Leucopis argenticollis Zetterstedt and Leucopis atrifacies (Aldrich) have been collected from hemlock woolly adelgid, Adelges tsugae Annand, infested western hemlocks, Tsuga heterophylla (Raf.) Sargent, in the Pacific Northwest (PNW). L. argenticollis and L. atrifacies are being investigated as candidates for A. tsugae biological control in the eastern United States. Abundance of L. argenticollis adults and chamaemyiid larvae were correlated with A. tsugae density on T. heterophylla in the PNW. Both Leucopis species have been reared from A. tsugae in the laboratory. L. argenticollis and L. atrifacies have been reported to feed exclusively on adelgids; however, they have not been previously reported on A. tsugae. Observations of L. argenticollis and L. atrifacies life histories and their synchrony with A. tsugae in the PNW are reported. Feeding behavior and host range of L. argenticollis in the laboratory will also be discussed.