Observations by other authors showing that T. interruptor attacks more host larvae that had been previously parasitized by O. obscurator than unparasitized hosts were confirmed in the present study. Preference for buds previously visited by females of O. obscurator, as compared to unvisited buds, was demonstrated. Preliminary experiments to determine the factors involved in this preference and their implication are reported. These studies indicate that the policy of importing and releasing T. interruptor in North America to assist in the control of R. buoliana was unwise.