Introductions of nonindigenous organisms into the United States have been linked to international trade. The individual contributions of imports, immigration, and international travel, however, are poorly understood because introduction dates are unavailable. We examine relationships between economic trends and discoveries of nonindegenous insects and use these relationships to infer the timing and determinants of introductions. We find that a few variables can explain much variation in species introductions and identifications. The most significant contributor to the introduction appears to be agricultural imports. Currently available proxies for academic effort are weak determinants of the probability that introduced species are identified.