As winter moth green inch worm caterpillar larvae continue to feed on trees, here’s what you can do.

If you live along the coast of Maine south into Massachusetts and west of Boston you have likely noticed small green caterpillars eating some or maybe all of your trees. Some of you have asked about what these small green inch worms are and how to control them. These green caterpillars are the larvae form of the winter moth. This insect entered the northeast during the past decade and continues to spread west and south. It was found in Canada back in the 1930s after arriving from Europe.

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The green worms don’t eat everything, but they do chew on a lot. Their favorite species are maples, oak trees, cherry trees, crab apple trees. I have seen them eating the new growth of rhododendron, beech and azalea shrubs as well.

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If you stand under a tree you can often hear the fecal matter hitting the ground as the insects eat away. You might even notice little black dots all over anything under the trees. Kinda gross right?

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Last fall, you might have noticed the moths flying around on the few mild evenings we had between Thanksgiving and Christmas. These were the males looking to mate. The female is wingless and lays about 150 eggs in the crevices of trees. After enough warm days, this didn’t happen until late April and early May this year, the eggs hatch. The insect then crawls into the buds of trees and begins feeding. Once the caterpillars get bigger along with the leaves, they can devour the foliage of entire trees in a very short period of time.

If a tree is defoliated it will grow a second flush of leaves, but this puts a lot of stress on the plant. It’s critical the tree or shrub have enough water in order to put out a new set of leaves.

Controlling Winter Moth

You can call certified arborists to come spray your property. After the caterpillars hatch, professionals often use Entrust® to spray. As a homeowner, you can purchase a product with spinsod as the active ingredient and spray smaller trees and shrubs on your own. This product is safe and approved for organic gardening. There are other chemicals you can use, but I don’t recommend them as they are harsh on the environment including people and pets.

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Don’t bother to hire someone to spray your property after the first week of June at the latest. By then, most of the feeding is done and the caterpillars will be pupating. They will remain in that state for the summer and fall until they hatch as moths and the cycle begins again.

Biological Control
There is some good news about the control of this pest. Back in 2005 and 2006, in a cooperative effort by the Department of Entomology at the University of Massachusetts and the Forest Health Program at the Massachusetts Department of Conservation and Recreation, hundreds of parasitic flies known as Cyzenis albicans were released in Hingham. In the past several years, more and more release sites have been added.

Research shows Cyzenis albicans has been effective in combating winter moth populations in eastern Canada. I have had arborists on my own property today collecting data to see if the moth is becoming better controlled.

You can follow more of my musings about weather and gardening on Twitter @growingwisdom.


I suspect over the next several years as more and more information is learned and the parasitic wasp takes hold, we will hopefully see less impact by the very destructive winter moth caterpillar.

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