Disease mitigation and pathogenic control in German and common wasps, Vespula germanica and V. vulgaris (Hymenoptera: Vespidae) : a thesis completed in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University, Canterbury, New Zealand

Abstract

Vespula germanica (F.) (German wasp) and V. vulgaris (L.) (common wasp) are major pests in New Zealand. They damage crops, have a significant impact on the biodiversity of native ecosystems and present a significant health hazard by stinging people and animals. Efforts to control them, both chemically and biologically, have not been as effective as initially hoped. This research contributes to a larger programme studying the potential of entomopathogenic microbes for the control of German and common wasps in New Zealand. This study has demonstrated the potential and limitations which exist for control of wasps using microbial pathogens. Effective pathogens were identified and disease transmission quantified in bioassays. Behavioural and physiological adaptations of wasps were investigated as resistance mechanisms to disease.

Three species of fungi (Beauveria bassiana, Aspergillus flavus and Metarhizium anisopliae) were identified as having potential to kill workers and larvae and severely disrupt the colony. In laboratory trials, fungi and bacteria were readily transmitted between workers and larvae and among workers. Workers and larvae, individually, possess few resistance mechanisms capable of suppressing a mass infection of pathogenic fungi. No tested bacteria were pathogenic to adult wasps.

Although B. bassiana was extremely pathogenic in bioassays, behavioural adaptations of wasp workers and larvae restricted proliferation of fungi in a healthy nest. Anomalies in behaviour when removing sporulating or infectious cadavers indicate a 'window-of-opportunity' for the dissemination of disease if the fungus could achieve maturity. However, getting sufficient inoculum into the nest and achieving widespread sporulation to establish an epizootic will be a significant challenge.

All of the bacteria tested were susceptible to antibiotics in larval saliva, and therefore not significantly pathogenic to wasps. Significant inhibitory effects of venom against A. flavus were identified, but no evidence of its use in the nest could be found.

A more cryptic and infectious disease, such as a bacterium or microsporidian, holds the greatest potential for future control efforts. It is apparent that, in order to establish an infection capable of developing into an epizootic in wasp nests, a microbe needs to be gradually invasive, readily transmitted, a prolific replicator, cryptic and have little impact on host behaviour leading up to maturation. Limitations to the current use of pathogens for wasp control may be overcome by strain selection and development of delivery systems. Pathogens have the potential to complement and enhance existing biological control programmes, but much research is needed to realise this potential.