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Alien Invasion

Gypsy MothGypsy moth (Lymantria dispar) egg mass counts in the north central and the northwest portions of Pennsylvania are elevated indicating the potential for significant outbreak. To date, 1770 gypsy moth egg mass plots have been done. As a result of these findings, the Bureau of Foresty, Division of Forest Pest Management, are preparing for a gypsy moth suppression program in the spring for the aforementioned areas.  


Defoliation by the gypsy moth has killed millions of oak trees across Pennsylvania. Although oaks are preferred, gypsy moth caterpillars also eat the leaves of hundreds of other tree and shrub species including apple, alder, aspen, basswood, birch, poplar, willow, hawthorn, hemlock, tamarack (larch), pine, spruce, and witch hazel. Gypsy moth typically avoids ash, butternut, black walnut, locust, sycamore, and tuliptree (yellow poplar). Although it usually takes two or more years of defoliation before trees die, conifers that are defoliated may be killed after a single season of defoliation.

Pennsylvania last experienced a gypsy moth outbreak in 2005 to 2009 (populations crashed in 2009), which included Blair County.  Blair County, at its peak in 2007, had 19,160 acres that were reported to have received moderate to heavy defoliation, determined from aerial surveys. 


The gypsy moth has had severe impacts on the Commonwealth’s forests since the early 1970’s. It has caused outbreaks at various intervals. On average, an outbreak has occurred every five to seven years since 1968.


Gypsy Moth CaterpillarThe degree of gypsy moth defoliation varied considerably over the years and regions. At its peak in 1990, gypsy moth defoliated 4.3 million acres of forest in Pennsylvania. However, due to costs, only a small proportion of infested areas are treated in any given year.


Gypsy moth treatments are typically aerial applications of  bacteria called BT (Bacterium thuringiensi). The actual mode of action of BT is simple. The bacterium produces a crystal protein toxin that kills the cells lining the insect gut. When ingested, the bacterial cell wall is digested which releases this toxin. Since insects have guts that are only one cell layer thick, this toxin literally "eats" a hole in the gut, causing an infection in the body cavity.


The interesting thing about BT, is that only certain insects digest and are affected by the protein toxins. In most insects, as well as people, birds, fish, and other animals (especially pets), BT is very safe with no effect.


The gypsy moth was brought to North America from France in 1862.  The purpose was to breed hybrid silkworms that would be hardier than the Chinese species and that could be used to establish a silk industry in the United States. In 1869 some of them escaped and were apparently scattered by a windstorm.  The population quickly expanded, and in 1932 were discovered in Pennsylvania.


Contributed by Christopher Jones, Service Forester with the PA DCNR Bureau of Forestry

Pennsylvania Bureau of Forestry foresters are available for assistance to private landowners free of charge.  Interested landowners can call 814-472-1862814-472-1862.

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