The spongy moth: an arborist’s take
The spongy moth (Lymantria dispar), recently renamed to avoid using a derogatory ethnic term, was one of the most serious insect pests of our eastern forests. It is a native of Europe, Asia and North Africa.
A French scientist hoping to produce a new race of silk-producing insects introduced it into Massachusetts from Europe in 1869. Some of the caterpillars escaped during his experiments, and in a few years, the insect became well established in the surrounding woodlands. By 1902, it was widespread in all of the New England states, in eastern New York and parts of New Jersey.
Today, the spongy moth has established populations from Canada to the Carolinas, from the coast to the Midwest. Eight to ten years ago the populations were at their peak in our area. The damage level was severe and it cost millions of dollars, both public and private to suppress the damage.
Spongy moth is prevalent this year. They could be noticed while driving the highways looking at the mountain sides. In certain areas, there were large brown patches of trees that covered serious acreage — this was spongy moth damage.
The spongy moth has historically fed on both deciduous and coniferous trees. Most people only recall that they were seriously damaging to our deciduous forest trees. This is the reason that I decided to discuss this topic. Due to the wide variety of host plants in their diet, spongy moths have the capability to build their populations slowly and quietly.
Recently, many people have misidentified this insect by discounting spongy moth on spruce. This will result in more serious infestations the following year if control measures are not taken.
Identification of this insect is easy. Spongy moth larvae are 2 inches long when fully grown. The body is brown to black and hairy. It has raised spots (tubercles) on its back, five pairs of blue and six pairs of red. These red and blue spots are present during all growth stages of the larvae. The adult moths are off-white to brown with a two-inch wing span. Egg masses are tan and oval, approximately 1 inch long and covered with yellowish hairs, appearing fuzzy. They are usually laid on the bark or limbs of the trees that they have infested. However, it is not uncommon to see egg masses on houses, fences, barns, cars, etc. Each egg mass could contain 200 to 800 eggs.
The spongy moth has one life cycle per year. Their eggs hatch around May and they immediately begin feeding. They go through five to six instars (developmental stages) before becoming fully-grown. By late June they pupate and emerge as adult moths in mid to late July. The males fly to the winged, yet flightless females to mate. By the middle of August the females lay their egg masses to overwinter.
The spongy moth is a permanent resident in our community. Their populations will fluctuate from low to significant numbers as environmental conditions dictate. We will have to wait to see what next year brings, but as of now it does not look good for trees.
Cody Kouneski is an arborist representative with Penn State Mont Alto Forestry and an International Society of Arboriculture Certified Arborist.