The Effect of Simulated Precipitation Change on Multi-trophic Interactions in a Cereal Crop

Climate models predict a 40% reduction in UK summer precipitation by 2080, together with increases in the frequency of drought and heavy rainfall events. These changes will have serious implications for UK agriculture as crop growth and yield as well as insect herbivores and their natural enemies will be affected by changes in soil moisture at a time when there is pressure to reduce inputs of chemical pesticides. To date, research has focused on the effect of continuous periods of drought on crop production, not considering the effect of changes in the frequency of rainfall events on plant growth and interactions between insect herbivores and their natural enemies in cereal ecosystems. The aim of this thesis was to investigate the impact of changes in the quantity and frequency of precipitation events on multi-trophic interactions in a barley ecosystem.

A 40% reduction in mean precipitation significantly reduced barley growth, yield, and changed plant chemical composition, whereas reductions in the frequency of precipitation events alleviated some of the negative effects of drought on barley plant growth, suggesting that changes in the quantity of rainfall may have larger negative effects on barley yield than changes in extreme rainfall events. Under future changes in precipitation patterns, below-ground insect herbivores, specifically wireworms, were found to have a smaller effect on plant biomass compared to under ambient watering regimes. In contrast, above-ground aphids increased in mass when feeding on plants grown under future precipitation patterns. Wireworm herbivory reduced the number of above-ground aphids on young barley plants but this interaction was removed under future precipitation patterns. These effects were evident in the third trophic level: Harlequin ladybirds increased in mass when feeding on aphids collected from plants subjected to future precipitation patterns.

This thesis provides the first experimental evidence that future reductions in mean precipitation and rainfall events can significantly affect multi-trophic interactions in a cereal ecosystem, with consequences for the abundance of pests and invasive species.