How parasitoid sex ratio, size and emergence time are associated with fruit tree cultivar, within-orchard tree position and ants

Abstract

The biocontrol potential of naturally occurring parasitoids is influenced by the parasitoids’ population and individual characteristics. We studied field determinants of characteristics of the parasitoid Scambus pomorum (Hymenoptera: Ichneumonidae) emerging from weevils (Anthonomus pomorum; Coleoptera: Curculionidae) hidden in damaged apple blossoms. The studied determinants comprised local-scale factors that can be managed by individual growers: tree cultivar, distance between apple trees and forest, and presence of ants. The studied parasitoid characteristics were sex ratio, body size and emergence time. Parasitoid sex ratio, in general female-biased, was significantly different for parasitoids emerging from hosts feeding on different apple cultivars. This finding suggests sex ratio adjustment driven by plant genotype-dependent variation in parasitoid host quality. The detected significant increase of sex ratio (more males) with increasing distance to forest might be explained by sperm depletion of ovipositing parasitoid females immigrating into the orchard. Exclusion of ants significantly increased female-bias in sex ratio in one of the studied apple cultivars. Body size of female and male parasitoids was significantly different between parasitoids emerging from different apple cultivars, supporting the view of cultivar-dependent variation in the quality of the parasitoid’s host. Distance to forest was positively correlated with parasitoid size, indicating farther dispersal of larger individuals. Emergence time varied significantly between apple cultivars, probably due to differences in plant phenology. By demonstrating that parasitoid characteristics vary widely within an orchard, this study shows that parasitoid characteristics that are relevant for biological control might be improved via appropriate management of the orchard and its immediate surroundings.