[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Climate change is causing an increase in drought events around the world. Water limitation affects the quality and quantity of plant resources for herbivorous insects, resulting in cascading effects for higher trophic levels. Physiological changes due to water limitation may make plants more nutritious for insect herbivores, but it comes at the cost of increased plant defensive responses, declining biomass, and restricted access to the phloem. These changes in the plant have direct effects on natural enemies of herbivorous insects by changing the attractiveness of the habitat, and indirect effects by changing the quality of their insect prey. However, the outcomes of these interactions are still up for debate. My research examined the broad effects of water limitation on insect communities and elucidated mechanisms driving the response. This research is novel in that I assessed insect responses to multiple levels of water limitation and across several levels of ecological organization: plant-herbivore, tri-trophic, and within the framework of the greater arthropod community. First, I examined effects of plant water limitation on the arthropod community associated with wheat (Triticum aestivum L.), with specific focus on aphids and their natural enemies. Second, I determined how plant water limitation affects bird cherry-oat aphid (Rhopalosiphum padi) performance and population growth, and investigated plant traits that influence the aphid response. Third, I assessed aphid behavioral responses to plant water limitation and their contribution to aphid performance outcomes. Finally, I determined the indirect consequences of plant water limitation for aphid population suppression by parasitic wasps. Across all studies, I found evidence that aphid populations may decline as droughts become more frequent and intense. I determined that all levels of water limitation reduced total insect abundance and shifted the composition of the insect community. Using structural equation modeling, I showed that aphids were affected by stress induced changes in plant properties other than plant biomass, whereas natural enemies were strongly affected by changes in plant biomass; these effects were exacerbated as water stress increased. Aphid population size, population growth rate, and individual fecundity were negatively affected by even mild levels of plant water limitation. Diminished aphid performance on stressed plants may be driven by reduced amino acid concentrations and increased sucrose concentrations, both conditions which reduce plant quality for aphids. In addition, aphid host-plant selection was affected by plant water limitation, and aphid feeding rate declined on stressed hosts. Finally, parasitic wasps were better at suppressing aphid populations on stressed plants, with the highest mummy production found under mild stress conditions. However, the mechanism driving improved aphid suppression by wasps on stressed plants was reliant on water stress intensity. Taken together, I found that plant water limitation negatively affects insect communities and influences tri-trophic interactions. These studies highlight the intricate ways plant water stress intensity can affect insect behavior, performance, and species interactions, which are critical for our ability to predict outcomes of a changing climate for insects.
Alternative developmental and transcriptomic responses to host plant water limitation in a butterfly metapopulation