We obtained historical numbers of established exotic insects from Sailer (1983) and data for plant pathogens and mollusks from the Office of Technology Assessment (1993). For trade data, we used U.S. Department of Commerce statistics of merchandise imports from 1800 onwards, converted to 1999 U.S. dollars. We then calculated cumulative imports to 1920, 1930, 1940, . . . 1990. Although other taxonomic groups were available for study (e.g., mammals), we restricted our analysis to those continuing to enter the United States as accidental introductions with trade.
This was one of the publications used: Sailer, R. I. 1983. History of insect introductions. Pages 15–38 in C. W. Wilson and C. L. Graham, editors. Exotic plant pests and North America agriculture. Academic Press, New York.