The Potential Geographic Diffusion of the Gypsy Moth (Portheria Dispar) into Southern Forests

Abstract

The gypsy moth is a forest insect native to Asia and eastern Europe, where it has caused moderate and infrequent damage. However, when taken from its native environment, the insect has displayed a capacity to be a costly and destructive menace to forests. Such a transition was begun in 1869 when a French biologist introduced gypsy moth eggs into Medford, Massachusetts. He hoped to produce a sturdy strain of silkproducing insects by crossing the gypsy moth with the silkworm moth and raising it in a favorable environment. As could be expected, some larvae escaped from the Medford experiment, thus creating the current gypsy moth infestation problem in the northeastern United States. (1) During subsequent years, gypsy moths spread over 200,000 square miles, defoliating forest and ornamental trees in the process. The moth has now become established in all or part of eleven states from Maine to Maryland (Figure 1). Records of outbreaks in this area have shown that preferred hosts consist mainly of hardwood, but older larvae successfully feed on pine, hemlock, and spruce. These conifers are usually killed by one season of complete defoliation, whereas hardwood species may be killed by two successive years of defoliation. In areas containing 50 percent or more hardwood trees (preferred hosts) there is generally an initial three-to-five year phase following introduction during which the insect becomes distributed at low densities throughout the region. Phase two may bring a population explosion throughout the entire area for several years in succession. Eventually, a population collapse occurs, followed by local abundance on the more susceptible upland oak sites, thus completing the cycle. (2) The potential gypsy moth threat to Southern forest resources has been analyzed by several persons over the past 40 years. If suitable hosts are present, the insect is expected eventually to occupy the entire region in which oaks are a component of the forest stands. Evidence is accumulating to indicate that Southern hardwood forests are highly susceptible to defoliation and, therefore, may experience greater mortality than that occurring in the Northeast. (3) This factor, combined with a milder climate and a virtual predator-free habitat in the Southeast, offers some very bleak possibilities. Even with the exhaustive efforts of the combined forces of local, state, and federal agencies, the gypsy moth has continued to gradually