Competition among parasitoid species on a stage-structured host and its effect on host suppression

An age-structured model in which two parasitoid species attack different developmental stages of a single host species is developed. Two versions of the model, differing in the outcome of larval competition, are presented. In the first version, hosts that are parasitized by the egg parasitoid cannot be attacked later by the larval parasitoid. In this version, the two species are not able to coexist. The egg parasitoid gains a competitive advantage through attacking the earlier host stage. The species that wins in competition is not necessarily the one that would produce the lowest adult host density. In the second version, hosts that are parasitized by the egg parasitoid can later be attacked by the larval parasitoid, which produces an adult of the larval parasitoid species. In this case, the two species can coexist because hosts that have previously been parasitized by the egg parasitoid serve as an additional resource for the larval parasitoid. When the two species do coexist, the density of each of the stages of the host is intermediate between the densities obtained when each of the two parasitoid species is present alone. These results suggest that the lowest host density in cases of biological control may be achieved by the release of only the most effective parasitoid rather than multiple parasitoid species.