The biological control of the red clover casebearer in New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Biocontrol of insect pests represents an important aspect of sustainable crop management that has potential to reduce the current reliance on chemical control measures. However, often the potential of all forms of biocontrol is undermined in contemporary productive systems and their resulting landscapes. In New Zealand seed three species of clover feeding moths of the Coleophora genus have seriously hindered red and white clover seed production during the last 100 years in New Zealand. From the 1920s until the 1960s two species of white clover casebearer (Coleophora mayrella and C. alcyonipennella) caused varying levels of seed yield reduction in white clover seed crops. During the 1960s efforts were made by government science agencies to establish parasitoid wasps of these species. This programme was highly successful, resulting in the total suppression of clover casebearer seed pests in New Zealand, until the arrival of the red clover casebearer (C. deauratella) during 2016. This pest quickly spread throughout the country’s clover growing regions, leading to yield reductions. This thesis explores the arthropod community of red clover seed crops under a main theme of ‘the biocontrol of the red clover casebearer’. The experiments employed are varied, although typically with a major aim being exploring the function of species within the red clover seed crop ecosystem. Three species of parasitoid wasp were able to successfully use C. deauratella as a host; Bracon variegator, Pteramalus puparum and Eupelmus (Macroneura) messene. The red clover casebearer was clearly undergoing remission as a pest and only two damaging populations of this pest could be located. The predominant parasitoid, B. variegator, had a field parasitism incidence of up to 75% on C. deauratella, but despite this these two populations of the pest persisted. Over the course of these experiments it was noted that a fungal growth was leading to mortality of C. deauratella larvae. From infected C. deauratella cases a potentially entomopathogenic strain of Fusarium pseudograminearum was isolated and identified through PCR sequencing. It is still uncertain how C. deauratella is being regulated in New Zealand red clover seed crops. The role of parasitoid wasps, a potential fungal pathogen and other unknown biocontrol agents is yet to be determined.