The Old World parasitesCotesia chilonisandC. flavipeswere compared for survival, parasitization, and utilization of the New World hostDiatraea saccharalis. Parasite cultures consisted of one collection ofC. chilonisfrom Japan and three collections ofC. flavipes, two temporally separate collections from the Rio Grande Valley of Texas, and a collection from Thailand. Adult females kept without food or water lived an average of 20.3 h and did not differ among parasite cultures. Egg-larval development averaged 13.1 d and did not differ among the cultures. Pupal development averaged 6.5 d forC. chilonis, 7.6 d for the two TexasC. flavipescultures, and 8.5 d for the ThaiC. flavipesculture. Numbers of progeny per parasitized host ranged from 73.9 for one TexasC. flavipesculture to 36.7 forC. chilonis. TheC. chilonisculture successfully parasitized 76% of hosts, versus 52 and 57% for the two Texas cultures, and 30% for the Thai culture. Seventy-nine of the 800D. saccharalishosts exposed to parasites encapsulated parasite progeny, as evidenced by a localized, dark spot on the posterior dorsum of host larvae. The magnitude of encapsulation differed greatly among cultures. Only one host exposed toC. chilonisencapsulated parasites (0.5%), whereas the threeC. flavipescultures averaged 13% of hosts that encapsulated the parasites. Host larvae exhibiting the characteristic dark spot had several outcomes: delayed complete or partial host pupation (22%): hosts remained in a terminal larval stage (50%): or hosts produced a few parasite progeny (28%). Few differences were found between the two parasite collections from Texas that differed only in the duration of time in culture. However, the results suggested biological differences amongC. chilonis, the Thai collection, and the two collections ofC. flavipesfrom Texas.