England

Joseph Elkinton
Joseph Elkinton

AMHERST, Mass. – Entomologists Joseph Elkinton, George Boettner and Hannah Broadley at the University of Massachusetts Amherst are declaring victory this month over the invasive winter moth – which begins life as a leaf-chomping, tree-denuding caterpillar that had threatened widespread defoliation of coastal New England shade trees and blueberries until the researchers introduced a natural parasitic fly that has greatly reduced moth numbers.

Elkinton says, “After 14 years of effort, we have successfully converted winter moth, a major new defoliator invading eastern New England, into a non-pest, presumably on a permanent basis. I’m pleased to report that we have now saved Massachusetts residents, who have been battling defoliation on their backyard trees and blueberry crops, millions of dollars of pesticide costs now and in the future. We have averted what was shaping up to be another major invasion calamity for the entire United States comparable to gypsy moth.”

He adds, “We accomplished this success by introducing a parasitic fly that attacks winter moth and no other species, an approach called biological control. It has vastly reduced the density of this insect pest.” This approach has been tried many times around the world against many invasive insect pests, Elkinton says, but complete success like this is “quite rare, at least on forest trees,” he notes. “In fact, I can’t think of any other example involving a major forest insect in North America.”

Winter moths are so named because adults emerge in late November, when males fly while temperatures are above freezing to find females and to mate. The wingless females then lay eggs on tree stems. These hatch in late April and the “inchworm” larvae bore into the host tree’s buds, causing significant damage even before leaves can emerge. After weeks of feeding on leaves, by late May the caterpillars drop to the ground and burrow into the soil where they spin a cocoon and pupate until adults emerge again in November.

Winter moths came to Nova Scotia, Canada, sometime before 1950 from Europe and arrived in New England in the 1990s, the insect researchers say. DNA analyses indicate that this was a new invasion from Europe and not from Nova Scotia. Outbreaks with defoliation were first noted at sites north and south of Boston before 2003 and these spread over most of eastern Massachusetts, Rhode Island, southeastern Connecticut and coastal Maine over the next decade. Once the infestation took hold, the caterpillars caused widespread damage to trees, orchards and blueberries across the region.

In 2005, Elkinton and Boettner began the biocontrol program, which had been earlier shown to successfully attack and reduce winter moth populations in Nova Scotia and British Columbia. They collected specimens of the moths’ natural enemy, a parasitic fly known as Cyzenis albicans, rearing them in the UMass Amherst lab and releasing them in spring at a couple of dozen sites in New England. By July 2018, they had released flies at 44 sites and had verified that the flies were established at 38 of them.

This accomplishment has gone unnoticed for the most part in western Massachusetts and interior New England, Elkinton notes, because winter moth was not established inland. They apparently cannot survive the colder winters away from the more temperate coast, he points out.

He, Boettner and graduate student Hannah Broadley, in cooperation with the USDA Forest Service, have just released a pamphlet, “Biological Control of Winter Moth in Northeastern North America” that is available to the public online. It contains a brief history of the invasive species, maps, photos and graphs. It advises coastal residents who see moths and caterpillars to do nothing and let the parasitic flies catch up and reduce populations.

But for residents in western New England who believe they have encountered the pest, the entomologists ask for samples to be sent to their UMass Amherst lab for DNA-based identification. If warranted, new releases of Cyzenis albicans can be requested, they note. Elkinton says that after the parasitic fly is established in an area with winter moths, after a few years both the moth and the fly will remain at lower densities. This is what happened in Nova Scotia in the 1950s.

“The object of biological control is to reduce density of the invasive species to non-pest status,” Elkinton notes. “That is what we believe we have achieved. One of the main features of Cyzenis albicans, the parasitoid that we have released, is that it appears to be an absolute specialist. It does not prey on anything but winter moth. Other native inch worms are not attacked.”

After the parasitic fly is established in an area with winter moths, within a few years both the winter moth and the fly will remain at lower densities. That is what happened in Nova Scotia in the 1950s, they point out.

This work had early support from Sen. Bruce Tarr (R-Gloucester) and then-representative Matthew Patrick of Falmouth and was funded for two years by the state legislature. More recently, the USDA Forest Service and its Animal and Plant Health Inspection Service have funded the project.

Winter Moth pamphlet and report are available from: