They’re back, the Winter Moths rise again

By Beth Sullivan

A couple of years ago we witnessed a new phenomenon: small moths in abundance, swarming and landing on garage doors and porch lights in December. Hardy little critters. We didn’t realize the full impact of this infestation until the following spring when many species of trees were leafing out, or more correctly, not leafing out. The leaves were distorted and chewed before they unfurled. Many blossoms were destroyed before the flowers could even open. In the season that followed, the pollinators were robbed of major nectar sources and without flowers and pollination, there were no fruits on many trees. The birds that relied on the fruits and berries in autumn lost out as well. Many trees were weakened significantly.

For whatever reason, last fall and winter there seemed to be fewer moths, and the trees had a chance to rebound a bit this year. At least those that were lucky enough to avoid the gypsy moth caterpillars.

The Winter Moth.

Very small caterpillars go dormant in the soil in masses, then pupate to emerge at this time of year.

Moths are attracted to lights and swarm on warmer evenings.

Winter Moths return

Driving home the other evening, it was balmy and wet. The kind of night I might have been looking for salamanders or frogs on the road if it was late March. But instead I witnessed clouds of moths in my headlights. They are back. Several nights of warmth again created perfect conditions for the emergence of the winter moths.

The life cycle of these moths is only now being understood. Right now the moths are flying in clouds, and those are only the males. They are small, boring and light brown. They usually land with wings spread, but they may be held together, which is unusual for moths. The females have small non-functioning wings, and when they emerge from the soil, they climb up the base of the trees where they are found by the males and mate. They then crawl up the tree to lay their egg masses in cracks and crevices in the bark, close to leaf buds. The moths die ,and the eggs overwinter. The caterpillars emerge very early and begin eating the leaves and flowers while in bud. They are not terribly fussy, and they will feed on many tree species, from mighty oaks, to blueberries and garden plants. When the caterpillars are done feeding, they drop to the ground where they remain dormant through the summer, to pupate in the fall. I discovered masses of these dormant larvae just under the leaf litter this fall as I raked out a new trail in the woods. They emerge, to continue their cycle, after a hard frost period and rewarming in mid-November and December.

During that last outbreak, the trees that had significant leaf damage were stressed for the entire season. Some were able to send out a second set of leaves later, but that is an enormous expenditure of energy. Then we had the later summer drought and heat which literally dried out the tender leaves well before they were due to fall. The affected oaks produced very few acorns locally, in an otherwise huge acorn year. We have seen a lot of die-off in the forests due to the combined attacks of winter moths and gypsy moths.

Several years of defoliation can kill many trees.

When leaves emerge in Spring, they are already damaged by the larval Winter Moth.

They can't be stopped

Until entomologists and landscape contractors can better understand the full life cycle, there may be no way to interrupt the onslaught. If you can check the bases of your trees now, it may be possible to find and destroy the females before they ascend the trunk. Smaller trees in the home landscape can be treated with a horticultural oil or insecticidal soap to kill the eggs or larvae early on. I go out and squash the moths by the hundreds, even thousands, when they collect by lights I leave on purposely to attract them. But that is a drop in the bucket. We cannot protect the entire forest. For now we hope that some natural predator or disease will be found that will stop their march.

Several years of stress from infestation and drought are causing trees to die, and we are seeing large areas affected, like after the Gypsy Moth invasion of decades ago. A disease evolved to help kill off Gypsy Moth caterpillars. Their egg masses are easier to find on tree trunks and can be scraped off. But those moths also reappeared last year in greater than expected numbers.

It seems to be yet another round of bad news for our already stressed woodland habitats.

By comparison Gypsy Moths are far larger and make their cocoons above ground.

Gypsy Moth egg masses are easy to spot now and can be scrapped off bark.

Photographs by Beth Sullivan.