The effect of the aphid sex pheromone on the aphid Myzus persicae and its parasitoid Aphidius colemani

Abstract

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.

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