Microplitis demolitor Wilkinson were reared from Heliothis zea larvae fed either an artificial diet or a diet of cowpea seedlings. Responses of females from these two sources to semiochemicals from the insect host-cowpea complex were compared. Very limited responses were obtained from females reared from hosts fed artificial diet unless they first had preflight contact with frass from plant-fed hosts. Female parasitoids reared from plant-fed hosts were generally responsive without preflight contact. Contact with cocoons containing frass of the plant fed host at the time of emergence was an important source of stimulation. none