In 1869, gypsy moths, Lymantria dispar, were brought to Massachusetts from Europe. They were brought to be crossbred with silkworms. Several caterpillars escaped and gypsy moths began to spread thoughout New England. Today, gypsy moths can be found feeding on hardwoods, especially oaks, in all Maryland counties.
Gypsy moths have great reproductive potential with egg masses containing over 1,000 eggs. Natural predators, parasites, and diseases that normally feed on these egg masses and caterpillars are not as prevalent in the United States or are not as effective as in their native habitats. As a result, gypsy moth populations are not stable. They fluctuate widely; building up until defoliation is so severe that gypsy moth caterpillars starve and populations crash.
Most young, healthy trees can withstand a single episode of defoliation but many older trees or trees under stress cannot withstand even one defoliation. Since, gypsy moth populations often remain high for several consecutive years, trees are defoliated several times and mortality can occur at high rates. In 1990, the Maryland Department of Agriculture counted the number of trees defoliated and killed by gypsy moth caterpillars in several small areas. They found that as many as 34% of the trees counted were dead in areas where defoliation occurred without treatment.
The Cooperative Gypsy Moth Suppression Program is conducted by the Maryland Department of Agriculture, in cooperation with the USDA Forest Service and county governments. This joint effort works to prevent tree defoliation in excess of 50% of the forest. Also, they work to reduce gypsy moth populations to limit the need for future treatments.
In 1995, nearly 64,000 acres of oak-hickory forests were treated. The treatments included aerial spraying of Dimilin (diflubenzuron) and B.t. (Bacillus thuringiensis). Less than 2% of the treated acres were defoliated. This was the fifth consecutive year that less than 2% of the acres treated for gypsy moths were defoliated.
In 1995, 93,864 acres were defoliated. These forested areas did not meet the criteria for treatment. Since 1993, over 90% of the treated and defoliated areas were on the Eastern Shore.
All Eastern Shore Counties saw an increase in defoliated acres since 1993. Populations of gypsy moths in the rest of the State remain relatively low due to the successful Cooperative Gypsy Moth Suppression Program. Natural cycles, a fungus, and a parasite that kill gypsy moths have played important roles in controlling gypsy moth populations. However, reports suggest that populations in central Maryland may be slowly building. It is not clear whether this is a short term increase or the beginning of a trend toward a severe population outbreak.