Investigating the influence of endosymbionts and population genetics on the predacious ladybird Chilocorus nigritus: implications for biocontrol

The scale insect predator Chilocorus nigritus is considered as one of the most successful and important biological control agents in classical biocontrol. The ladybird is currently a commercial product, used for scale insect control in glasshouse environments, but despite widespread success in wild field releases, use of the predator in glasshouses has achieved only moderate success. This study aimed to find out which factors may affect the success of C. nigritus in glasshouse pest control. Two key factors with potential to have a dramatic impact on the predator were identified; genetic variability within and between insect populations, and male killing endosymbiotic bacteria, known to have a diverse range of effects on up to 70% of all insect species.

Beetle strains were sourced from several insectaries and geographical locations. DNA sequencing determined significant genetic differences between biotypes of C. nigritus from different localities, indicating that C. nigritus exists as a series of functional biotypes across its range. All biotypes were tested for Wolbachia, Rickettsia and Spiroplasma infections, identifying Rickettsia and Wolbachia presence in most populations. Uninfected sub-lines of these strains were therefore created via tetracycline treatment. A number of bionomic characteristics of the beetle were compared across these strains and infection types in order to assess the influence of genetics and bacterial endosymbionts.

Significant effects of endosymbionts were noted in fecundity and prey consumption, and genetically distinct biotypes varied in their prey consumption. A combination of biotypes and infection types also provided successful suppression of scale insects in glasshouse trials at Royal Botanical Gardens, Kew, and beetles were shown to produce a defence pheromone similar to hippodamine, which could potentially be another strong influence on the species’ ecology. This thesis suggests that variability within a predator population may be an asset, rather than a hindrance.