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'Spongy Moth' has been adopted as the new common name for Lymantria dispar (formerly known as Gypsy Moth). Over one hundred years ago, spongy moth caterpillars were brought into the U.S. for research as a possible source for silk production. Escapees found a welcoming habitat, and masses of caterpillars have been plaguing our forests and landscapes on and off for over a century.
Adult spongy moth females are white with brown markings that resemble an inverted V pointing toward the head. Males are brownish with black markings and have a wingspan of 1 to 1 1/4 inches (25-32mm) - they’re the ones you’ll see flying. Adult moths don’t feed, but the larval stage is significantly destructive.
In July and August, females lay dark brown masses of 100 to 600 eggs protected by a light-colored, almost hairy covering. The older the mass, the softer they are to the touch. They are often on tree trunks, but can be found on the side of buildings, signs, trailers, are other outdoor surfaces.
The following spring, ¼ inch (6.3mm) long hatchling caterpillars move away from the egg mass to feed on leaves. To do so, they often produce silken threads that catch in the wind and send them aloft to other trees—a practice called ‘ballooning’ which enables their spread. Early instar larvae are small, dark brown-to-black, and very fuzzy. Later instars lighten in color and have a showy display of two rows of colored spots: five pairs of blue and six pairs of red. At about seven weeks, larvae are fully grown at 2 – 2 1/4 inches long (50-56 mm).
In June and early July, larvae pupate in hiding spots under bark or similar protection. The dark brown, hairy pupae are about 2 inches long.
Once they complete pupation, adult male spongy moths emerge and fly erratically during daytime in search of mates. Heavy-bodied adult females have wings but don’t fly. They rest on trees and wait as males follow female pheromone trails to find them.
A healthy tree can withstand some defoliation, finding spongy moths on your property is serious but not necessarily the end for affected trees. Two factors must be considered—the number of larvae, and whether this is a repeated defoliation from a prior year. Management of spongy moth using IPM always requires scouting. Take a good look at your trees, take photos, and make note of your findings to see if numbers are getting higher and damage worse.
If you’ve noticed defoliation on trees in your landscape, don’t worry. They often will bounce back and put out new growth after the spongy moth caterpillars have stopped feeding for the season. To help these stressed trees rebound, it can be beneficial to water the trees if there are dry spells over the summer.
Spongy moth outbreaks are naturally reduced by birds, rodents, parasites, and disease. What looks like a problem in one year does not indicate the same problem will occur the following year. It is typical, in fact, to see an outbreak build for a year or two only to have the population collapse due to spongy moth diseases in a subsequent year.
Spongy moths are an invasive exotic pest with a broad diet. If you have the following preferred plants, your property becomes that welcoming habitat for egg laying and subsequent feeding of plant leaves by masses of Spongy moth larvae.
Plants preferred by spongy moths:
- oak, willow, apple, crabapple, white birch, witch hazel, mountain ash, basswood, linden, pine, and spruce
If preferred plants are not available, spongy moths will also feed on:
hardwoods (maple, cherry, walnut, hickory and chestnut) and hemlock, elm, hackberry, yellow birch, beech, cottonwood, box elder, and ironwood
They generally avoid:
- ash, balsam fir, locust, Scotch pine, red cedar, tulip poplar, catalpa, sycamore, and dogwood
- You may want to protect individual trees by using a simple burlap cloth flap trap. This involves encircling a tree with a foot-wide length of burlap tied with twine right in the middle and pulling the top portion over the lower portion. This technique takes advantage of the caterpillar’s drive to crawl upwards and not turn around if they encounter the barrier of the burlap. The accumulated caterpillars can then be periodically removed and destroyed.
- During the summer, look for caterpillars and adult moths. Squish them or knock them into soapy water. (Note--some people have an allergic reaction when coming in contact with spongy moth caterpillars so it's a good idea to wear gloves.) During outbreaks, high numbers make this impractical. Take notes of what you find.
- If you note diseased larvae, leave them be so that other larvae will also be infected. Larvae killed by viruses look like V's. Larvae killed by fungi hang head down.
- If you’ve seen spongy moth caterpillars or defoliation during the growing season, consider going out on a dry day between September and late-April to look for egg masses on trunks. Scrape any you find into soapy water.
- If your scouting confirms high concentrations of spongy moth caterpillars, a decline in tree health, or threats to maple sap production, insecticide sprays may be an option.
- There are two kinds of pesticides: traditional chemical sprays or biological sprays containing naturally occurring organisms. The most common biological used on caterpillars is Bacillus thuringiensis kurtaki (Btk). Bt is most effective on small caterpillars, and harmless to people, animals, and plants. By introducing it into their digestive tract, caterpillars become paralyzed and are unable to feed. Note: Bt affects other young moth and butterfly larvae, too.
- Traditional insecticides can impact many insects including beneficial, native insects (such as bees). Always use, or allow their use, only when necessary and follow all pesticide label instructions.
- Hire professional pesticide applicators trained to legally and correctly use registered products. Because of the area involved — including height of tree — pesticide spraying is a job best left to trained professionals with the proper equipment. However, a well-informed homeowner can apply certain products in some instances. Note: spraying is not effective against pupae, egg masses, or caterpillars over one inch in length.
- Contact your local cooperative extension if you have questions about what a proposed pesticide applicator is telling you and never agree to unnecessary applications.