Leucopis spp. (Diptera: Chamaemyiidae) from the Pacific Northwest previously were identified as potential biological control agents for the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), in the eastern United States. We collected Leucopis spp. larvae from A. tsugae infested western hemlocks in Oregon and Washington and reared them on an unidentified Pineus spp., Pineus strobi (Hartig), Adelges cooleyi (Gillette), Adelges piceae (Ratzeburg), and A. tsugae in three no-choice tests. Leucopis spp. survival on A. tsugae was significantly higher than on A. piceae during the 2010 progrediens generation test and significantly higher than on P. strobi and A. cooleyi during the 2010 sistens generation test. However, across all three tests, some larvae completed development to adult on all four of the alternative adelgid species. Larvae that survived to the adult stage were identified as Leucopis argenticollis Zetterstedt and Leucopis piniperda Malloch. These results suggest that populations of L. argenticollis and L. piniperda in the Pacific Northwest may not be specific to A. tsugae. We also studied the phenology of Leucopis spp. on fourteen A. tsugae infested western hemlock trees in Oregon and Washington over a period of 14 mo. Leucopis spp. larvae were collected year-round, but highest densities coincided with the presence of progrediens and sistens eggs and adults of A. tsugae. There was a positive correlation between Leucopis spp. and A. tsugae abundance.