Studying a winter moth invasion

Winter moth caterpillars from one infested oak tree

By Zoe Hufnagel and Amy Palmer

Winter moth is an invasive species of moth from Europe. An invasive species is a living organism that comes from a different place and takes over. Winter moth has invaded places like Nova Scotia, British Columbia, and Eastern Massachusetts. Most recently it has come to Vinalhaven and Harpswell. When people move or have a summer home, they sometimes bring their favorite garden plants with them. Little do they know, the winter moth cocoons are in the soil from June through November. They emerge at Thanksgiving time and spread around their new home and lay eggs. Those eggs overwinter on the bark of our trees, and become voracious caterpillars in June. Since they have no predators here, they can defoliate trees quickly.

We noticed an area in Tenants Harbor where tree leaves looked like Swiss cheese. We wondered if it was winter moth causing the problem. We also wanted to know how many winter moths were populating the area. Last November and December, we set out to investigate.

We put pipe insulation with sticky tanglefoot on four different deciduous trees around town. When the winter moths emerged from their cocoons in the soil, the wingless female moths climbed up the trees and got stuck in the tanglefoot. The males meanwhile flew over to meet them and got caught in the tanglefoot as well. We took photos of the moths and shared them with state entomologists, who confirmed them as winter moth. We found that on Sea Street, winter moths were really bad. About 800 winter moths got caught in our trap on that tree, as compared with 175 on the tree at 20 Watts Avenue, 20 at the school tree, and eight on the tree at 90 Watts Avenue.

This spring we did the second part of our investigation. We went to Sea Street and got leaves off of some maples and oaks.

Then we counted how many caterpillars were on each leaf. On one infested oak tree there were 53 caterpillars on just 31 leaves! When you go up the hill on Sea Street, you can literally see the holes in the leaves from all the caterpillars. If the trees have to endure this for several years, they will die.

Zoe Hufnagel and Miles Bartke with a tanglefoot trap they placed on a tree on Watts Ave. as part of a project on winter moths they worked on with STEAM teacher Amy Palmer throughout this past school year.

In order to help, we collected winter moth caterpillars and reared them until they became cocoons. Now we are sending them to Tom Schmeelk, who is a Maine state entomologist. He will dissect them to see if there are any parasitic flies in them. If there are, that is great news. The parasitic flies will control the population of winter moth. If there aren’t, we will try to raise money to get parasitic flies released here next spring. (They were released in Harpswell and Vinalhaven a few years ago.)

If you want to help, you can let people know not to transplant their plants from other places that have winter moth. In the meantime, if you want to prevent winter moths from laying eggs on your favorite deciduous tree, you can put up a tanglefoot trap in mid-November. If you have questions, you can contact Amy Palmer at the St. George School: a.palmer@stgeorgemsu.org

(Zoe Hufnagel just finished 4th grade at the St. George School, where Amy Palmer teaches STEAM (Science, Technology, Engineering, Art, and Math).)

PHOTOS: Amy Palmer

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