Plan Now for the Coming Winter Moth Infestation

Submitted by Melinda C. Dietrich for BARC,  Bedford’s Arbor Resources Committee

Winter moth caterpillar - Image (c) Burlington Science Center -
Winter moth caterpillar – Image (c) Burlington Science Center,

Now is a good time to begin planning for what may be a bad spring infestation of the Winter Moth. This insect has a voracious appetite for our northeastern native trees and shrubs—especially red maples and oaks– and it kills them quickly.

Is it a real concern?
Discovered in Massachusetts in the late 1990’s, by 2005 the winter moth had expanded its range, defoliating over 30,000 of acres in eastern coastal Massachusetts, both north and south of Boston; near the western border of Massachusetts; and, most important to us, in the town of Bedford. By last year, it had defoliated 89,000 acres in Massachusetts alone.  Native to Europe, here it is an invasive species with no natural predator other than cold weather. It is believed they will remain a hazard for ten more years, inexorably expanding their terrain west and south.
Adult moths emerge around Thanksgiving and can remain actively mating into January. Ken Gooch, Supervisor of the Forest Health Program at the MA Department of Conservation and Recreation, warned that next spring homeowners and towns could see large winter moth infestations in native trees and shrubs. Many of us thought last spring’s infestation was bad, but it was less severe than expected, because there was an early thaw followed by a cold snap: only 10,000 acres in Massachusetts were affected.

What do they look like? How do they behave?
An estimate is that about 100,000 caterpillars can hatch in one tree. Males, small, light brown to tan in color with fringed wings, are attracted to lights at night, which is why you see them flying in the headlights of your car and gathered by your house’s front door and garage lights. At dusk, clouds of male winter moths fly around tree trunks, resting on the bark and mating with females. After mating, females scurry up the tree to lay egg clusters of up to 150 eggs each in protected areas under lichens and bark scales and in bark crevices on the tree trunks and branches. You can see the small, gray, wingless females crawling up the tree trunks, sides of houses and other vertical surfaces. Both males and females die soon after mating and laying egg deposits.

The pale green eggs turn reddish orange prior to hatching when temperatures average around 55° F., usually early March to mid-April. Larvae appear as pale green caterpillars with a white stripe running down each side of the body. Known as “loopers,” or inchworms, larvae “weasel” between the bud scales, bracts, etc. to get into the buds, feeding inside one, then another, until they reach about 1 inch at maturity when they are able to feed on the unfurling foliage, leaving “lacy skeletons.” When tiny (less than 1 mm), if the buds are not yet swollen enough for them to gain access, the caterpillars can spin down from the tree on a silken thread to then float upwards on air currents into the tree canopy in a dispersal process known as “ballooning.”  This stage causes high levels of injury to the host plant. The caterpillars eat so much that their droppings can be so numerous that it sounds like rain when they fall. Winter moths also drop down from trees on to bushes below, where they feed on perennials, such as roses. Around mid-June they migrate to the soil to pupate until the fall, when the cycle begins again.

What can you do about the winter moth?
Homeowners can hire a professional company, or apply their own treatment in spring when the larvae first appear. While still on the host plant, winter moth larvae are exposed and very treatable with a variety of products. Timing treatment while the insects are small is important to ensure protection for the growth of new buds. Dormant oil sprays typically applied in the fall are of no use for winter moth, because the oils work by suffocation and are only effective when they can completely cover the eggs. Since the female moth protects her eggs by laying them in crevices and under lichen, the eggs will not be completely covered, and, therefore, not killed.
Do-it-yourselfers can purchase off-the-shelf insecticides available at local retail stores. However, Robert Childs of the UMass Center for Agriculture warns that it is extremely important to know what chemicals can or cannot be mixed together and which can be used on a particular plant to avoid serious injury to the plant, the environment—and the applicator. A subsequent article will address how we can control the winter moth in more detail.

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James Hudzik
James Hudzik
9 years ago

Now is too late! Unless you have small trees/bushes that you can spray exactly when the larvae emerge you can’t stop them. You have to interrupt the life cycle in the late fall and wrap your tree trunks with sticky stuff to stop the females from crawling (can’t fly) up the trunk to lay their eggs in crevices which hatch in the spring and come out hungry for your leaf buds. I did it this past fall and we will see how it worked. For reference look at UMass Amherst Winter Moth

9 years ago

The problem is, you can treat the larvae on the host plant- your apple tree, your blueberry bushes- hopefully with something biological, but a little later larvae will swing off the neigboring street trees and woods (maples especially) to your garden nevertheless. I noticed less mating and egg laying last November around the trees in my street and am hoping that this is all part of a cycle.

Stephan Hovnanian
9 years ago

Good to know; I actually have lots of bees, not moths, but will definitely be doing some pre-treating this spring.

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