The effects of wildflowers on the longevity and fecundity of Diadegma insulare (Cresson), one of the major parasitoids of diamond back moth, Plutella xylostella (L.), in North America, were studied in the field. Wildflowers provided nectar sources for D. insulare. Longevity and fecundity of the parasitoid female varied with wildflower species and the morphological characteristics of the flower. Several flowers. including Brassica kaber (D.C.) Wheeler, Barbarea vulgaris R. Br., and Daucus carota L., supplied nectar and resulted in D. insulare longevity and fecundity equal to when honey–water was supplied as food. Others, including Erysimum cheiranthoides L. and Thlaspi arvense L., were not significantly better than no food at all. Chenopodium album L. and Sonchus arvensis L. did not provide available nectar, however, adult parasitoids fed on honeydew excreted by Aphis fabae (Scopli)feeding on the plants. Fecundity of D. insulare generally peaked 6–15 d after adult emergence. An increase in longevity and fecundity was correlated with flower corolla opening diameter. Shading also increased longevity and fecundity of D. insulare. The oviposition behavior within 1 min of exposure to diamond back moth larvae was highly correlated with longevity and fecundity of D. insulare, whichwe considered indices of food quality. Seasonal manipulation of the diversity and distribution of wildflowers in the cabbage field and adjacent habitats, as well as providing shade for D. insulare, could increase D. insulare effectiveness in management of diamondback moth.