noneEnter Black-headed caterpillar, Opisina arenosella Walker (Lepidoptera: Crytophasidae) is an endemic, frequently outbreaking pest of coconut in India and Sri Lanka. It also occurs in Bangladesh and Myanmar. Two larval parasitoid wasps Goniozus nephantidis (Muesebeck) (Bethylidae) and Bracon brevicornis Wesmael (Braconidae), are widely employed, either singly or in combination, for the biological control of O. arenosella in India. The co-occurrence of G. nephantidis and B. brevicornis in the field indicates the potential for competitive interactions which may interfere with pest suppression. In order to study the outcome of competitive interactions between these parasitoids, host larvae were exposed to the parasitoid species either alone or in combination. Females of G. nephantidis and B. brevicornis did not discriminate between unparasitised hosts and hosts parasitised by the other species. Survival of G. nephantidis and B. brevicornis from eggs to adult eclosion was 89.6 and 81.8%, respectively, when larvae of O. arenosella were exposed to these parasitoids individually. When both species were released simultaneously, the survival of G. nephantidis was 86.8% and females of that species chased B. brevicornis females, attempting to bite and sting them. The mean percentage of progeny obtained was significantly higher for G. nephantidis than for B. brevicornis. G. nephantidis dominated due to its parental care behaviour producing 87.0% of progeny and a sex ratio (proportion of females) of 0.79, whereas B. brevicornis produced 84.0% progeny and a sex ratio of 0.36. Although parental care by G. nephantidis females serves to protect the progeny from intruders, it may be disadvantageous from the standpoint of pest suppression under field conditions especially, when the pest population density is high BR Animals and man. Control. Parasites diseases and disorders. Parasites. Insect parasites. Hosts. Insect hosts. Ecology. Competition. Goniozus nephantidis (Bethylidae): Parasite; Bracon brevicornis (Braconidae): Parasite; Opisina arenosella (Oecophoridae): Host. Bracon brevicornis, Goniozus nephantidis: Biological control, Competition between biocontrol agents, Lepidopteran hosts, Opisina arenosella, Interspecific competition; Opisina arenosella: Biological control, Competition between biocontrol agents, Hymenopteran parasites, Bracon brevicornis, Goniozus nephantidis. ST Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Aculeata, Chrysidoidea, Bethylidae; Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Braconidae; Animalia, Arthropoda, Insecta, Lepidoptera, Glossata, Heteroneura, Gelechioidea, Oecophoridae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans.