Thursday, June 9, 2011

This Year's Inchworm Damage

 Just as the temperature began to warm about 10 degrees above freezing and the buds began to break this spring, I spotted this nest of caterpillars.

On a branch above the nest, there was a large insect poised unmoving, intently waiting. . . The beetle's diligence must have paid off with a delicious caterpillar meal, but there were many others that escaped him and his ilk.

That much has been evident for weeks, as I bumped into thread after thread of the ones that got away. These threads are actually a second line of defense against predators for the caterpillars. If the caterpillar is threatened by a bird or another insect, it will let go of the branch with all of its legs except for the rear prolegs and make like a small green twig. If that jig fails, then it lets go with those prolegs and relies on a thread that it fastened to the branch with the spinneret in its head when it originally let go.

This year these caterpillars have left incredible damage in their wake. I took some pictures of a Norway maple on which the leaves have turned to rags, from top to bottom. Other foliage of choice for these worms that move inch by inch have been ash, elm, cherry, dogwood, crabapple, and even oak.

According to University of Massachusetts, the tree must put on a second flush of growth in order to recover. To aid regrowth, it is recommended that sufficient water be supplied, but no application of fertilizer. Drought and other leaf eaters may worsen the outlook of the tree, and if defoliation is incurred just three years in a row, all of the affected branches die.

The inchworms that cause this damage were first thought to be fall cankerworm, which have a similar life cycle to the real culprit, winter moth. Winter moth was introduced in Nova Scotia from Europe in the 1950's. In Europe,there are plenty of natural predators to regulate the winter moth population. As the moth spread to Massachusetts, it became an unregulated pest, killing numerous whole trees. Researchers at the University of Massachusetts have used pheremone traps to determine the range of winter moth over the 11 years it has been present in this state. After it was introduced in Nova Scotia, it was introduced separately in Vancouver, Canada where it also became a pest.

If you see damage to a plant by the winter moth inchworm, either trees or shrubs or perennials underneath a tree where they may have hatched, water the plant to help replenish the growth before any further damage is done by other leaf-eaters. If there are inchworms present, they can be killed on contact with Spinosad.
I haven't seen any of the inchworms for about a week now. By this time of year, late May to early June, they have dropped to the soil to pupate. Research on the west coast has determined that some native parasitic wasps and flies will prey on the pupae.

After pupation they join with a few other species of  familiar brown moths that flock to our porch lights. Last fall was a particularly abundant year for them. As adult moths, they climb up tree trunks to lay eggs which will in turn hatch into caterpillars. The moths can be thwarted from climbing up tree trunks somewhat effectively with sticky bands.

During the last phase before they hatch, dormant oil can be applied to the bark of trees to kill the eggs. With any luck, trees and will break bud and leaf out to provide us with shade and fruit undivided by holes!


  1. When I bring my dogs for walks in the morning, I meet these inchworms. If I am not careful and watch where I walk, I collide into them. Yikes!

  2. Yeah, I've been teaching high school kids some natural history in one of the local forests and all of them seemed to recognize which pest made all the holes in the leaves. Many of them had been colliding with caterpillars in the same way!


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