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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.

Multitrophic and intraguild interactions influence the success of biological control. The interactions between Acyrthosiphon pisum, three natural enemies (Pandora neoaphidis, Coccinella septempunctata and Aphidius ervi) and the host plant, Vicia faba, were assessed. Volatiles released from aphid-damaged plants had a direct effect on P. neoaphidis indicating they may act as synomones. However, volatiles did not increase efficacy of the fungus suggesting it is not a bodyguard species. Transmission was greatest during plant colonisation by aphids and was not affected by plant condition. Infection by P. neoaphidis had a direct negative effect on the fitness of the aphid through reduced reproduction and early host death. Avoidance of infected colonies by predators and parasitoids could reduce the effectiveness of guilds of natural enemies for biological control. However, at the laboratory scale A. ervi and C. septempunctata did not detect infection and entered and foraged in infected aphid colonies. Aphidius ervi spent longer searching for hosts on plants that had been damaged by aphid feeding (and were emitting aphid-induced species-specific volatiles) and this may increase fungal transmission. Coccinella septempunctata and A. ervi significantly reduced populations of A. pisum when introduced as individual species whereas P. neoaphidis had no effect on aphid population size. Foraging by both C. septempunctata and A. ervi increased the abundance and distribution of P. neoaphidis which may be sufficient to initiate an epizootic. The benefits of increased transmission by C. septempunctata outweighed the fitness costs to the fungus of intraguild predation. Pandora neoaphidis was associated with a decrease in the reproductive success of A. ervi, which was further reduced as the competitive advantage of the fungus increased. This could result in competitive exclusion of the parasitoid. Poly tunnel experiments confirmed that A. ervi did not discriminate between infected and uninfected aphid colonies at this spatial scale. However, A. ervi did not incur a fitness cost from foraging in patches containing the fungus. These results indicate that C. septempunctata and P. neoaphidis may be effective as multi-species biocontrol agents. In contrast, competition between P. neoaphidis and A. ervi may reduce their overall effectiveness as control agents. Further work is required at larger spatial scales and over several generations of both the pest and natural enemy species to confirm these interactions. The implications of these results for the use of P. neoaphidis as part of a multi-species biological control program are discussed.

Aphids remain an enormous threat to the sustainability of crops in glasshouse and field environments around the world. It is known that the aphid sex pheromone is used as a kairomone by its natural enemies, such as parasitoids. The focus of this research was how the aphid sex pheromone component, (4aS,7S,7aR)-nepetalactone, affects a host, its parasitoid and the host-parasitoid interaction in a tritrophic system. A model system of Chinese cabbage Brassica rapa sp. Pekinensis Cv. Wong bok, the peach-potato aphid Myzus persicae and the generalist parasitoid Aphidius colemani is applied with a particular emphasis on understanding parasitoid foraging and how it may be affected, and potentially manipulated, by nepetalactone. Firstly, it was demonstrated that asexual M. persicae are capable of detecting the sex pheromone components, despite their components having no previously known ecological function in parthenogenetic populations. Although it was found that they avoid the odour in high concentrations, it was concluded that performance on an individual or population level were unlikely to be affected. The ability of the parasitoid A. colemani to detect nepetalactone was confirmed at the electrophysiological level. Nepetalactone did not elicit any behavioural response when presented in isolation but was found to increase retention of the parasitoid within a patch if other host cues were also present. It was found that Nepeta cataria oil, from which nepetalactone can be isolated, increased the success of parasitoid oviposition in the host. To enhance parasitoid foraging, it was investigated whether learning was possible with nepetalactone; an odour already known to elicit an innate response. Learning through emergence conditioning was ineffective in altering parasitoid behaviour; however, ovipositional experience did induce a change in foraging patterns. This change in foraging pattern did not translate to more effective host location when tested in the laboratory, which led the research towards experimentation in a more complex spatialtemporal environment. Nepetalactone, or the learning of nepetalactone, were not found to have an effect on parasitoid success at this scale. It was found that the introduction of parasitoids into a glasshouse environment reduced aphid population growth at a rate disproportionate to the rate of mummification. This highlighted the importance of indirect consequences of parasitoid visitation on aphid population control. In a separate assay it was identified that aphid population size affects plant fitness, such that smaller aphid populations result in greater plant fitness, thus demonstrating benefits of parasitoids in biological control which are often overlooked. This work provided a greater insight into the role of nepetalactone in a tritrophic system and how odours may be used by parasitoids during foraging. Finally, the key findings of this study are discussed and the possible direction of future work. A new interpretation of parasitoid foraging is discussed, by the integration of information provided by this study and knowledge generated by previous work.