Bottom‐up effects shift top‐down community structure of the parasitoid guild that attacks Asphondylia borrichiae (Diptera: Cecidomyiidae) on Borrichia frutescens (Asteraceae)

Abstract

Abstract Asphondylia borrichiae induces galls at the apical meristems of its host plant, Borrichia frutescens, and the larvae develop and pupate within the gall. During development, immature stages of the midge are attacked by four species of parasitic wasps; two of these parasitoids, Galeopsomyia haemon and Torymus umbilicatus, are the most common members of the guild, and both are hyperparasitic. In the current study, plots of B. frutescens were randomly assigned to control (unmanipulated), salted or fertilised in a long‐term press experiment. Although plots initially exhibited no differences in stem length, gall and flower densities, after 3 months, they began to exhibit significant differences, suggesting stress (salted plots) and vigour (fertilised plots). Galls produced on plants from fertilised plots were approximately 20% and 31% larger than those from control and salted plots, respectively. Larger gall diameter in fertilised plots resulted in a shift of the parasitoid community towards the largest parasitoid, T. umgbilicatus, whereas smaller galls produced in salted plots became biased towards the smallest, but gregarious, G. haemon; interestingly, this pattern held for T. umbilicatus across treatments, and the effect for G. haemon was not significant when all treatments were combined. This study suggests that abiotic factors that alter host plant characteristics, such as gall diameter, affect the composition of the parasitoid guild that attacks A. borrichiae, which has implications for the acquisition of enemy‐free space during host range expansion of the midge.