2004 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 collaborative effort 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, 17 populations/species of parasitoid and one species of predatory fly have been imported into quarantine and established in culture. Most material is in the Aphelinus-varipes complex. To reduce the work involved in measuring host specificity of each population, we measured reproductive compatibility between selected populations from Japan and China and others in the Aphelinus-varipes complex for which we already had host specificity measurements to test whether reproductively compatibility would predict host specificity. Unfortunately, reproductively compatible populations sometimes showed differences in host specificity. Thus we are now measuring host specificity for the remaining 6 populations in the Aphelinus-varipes complex.