Effects of parasitoid/host adaptive asymmetry on biocontrol stability

Abstract

Recent simulation modelling has strongly supported this success of classical biological control may depend on a control agent reaching an evolutionary e­quilibrium with its target pest species. In New Zealand, over the last 5 years, evidence has been accumulating showing that there has been a very significant decline in attack rates of the sexually reproducing pest weevil, the Argentine stem weevil Listronotus bonariensis, by its asexual braconid endoparasitoid Microctonus hyperodae. Indications have continued to point to resistance having occurred in the Argentine stem weevil population probably as ­the result of asymmetries in reproductive strategies of the weevil and para­sitoid. This has led to an 'unequal evolutionary arms-race' causing host resistance. At the same time the modelling also tested the likely equilibrium outcome when both the parasitoid and the host reproduce sexually, assuming adaptive potential in both species. This showed empirically that a decline in parasitism would not occur. Usually there are New Zealand data from the 1980s that showed that the sexu­ally reproducing parasitoid Microctonus aethiopoides had supressed the damage caused by the lucerne weevil Sitona discoideus. Sampling from the same ecosystems as those in the 1980s S. discoideus populations were again taken on 10 occasions between 2014 and 2017 and showed no sign in parasitism. This is in complete contrast to the L. bonariensis/M.hyperodae dynamic, thereby conforming to the predictions of the model.