Credit: Cornell College of Agriculture and Life Sciences
They’re back. Last week, spongy moth eggs began to hatch, each one releasing hundreds of quarter-inch-long caterpillars. These tiny caterpillars drop from trees and, once on the ground, begin their search for food.
Male spongy moth caterpillars undergo five instar stages, molting at each stage to facilitate growth. Females molt six times, growing up to 2.5 inches, slightly larger than males. During the molting period from May through June, the caterpillars will demonstrate their voracious appetites by climbing trees and feasting on spring leaves.
Credit: Priscilla Herdman
Wrapping sticky tape (with the adhesive side out) or burlap around the base of a tree can impede these climbs and protect the leaves. Another method involves encircling the base of a tree with plastic and applying a sticky substance, such as petroleum jelly. These methods should be implemented quickly as the caterpillars begin climbing as soon as they find a tree. While these tactics are impractical in a densely forested area, they may help to protect a favorite tree. However, there are concerns about the sticky material harming other insects, and even birds can be endangered.
Protecting a tree in this manner does not guarantee that the caterpillars won’t reach the upper limbs by another method: borne by the wind. The small caterpillars are very light and are easily blown from tree to tree.
How devastating the spongy moth infestation will be in 2024 remains uncertain. Typically, the outbreaks last two or three years, and the severity can vary from location to location. In 2023 the Pine Plains area experienced significant defoliation by early July and it is likely that this will occur again in some places this year. But, for the most part, trees are resilient and will produce a new set of leaves once the caterpillars have begun their metamorphosis into moths during their pupate stage.
It is possible that this year’s infestation will be milder than last year’s. There are several factors that can contribute to a decline in the spongy moth population. A large infestation can collapse due to the moths simply running out of food to support their large numbers. Additionally, a naturally occurring virus, the nuclear polyhedrosis virus (NPV), which is deadly to caterpillars, could curb outbreaks by spreading among caterpillar populations in the treetops. However, there were no reports of NPV impacting spongy moth caterpillars in the region in 2023.
The fungus Entomophaga maimaiga, introduced from Japan in 1910 or 1911, is also deadly to the spongy moth caterpillar, while not killing other insects. This fungus thrives in spring, in cool, wet conditions. The weather in late winter may have been favorable for the fungus.
In January, Cary Institute of Ecosystem Studies scientists Clive Jones and Charles Canham presented a program on spongy moths. They discussed a study of spongy moths (called “gypsy moths” when the study began) between 1980 and 2010 on the institute’s forest property in Millbrook, which established a correlation between spongy moths and populations of white-footed mice. The research found that these mice feast on spongy moth pupa, climbing trees and devouring them before they emerge to reproduce. The greater the population of mice, the more spongy moth pupa devoured.
Credit: Priscilla Herdman
The population of white-footed mice rises and falls with the acorn production of oak trees. In 2023 throughout the Northeast and Midwest, there was a noticeably large acorn crop. According to the Cary Institute analysis, more acorns should result in more mice to eat pupa this year. This, in turn, suggests that mice will help make this the final year of the current spongy moth invasion. The tradeoff is that with more mice, there are apt to be more ticks in the woods this summer.
Regardless of the level of spongy moth tree damage this year, the New York State Department of Environmental Conservation and the United States Forest Service do not intend to intervene. It is the position of these two agencies that spongy moths are a natural phenomenon whose impact is short-lived and that the trees affected will survive the experience.
Originally imported from France to Medford, Mass., in 1869 for silk production, some moths escaped. Despite an effort after World War II to keep the moths from spreading west of the Hudson River using DDT, spongy moths, with large outbreaks occurring every 10 to 15 years, have spread as far south as Virginia and as far west as Minnesota. Spongy moths are here to stay.