Host-parasitoid avoidance behaviour in the context of contemporary evolution in insect classical biological control : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Pests are increasingly invading novel environments due to global trade and travel and their management requires a greater emphasis on classical biological control than has previously been the case. This approach has been particularly successful in New Zealand pasture such as with the Argentine stem weevil, Listronotus bonariensis (Coleoptera: Curculionidae) (ASW). This pest was successfully managed by releasing the parthenogenetic parasitoid, Microctonus hyperodae (Hymenoptera: Braconidae), in the early 1990s with ASW parasitism rates quickly reaching over 75 %. However, these rates have substantially declined in the last decade. This decline was hypothesised to be due to contemporary evolution of enhanced avoidance behaviour by the sexual ASW and that these behaviours are influenced by the host plant. The work in this thesis used microcosms in laboratory experiments to investigate whether there was any behavioural evidence supporting the above hypotheses, which if confirmed, may explain recent low parasitism rates. Experiments were conducted to examine the ASW behavioural responses to M. hyperodae and investigate whether these ASW responses differed between the ryegrass host plants; diploid hybrid (diploid Lolium perenne x diploid Lolium multiflorum), diploid L. perenne and tetraploid L. multiflorum. Furthermore, ASW behavioural responses to the presence of M. hyperodae were compared between ASW populations collected from areas with different parasitism rates, history and latitude. The sensitivity of ASW responses to different parasitoid species was also investigated. The work in this thesis showed that the weevil’s avoidance behaviour differed depending on the host plant, with ASWs on the diploid hybrid having the most consistent reduced feeding and plant abandonment responses to M. hyperodae. ASW on diploid L. perenne had similar responses to weevils on the diploid hybrid but these responses were delayed. ASW on tetraploid L. multiflorum showed a reduced feeding response but no plant abandonment. These results reflected recent ASW parasitism rates that were low on the diploid host plants compared to those in the 1990s and may explain the behavioural mechanisms behind the biological decline. Similarly, behavioural responses to M. hyperodae differed between ASW regional populations, which reflected current parasitism rates and history. ASWs from the Waikato (Ruakura) region had the strongest behavioural responses to M. hyperodae where the greatest parasitism decline has occurred. There were delayed feeding reduction and plant abandonment responses to M. hyperodae from ASW populations (Canterbury: Lincoln; North of Auckland: Wellsford) with a medium decline in parasitism. There was minimal response from southern ASW populations (Otago: Invermay; Wiapiata) that have always had low parasitism rates with no decline detected. When comparing ASW responses to different parasitoid species, M. hyperodae showed the strongest responses with increased crouching and reduced feeding compared to other parasitoid species. There was some behavioural response to Microctonus aethiopoides (Hymenoptera: Braconidae), a parasitoid of the clover root weevil (Sitona obsoletus) (Coleoptera: Curculionidae), suggesting that ASW has pre-adaptation to this novel interaction. The findings in this work strongly suggest that ASW behavioural responses are the mechanism behind the potential contemporary evolution causing the parasitism decline. Future work could investigate the behavioural effects of M. hyperodae from different populations, and what cues ASW uses to detect M. hyperodae. Furthermore, identifying the underlying genetic or symbiont mechanism for the increased sensitivity of ASW behaviour is essential to confirm contemporary evolution, which would be novel in the literature. This work also has implications for the persistence of other long-term successful insect biological control systems and points to what factors may lead to their future decline in efficacy.