Meteorological Influences on the Dispersal of the Gypsy Moth: Spread to the Arrowhead of Minnesota

The gypsy moth, Lymantria dispar, is a non-native forest pest that continues to invade new areas in North America. Spread usually occurs through stratified dispersal in which local growth and diffusive spread are accompanied by long-distance jumps ahead of the expanding leading edge. Anthropogenic movement of life stages is a well-documented, stochastic mechanism of gypsy moth spread dynamics. Another mechanism is through the atmospheric transport of early instars and adult males over both long and short distances. Pheromone-baited traps have been deployed across Minnesota since 2000 to monitor populations. Gypsy-moth-trap catches have been particularly high in the arrowhead region of northern Minnesota (St. Louis, Lake, and Cook counties). The time series of male moth trap catch data in northern Minnesota shows a temporally continuous pattern of low male moth counts with a spatial pattern that suggests adult males, or immature life stages, were introduced at various spatial locations nearly simultaneously, such as through a discrete atmospheric transport event. This type of pattern was also observed in Wisconsin as gypsy moth was first invading. These patterns are in contrast to observations of the patterns of newly-established gypsy moth populations elsewhere in which there is a structure that generally originates from a single location, such as one that is initiated through the anthropogenic movement of egg masses. The Minnesota trap catch data also reveal a bimodal structure over the course of a season, in which one peak phenologically corresponds to climatic conditions in northern Minnesota while an earlier peak phenologically corresponds to climatic conditions elsewhere. Furthermore, there is evidence from measurements of wing length from male gypsy moths collected in traps from 2007–2009 that the individuals trapped in Northern Minnesota may be comprised of different populations. This study uses a probabilistic climatological model to explore the question of whether there exists a reproducing population established at low density (resident population), or if trap catch data are the result of yearly re-introductions (immigrating populations), or a combination of both. The ability to ascertain the degree of introductions through atmospheric transport would greatly improve management decisions and better target treatments against those areas most likely to contain established populations. This project expands understanding of the dispersal mechanisms in the Great Lakes region, and perhaps nationwide, and works toward validation of the previously-developed meteorological model to predict atmospheric transport of the gypsy moth.