Simultaneous use of parasitoids and entomopathogenic nematodes for codling moth (CM) control could produce an antagonistic interaction between the two groups resulting in death of the parasitoid larvae. Two ectoparasitic ichneumonid species, Mastrus ridibundus and Liotryphon caudatus, imported for classical biological control of cocooned CM larvae were studied regarding their interactions with Steinernema carpocapsae. Exposure of M. ridibundus and L. caudatus developing larvae to infective juveniles (IJs) of S. carpocapsae (10 IJs/cm2; [approximately equal]LC80-90 for CM larvae) within CM cocoons resulted in 70.7 and 85.2% mortality, respectively. However, diapausing full grown parasitoid larvae were almost completely protected from nematode penetration within their own tightly woven cocoons. M. ridibundus and L. caudatus females were able to detect and avoid ovipositing on nematode-infected cocooned CM moth larvae as early as 12 h after treatment of the host with IJs. When given the choice between cardboard substrates containing untreated cocooned CM larvae and those treated with an approximate LC95 of S. carpocapsae IJs (25 IJs/cm2) 12, 24, or 48 h earlier, ovipositing parasitoids demonstrated a significant preference for untreated larvae. The ability of these parasitoids to avoid nematode-treated larvae and to seek out and kill cocooned CM larvae that survive nematode treatments enhances the complementarity of entomopathogenic nematodes and M. ridibundus and L. caudatus. Animals and man; Control; Biological control; Parasites diseases and disorders; Parasites; Helminth parasites; Insect parasites; Hosts; Insect hosts Steinernema carpocapsae (Nematoda); Control of; Cydia pomonella (Lepidoptera); Lepidopteran hosts; Cydia pomonella; Interactions with hymenopterans, biocontrol potential implications Cydia pomonella (Tortricidae); Control by; Ichneumonidae (Hymenoptera) & Steinernema (Nematoda); Nematode parasites; Steinernema carpocapsae; Interactions with hymenopteran parasites; Hymenopteran parasites; Liotryphon caudatus & Mastrus ridibundus; Interactions with nematode parasites, biocontrol potential implications Liotryphon caudatus; Mastrus ridibundus (Ichneumonidae ); Control of; Cydia pomonella (Lepidoptera); Lepidopteran hosts; Cydia pomonella; Interactions with nematodes, biocontrol potential implications none