Phacelia tanacetifolia Bentham is known worldwide as a kind of insectary plant and is used in Japan as a green manure crop before Welsh onion planting. To explore the possibility of using P. tanacetifolia as a banker plant or an insectary plant for parasitoids of Liriomyza chinensis Kato, we investigated the seasonal abundance of agromyzid leafminers and their hymenopterous parasitoids on infested leaves and blooming flowers of P. tanacetifolia collected in Chiba Pref., Japan, in 2009, 2010, and 2012. We also investigated longevity increase in Diglyphus isaea（Walker）adults with a blooming P. tanacetifolia plant in the laboratory. Almost all agromyzid leafminers reared from leaves were Chromatomyia horticola（Goureau）. Twenty-one parasitoid species including seven species reported from L. chinensis mines by different authors in Japan were reared from leaves, and nine species including eight species reared from leaves were caught on flowers. The most abundant parasitoid species was D. isaea. In this study, Diglyphus crassinervis Erdös was reared from C. horticola for the first time in Japan. The longevity of D. isaea adults provided with a blooming plant and water was about 18 days at 24°C with a 16L : 8D photoperiod. The log-rank test, used to compare survival curves from the 2 treatments; with and without blooming P. tanacetifolia plant, showed a statistically significant difference between treatments. These results suggest that P. tanacetifolia has potential as a banker plant or insectary plant for parasitoids to control L. chinensis in Welsh onion fields.