[AGRIFOOD] Trophic cascades in a changing climate - effects of elevated CO2 on breakdown of plant defences

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Atmospheric carbon dioxide concentrations are predicted to rise to 550ppm by 2050 with concomitant increases in plant productivity. Such predictions seldom account for plant-insect interactions that under climate change may undermine projected increases in primary production by altering plant resistance to herbivory. This has potentially major implications for future food security. Climate change has the potential to modulate plant resistance to herbivory. Elevated CO2 concentrations (eCO2) have been shown to compromise both direct and indirect plant defences to insect herbivores, for example, by down regulation of plant resistance genes leading to enhanced herbivore performance. In many agro-ecosystems, plant defences arise through selective breeding which means plants are unable to adapt resistance mechanisms quickly enough to counteract the compromising effects of eCO2. Moreover, it remains unclear how higher trophic levels will respond to increases in herbivore abundance when plant defences are compromised. Using a multi-trophic system comprising red raspberry (Rubus idaeus), the large raspberry aphid (Amphorophora idaei), a predatory ladybird (Coccinella septempunctata) and an aphid parasitoid (Aphidus ervi), this PhD will investigate the effects of eCO2 on multi-trophic interactions, and specifically plant defence breakdown. Amphorophora idaei is the most significant pest (virus vector) of raspberry production in Europe. Plant resistance to aphid feeding in raspberry cultivars is underpinned by A1 and A10 genes, with A10 conferring stronger resistance, probably through altered leaf wax composition. Preliminary findings suggest aphids overcome resistance in raspberry under eCO2 because of altered gene expression. This project aims to: (1) characterise the effects of eCO2 on plant resistance to different aphid biotypes and identify which genes are implicated; (2) measure the phenotypic changes in plant defence mechanisms underpinning resistance breakdown; and (3) determine how population dynamics of higher trophic levels are affected by climate-induced changes in herbivore abundance. With experimental microcosms in controlled environment facilities, this project will test whether: Hypothesis 1: eCO2 accelerates resistance breakdown, with partially adapted aphid biotypes responding most rapidly to the down-regulation of resistance genes; Hypothesis 2: eCO2 alters composition of leaf waxes associated with aphid resistance; and Hypothesis 3: predator and parasitoid populations will follow a time-lagged increase corresponding to larger aphid populations under eCO2, but mutual interference will impede foraging behaviour at highest aphid densities. This PhD proposal is strongly aligned to the NERC Open Case Priority area of Agrifood research, with particular emphasis on the effects of climate change on proliferation of pests through resistance breakdown in a model crop. The impact of this study will be to provide mechanistic evidence of how multi-trophic interactions are likely to alter under climate change. This will enable crop breeders to target particular plant resistance traits and biocontrol measures for the adaptation and 'future proofing' of crop production under climate change. This contributes directly to the NERC Strategic Plan to enable society to respond urgently to global climate change. The project will meet key objectives of LWEC by investigating how climate change will affect crop-herbivore-enemy interactions and provide timely evidence-based recommendations to policy makers charged with climate change adaptation and mitigation.