Analysis of gypsy moth (Lepidoptera: Lymantriidae) population dynamics in Michigan using geographic information systems

We studied the 9-yr (1986-1994) statewide gypsy moth, Lymantria dispar (L.), population distribution dynamics in Michigan using geographic information systems (GIS) based on an analysis of pheromone trap data of male moth catch from more than 3,000 permanent sites. A time series of male moth contour maps with 7 density categories (0, 1-25, 26-100, 101-200, 201-300, 301-400 and 400 moths (.) trap-1 (.) year-1) was created using the GIS. The statewide gypsy moth population expanded at an alarming rate of 6,053 km2/yr with the largest area infested being 128,164 km2 (85% of the state) in 1993. The population-weighted mean center stayed in the Lower Peninsula, but a low density population was developing in the Upper Peninsula during the 9-yr period. Map analysis showed that the statewide population gradually increased from 1986 to 1990 and then became relatively stable thereafter. Both the highest weighted average density (191 moths (.) trap 1 (.) year-1) and the greatest coefficient of relative dispersion (307%) occurred in 1990. Linear regression slopes between successive years were 1.0 before 1990 and 1.0 thereafter, indicating an increasing and then decreasing population density change. Regression results between successive years indicate that the previous years' population map is a good linear approximation for the following (r2 70%). In the 9-yr study, 55% of the population cells stayed in the same density class in the following year. A general pattern of population density shifts was that increase prevailed over decrease. Research methodology, population distribution, temporal dynamics, and density shifts are discussed