Ruth N. Wade, Alison J. Karley, Scott N. Johnson and Sue E. Hartley
Climate change is predicted to cause more variable rainfall in the UK, including increased incidence of extreme drought and heavy rainfall events, and overall reductions in summer precipitation of up to 40% by 2080. These altered rainfall patterns will mean periods of stress and recovery for plants, with dry days followed by heavy rainfall events potentially impacting plant growth. Herbivores and their predators will also be affected by changes in rainfall through altered availability and quality of their food. Interactions between species in a food chain underpin the functioning of an ecosystem; therefore, understanding the impacts of future changes in precipitation on such interactions is important for food security, pest management and for constructing more accurate predictions of global change impacts. Despite the importance of this knowledge, little research has been conducted to investigate whether changes in plant growth arising from predicted rainfall patterns affect interactions between above- and below-ground herbivores and insect predators.
We conducted two experiments, one in controlled conditions and one outside under rainshelters, to test the effect of a 40% reduction in water supply, at two different watering frequencies, on the interactions between root and shoot feeding herbivores and their natural enemies. A continuous drought regime reduced plant growth significantly whereas plant growth was unaffected under a drought regime where water was delivered by infrequent ‘ deluge’ events. However drought/deluge watering events changed the chemical composition of the plant, which benefitted the growth of shoot-feeding aphids and their predator, the invasive harlequin ladybird. Furthermore the drought/deluge watering regime reduced the negative effects of root-feeding wireworms on plant growth and aphid numbers above-ground.
Therefore, changes in the quantity and frequency of rainfall events had a significant impact on insect herbivores above- and below- ground and these effects transferred up the food chain to insect predators, indicating consequences of altered rainfall patterns for crop production, control of agricultural pests and success of invasive insect species.
Image caption: Wireworm foraging for roots. Photograph taken by Ruth Wade.