Technical Abstract: Nematodes that selectively attack insects, such as Steinernema carpocapsae, have good potential for biological control of cocooned CM larvae and pupae. Their use along side of parasitic insects could produce antagonistic interaction between the two groups resulting in death of parasitoid larvae. Two parasitic wasps, Mastrus ridibundus and Liotryphon caudatus, that were collected in Central Asia and imported into the United States for classical biological control of cocooned codling moth larvae were studied regarding their interactions with S. carpocapsae. Exposure of M. ridibundus and L. caudatus developing larvae to infective nematodes within codling cocoons resulted in mortality that was comparable to that of codling moth larvae. However, overwintering full grown parasitoid larvae are almost completely protected from nematode penetration within their own tightly woven cocoons. Our investigations also reveal the ability of M. ridibundus and L. caudatus females to detect and avoid laying eggs on nematode-infected cocooned codling moth larvae as early as 12 hours after treatment of the host with nematodes. The compatibility of the two groups of biological control agents for codling control could be facilitated by careful timing of applications. The ability of parasitoids to avoid nematode-treated larvae and to actively seek out and kill cocooned codling larvae that survived nematode treatments enhances the complementarity of entomopathogenic nematodes and M. ridibundus and L. caudatus.