Characteristic Permitting Coexistence Among Parasitoids of a Sawafly in Quebec

Abstract

–Six poarasitoid species (Hymenoptera: 5 Ichneumonidae, 1 Eulophidae) coexist on cocoon populations of the Swaine jack pine sawfly, Neodiprion swainei Midd. in Quebec. Their distribution between plant communities was related to host availability except for Gelis urbanus (Brues) which has alternative hosts. Pleolophus basizonus (Grav.) was dominant at high host densities, tending to displace Pleolophus indistinctus (Prov.) which was dominant at low host densities. Mastrus aciclatus (Prov.) occupied dry open sites at high host densities, where other species were less numerous. The highest species diversity of cocoon parasitoids occurred at the ecotone around jack pine stands and at moderate host densities in increasing host populations. Furthermore, parasitoids attacking larvae were well represented in moderately dense host populations. As the host density increased, P. basizonus became so dominant that it reduced species diversity, particularly among the parasitoids attacking the host larvae. Species diversity can be predicted from the proportion of P. basizonus present. This parasitoid acts as an organizer species within the parasitoid complex because of its superior coimpetitive ability. Parasitoids of larvae have a conservative strategy for host exploitation that appears adapted for utilizing low host populations. In contrast, the parasitoids attacking cocoons gain dominance only when host—finding is easy in high host populations.