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The foraging strategies of two natural enemies of the peach-potato aphid Myzus persicae: the seven-spot ladybird Coccinella septempunctata and the parasitoid wasp Diaeretiella rapae, were investigated. Specifically the roles of plant semiochemicals in the location of plants infested with M. persicae by these natural enemies were examined. I investigated the olfactory responses of female C. septempunctata to volatiles collected from M. persicae and four Brassica cultivars; Brassica rapa, B. juncea, B. napus cultivar ‘Apex’ and B. napus cultivar ‘Courage’ and wild-type Arabidopsis thaliana that were: undamaged, previously infested by M. persicae and infested with M. persicae. C. septempunctata showed no attraction to volatiles from M. persicae alone. C. septempunctata significantly changed its searching behaviour in response to plant volatiles from B. rapa, B. napus cv. ‘Apex’ and Arabidopsis infested with M. persicae. C. septempunctata was also found to display a significant turning bias when foraging on a branching horizontal wire stem. A model was developed to investigate how turning biases affect the foraging efficiency of C. septempunctata in dichotomous branched environments. Simulations using this model indicated that turning biases could potentially increase searching efficiency. D. rapae showed a significant preference for volatiles from M. persicae infested wild-type Arabidopsis but no preference to volatiles from M. persicae alone or M. persicae honeydew. Volatile emissions by Arabidopsis were shown to be localised to the area of aphid-infestation rather than systemic. Using gas chromatography plants infested with M. persicae were shown to emit a quantitatively different volatile blend than undamaged plants. In experiments with jasmonate mutants of Arabidopsis the jasmonate (octadecanoid) wound response pathway was implicated as being important for the production of M. persicae induced volatiles, attractive to D. rapae. Other wound response pathways were also found to be involved in the production of the full blend of M. persicae induced volatiles.

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The mechanisms that enable plants to perceive and respond to aphids remain poorly characterised, particularly with respect to systemic signalling pathways. The aim of the studies reported in this thesis was to characterise the local and systemic signalling pathways induced by Myzus persicae infestation of Arabidopsis thaliana with translational aspects to potato (Solanum tuberosum). Particular emphasis was placed on the role of redox signalling pathways, which were studied in wild type A. thaliana (Col0) and in mutants that were deficient either in the major low molecular weight antioxidant ascorbic acid vitamin C defective 2 (vtc2) or in the abscisic acid (ABA) Insensitive-4 (ABI4) transcription factor or both components. Transcriptome analysis and metabolite profiling of leaves infested by M. persicae and on leaves from the same rosettes that were remote from the site of aphid attack revealed that the plant responses to aphids involved rapid local and systemic transcriptome re-programming in the absence of marked changes in the metabolite profiles. Moreover, the transcriptome reprogramming observed in infested leaves was different from the systemic response. The aphid-induced transcriptome signature of the infested leaves bore strong hallmarks of redox-signalling, salicylic acid (SA) signalling and ABA signalling, while that of the systemic leaves revealed a transcript profile where redox signalling was present but SA signalling was decreased. The vtc2 mutant showed decreased aphid fecundity, while aphid numbers were increased on the abi4 mutants. The differences in resistance to aphids between these genotypes are linked to alterations in ABA-dependent jasmonate-signalling pathways. When potato leaves were enriched in asorbate aphid fecundity was increased. These findings demonstrate the central role of redox signalling pathways in plant responses to aphids. Further characterisation of genes crucial for maintenance of redox homeostasis following aphid attack will inevitably facilitate development of aphid-resistant crops through GM technologies and marker-assisted selection.