Gypsy Moth Invasion in North America: A Quantitative Analysis

The gypsy moth, Lymantria dispar (L.), was accidentally introduced to North America in 1868 or 1869. Since that time, the range of this insect has spread to include most of the northeastern states in the US and eastern provinces of Canada. We compiled historical records of gypsy moth invasion in North America and as- sembled these data in a geographical information system (GIS). Individual US counties and Canadian census dis- tricts were used as the smallest spatial unit in this data- base. Data indicated that three distinct periods occurred during which spread rates differed: a high rate (9.45 km/ year) from 1900 to 1915, a low rate (2.82km/year) from 1916 to 1965, and a very high rate (20.78 km/year) from 1966 to 1990. Furthermore, expansion was slower (7.61 km/year) during the period of 1966-1990 in counties where the mean minimum temperature was less than 7? (C). The rate of range expansion was independently cal- culated as 2.5 km/year from estimates of r, the intrinsic rate of increase, and D, the diffusion coefficient (disper- sal magnitude) and a simple spread model. This estimate was substantially less than the empirically derived expan- sion rates. The higher observed rates of expansion may be due to human-caused movement of gypsy moth life stages which was not incorporated in estimates of D made here.