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

THE POTENTIAL GEOGRAPHIC DIFFUSION OF THE GYPSY MOTH (PORTHERIA DISPAR) INTO SOUTHERN FORESTS Glenn R. Pritchett* 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 *Mr. Pritchett is regional planner at the Land-of-Sky Regional Council in Asheville , North Carolina. This paper was accepted for publication in January 1975. 48 Southeastern Geographer • Original Infestation - 1869 Infested - 1914 [•SSSv··! Infested - 1934 *·'·'·*-':':'-'-·-t \ ppHHJ Infested - 1952 J I I I Il I I . I Infested 1961 [:&&$Ìl Infested - 1972 Miles Source Animal and Plant Health Inspection Service, March, 1973 Figure 1. Gypsy Moth Spread (1869-1972). extend its numbers and range. Thus far, such methods as the creation of a barrier zone, the attempt to starve the larvae, the use of artificial sex attractants, the importation of predators and parasites, the use of Vol. XV, No. 1 49 exotic disease organisms, and the development of new insecticides have all failed to halt or even slow the diffusion. Without a workable and ecologically feasible check, an assault on new and distant territory appears inevitable. The gypsy moth is an example of an animal whose density distribution varies widely, from a few individuals per acre of woodland to massive numbers whose quest for food results in complete defoliation. Therefore, the ability to predict the moth's population dynamics is a prime asset in any control program. (4) This study is an attempt to predict the moth's geographic diffusion. Of primary significance is the specification and analysis of inherent and extraneous factors that historically have influenced the gypsy moth's propulsive population growth rates. Just how these conditions should influence the future spread of the moth populace, and the anticipated results of such activities, constitute the primary subject matter of the following section. CHARACTERISTICS OF POTENTIAL MIGRATION. On the basis of historical documentation, it can be assumed that significant natural and artificial factors exist which have modified the ability of the gypsy moth to increase and spread its...