What is the spongy moth, and what is it doing to our forests?

MILLBROOK — One of the major factors in the cycle of the spongy moth and their proliferation — or lack of proliferation — turns out to be acorns.

Spongy moths, formerly referred to as Gypsy moths for their itinerant ways, were the topic of a lecture Thursday, Jan. 11, by scientists Clive Jones, who has studied the spongy moth for 30 years, and Charles Canham, who has studied northeastern forests for 40 years. Both are emeritus scientists of Cary Institute of Ecosystems Studies, which hosted the lecture.

Spongy moths, natives of Europe and Asia, came to North America in 1868 or 1869, to Medford, Massachusetts. Amateur entomologist Etienne Trouvelot believed he could crossbreed them with silkworms in an attempt to “make a hardy silkworm.”

“He got some egg masses of the spongy moth and set them on the sill of his open window. When they blew into his garden, he could not find them,” said Jones.

By 1891, there was mass defoliation in the Medford area and spongy moths were found in a 200-square-mile area. From there, they began to spread throughout the U.S. and Canada.

Alone, the spongy moth travels by ballooning on silk threads; assisted, humans unwittingly transport eggs laid on cut logs or vehicles.

The life of the spongy moth could be said to begin in July and August, when the flightless female moths emit pheromones to tell males that they’re ready to mate.

They each lay a few hundred eggs, often at the bases of trees. If the moth population is high, they lay eggs anywhere, on garden furniture or on the bumper of a car.

Then, the following May, the eggs hatch. The larvae spend May and June ballooning to new locations on silken threads. They rest under trees during the day, then go up into the tree canopy at night to feed. They prefer oaks but also eat maple, beech, apple, hickory, willow and birch trees, among others.

When the population is very dense and the competition for food is fiercer, they may eat all day and night. Once a tree is defoliated, they’ll move to the next canopy.

In late June and July, the larvae pupate — a caterpillar-to-moth metamorphosis that lasts a couple of weeks — then they emerge and breed and the cycle begins again.

Acorns and outbreaks

Jones explained that “outbreaks” — or high densities of spongy moths — “occur about every 10 years on average, but like any average, it does not mean there will be outbreaks like clockwork.”

There are two major causes of outbreaks, he continued. One is “high female fecundity,” for example: If one moth laid 500 eggs and 250 were female, 125 of those would live to maturity and lay eggs. So, in year two, there would be 125 females laying eggs, and the third year, over 15,000 would result.

The other cause of outbreaks is the collapse of the white-footed mouse population, he said. White-footed mice eat the pupae as a part of their omnivorous diet.

At this point, we reach the subject of acorns:

“The number of mice is determined by the number of acorns the previous fall,” Jones continued, saying that if there is a moderate to large acorn crop in the fall, more mice survive the winter and begin breeding earlier in the year, in late winter or early spring. When that happens, there is an extra generation of mice, meaning more mice to eat more moth pupae.

Studies at the Cary Institute concluded that “moderate to high mouse densities keep moth populations low,” while it was found “that when the mouse population declines, the spongy moth population increases.”

However, defoliation of oaks can cause the trees to produce fewer acorns, meaning less food for the mice, meaning fewer mice the following spring, meaning more moths, and so on, until the moth population collapses again.

Spongy moth population collapse

There are three major causes of population collapse among the moths.

One is a fungus, Entomophaga maimaiga, which kills the moth at moderate and high densities. It likes cool, wet springs and is more abundant in those conditions.

The more common cause of collapse is Nuclear Polyhedrosis Virus (NPV): When the moths are at high density, they are more stressed by the struggle to find food. Their immunity drops, they are more likely to catch the virus, and many are killed.

The third cause of moth population collapse is lack of food: When they defoliate a large area of trees and run out of food too early, they can’t mature and proceed to egg laying.

Jones said that Cary Institute scientists predict an NPV collapse in 2024: “Whether or not there’ll be a lot or a little defoliation [this year] will really depend on how fast the virus moves through the population. If it moves slowly, there’ll be complete defoliation. If it goes through fast, there’ll be incomplete defoliation.”

The defoliated tree

Canham continued the lecture by saying, “Defoliation by caterpillars is rarely a direct and immediate cause of tree mortality.”

Canham explained that this is due to “carbohydrate economy”: “Basically, photosynthesis during the growing season produces the sugars needed to produce new tissues, and the energy those tissues need for their metabolism.

“But the even more important outcome of a good growing season is the profit leftover after meeting those immediate needs, and that profit is in the form of sugars and starches that act as reserves for use next year.”

Usually, unless the tree is weakened by other factors, it will draw on its reserves to get through this defoliation and produce more leaves that same year.

An exception that Canham has observed at Cary Institute is the understory hemlocks, which don’t make as many reserves and, after a couple years of defoliation in a row, may not recover. Needle trees, DEEP has noted, can be killed if they lose more than 50% of their foliate.

The bottom line

Canham said he worries more about the devastating effects of the emerald ash borer, longhorn beetle, spotted lantern fly and hemlock wooly adelgid than about the spongy moth.

He said if a tree is in crisis, a way to help it is to water it to “help replenish its reserves.”

For more information or to view the lecture video, go to www.caryinstitute.org/news-insights/lecture-video/...

Cary Institute of Ecosystem Studies/Lori Quillen

Female spongy moths laying egg masses on a black oak at the Cary Institute of Ecosystem Studies in Millbrook, N.Y.