Pontania proxima (Tenthredinidae: Hymenoptera) : Natural enemies and defensive behavior against Pnigalio nemati (Eulophidae: Hymenoptera)

Abstract

The larval hymenopterous ectoparasitoid Pnigalio nemati (Westwood) was the dominant parasitoid and accounted for a nearly 4% reduction in population size in Pontania proxima (Lepeletier) in galls on the crack willow, Salix fragilis L., in south Dublin, Ireland, during April-November 1995. However, the observed population of P. proxima enjoyed the very high survival rate of 76.7%; this may be because of the relatively weak influence of the sawfly's natural enemies. In the laboratory, it was found that with increasing age and size, the frequency and effectiveness of P. proxima's defensive behavior against P. nemati also increased. The 4th and 5th instars and the prepupa were the most aggressive, leading to a lowered acceptance of these stages by the parasitoid and increased failure to complete oviposition once it was begun. Furthermore, the mean duration of attempted oviposition by the parasitoid was reduced by the defensive reactions of the host.