2003 Annual Report 4.What were the most significant accomplishments this past year? D. Progress Report: This report serves to document research conducted under a specific cooperative agreement between ARS and the University of Delaware. Additional details of this research can be found in the report for the parent project 1926-22000-013-00D, Population Biology of Agents introduced for Biocontrol of Arthropod Pests and Weeds. In a collaboration between the Beneficial Insects Introduction Research Unit and the University of Delaware, we are measuring the host range of biological control agents that are candidates for introduction against the soybean aphid, Aphis glycines, a major new pest of soybeans in the U.S. From China and Japan where this aphid originates, 10 populations/species of parasitoid and one predatory fly have been imported into quarantine and established in culture. The host ranges of three populations/species of parasitoids in the Aphelinus-varipes species complex and one species in the Aphelinus-asychis species complex were tested, and they were found to attack a wide variety of aphid species. From data in the literature, it appears that the remaining candidates in quarantine may also have broad host ranges, although this remains to be confirmed in experiments. Nonetheless, our results so far indicate that further exploration is needed to find biocontrol agents with narrower host ranges to avoid impact on non-target species.