The effects of host age, age of Cotesia marginiventris Cresson adults and temperature on interspecific competition between fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), larval parasitoids C. marginiventris, Microplitis manilae Ashm. and the egg-larval parasitoid, Chelonus insularis Cresson were studied. All parasitization results for C. marginiventris and M. manilae were from FAW larvae parasitized as eggs by C. insularis. The maximum reproductive potential for C. marginiventris was achieved in hosts initially parasitized by C. insularis and were 36 to 48 hours old and at a temperature of 25°C. No host discrimination was found for C. marginiventris between larvae containing a developing C. insularis and non-parasitized larvae. The optimum parasitoid age for C. marginiventris during the host exposure period was 48 to 96 hr. Younger or older parasitoids were not able to compete as successfully against C. insularis. A sex ratio of approximately 65% males was present in C. marginiventris progeny produced from parents ranging in age from 24 to 108 hr old. Egg to adult development times at 25°C were 17 and 26 days for C. marginiventris and C. insularis, respectively. In multiple parasitized larvae, C. marginiventris appeared to physically attack and destroy the larvae of C. insularis. However, M. manilae larvae died after 6 days without appearing to have been physically attacked in hosts containing C. insularis.

Interspecific competition within larvae of the fall army worm (FAW), Spodoptera frugiperda (J. E. Smith), by the larval parasitoids, Cotesia(= Apanteles) marginiventris Cresson and Microplitis manilae Ashmead, and the egg-larval parasitoid Chelonus insularis Cresson was studied. Chelonus insularis was not able to compete successfully against Cotesia marginiventris, but was a successful competitor against Microplitis manilae. Multiple parasitization of larvae by either Cotesia marginiventris or Microplitis manilae, which as eggs were parasitized by Chelonus insularis, did not result in additional host mortality. Percentage parasitization under greenhouse conditions by Cotesia marginiventris of larvae parasitized previously by Chelonus insularis was two-fold higher in corn compared to sorghum and more than four-fold higher compared to Bermuda grass and itch grass. The host finding and behavioural sequence of oviposition of Cotesia marginiventris in FAW larvae already parasitized by Chelonus insularis consisted of nine steps. Microplitis manilae females changed their behaviour significantly by displaying a reduction of ca. fifty per cent in host examinations, 45% in ovipositor probes, and 55% in apparent ovipositions when Chelonus insularis parasitized larvae were presented. Cotesia marginiventris and Microplitis manilae exhibited no significant behavioural changes in the per cent contacts, examinations and apparent ovipositions when attacking larvae parasitized previously by either Cotesia marginiventris or Microplitis manilae.