Lymantria dispar, the gypsy moth, is an invasive species in the United States, introduced from Europe in 1868 by Etienne Leopold Trouvelot, a French artist and astronomer. Trouvelot was interested in producing a hardy silk producing moth, and imported L. dispar eggs which escaped into his back yard in Medford, Massachusetts. Shortly after this, in 1889, the first gypsy moth outbreak in the United States occurred in the Boston area. Gypsy moths have since spread throughout the Northeast, Canada, and the Midwest, despite huge efforts to eradicate this pest. An outbreak of gypsy moth caterpillars can very quickly and effectively defoliate forests. Spread of this species represents a significant risk especially to hardwood trees, their preferred hosts, but since the gypsy moth has a wide diet, most types of trees are affected. Since 1980, the gypsy moth has defoliated about a million forested acres each year. Infestations occur cyclically, with populations reaching outbreak levels every 5-10 years.

The USDA has a coordinated Federal-state program to control populations and limit at least the human propagated spread of the Gypsy moth from currently quarantined states into new areas. The Gypsy Moth quarantine currently includes the District of Columbia and the entire states of Connecticut, Delaware, Maryland, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island and Vermont. As well as spreading in concert with humans, populations can naturally spread by female moths flying to uninfested areas, or at the larval (caterpillar) stage, which are carried on the wind by their silk threads.

The government has developed another interesting control program which sprays effected areas with an engineered baculovirus, which is very effective in killing the caterpillars. The baculovirus works by changing the nocturnally-feeding caterpillars behavior, so that they remain high in the forest canopy instead of their usual return to daytime hiding places on the ground. When the virus then kills the caterpillar, the caterpillar's flesh dissolves and the virus rains down from the top of the tree and is widely spread to other caterpillars below.

The asian subspecies of Lymantria dispar, although similar to the European subspecies described above, has never become established in North America. Because it is a stronger flier than the European subspecies, and presumably could quickly spread throughout the US, it is considered a major threat and carefully monitored at likely entry pathways.

(Aphis-USDA 2003; Aphis-USDA 2011; Hamilton, 2011; Hoover et al. 2011; Liebhold 2003; McManus et al 1989)