Examining the role of plant defense traits in moderating effectiveness of biocontrol agents

Classical biological control is widely accepted as an efficient and cost-effective method to control invasive plants. However, mounting evidence shows that biocontrol agents (BCA) are not always effective at managing invasive plants. One underappreciated ecological factor that may help explain variability in the effectiveness of BCA is the role of plant defenses and how they vary across environmental gradients. Florida harbors a highly problematic invasive weed: air potato (Dioscorea bulbifera L.). The beetle Lilioceris cheni (Coleoptera), a specialist on D. bulbifera, has been successfully established as its BCA although its effectiveness varies across the latitudinal gradient in Florida. Since the plant reproduces asexually, environmental variation across Florida may alter the plant defense chemistry and is possibly the primary driver of the variation in beetle effectiveness. Therefore, we are investigating the role of different environmental conditions on plant defense chemistry among air potato populations through a series of lab-based chemical and feeding assays. We hypothesize that defenses will be higher in the south, due to longer growing seasons and higher resource availability, and will decline towards the north. We aim to understand defenses by quantifying carbon to nitrogen ratio, saponins (terpenoids), total phenols, polyphenol oxidase, and protease inhibitors in plant leaves collected across Florida. We also plan to test the dose-dependent effect of diosgenin- a saponin found abundantly in air potato, on beetle physiology and hypothesize that beetles will perform poorly at higher doses. This study will be instrumental in exploiting plant defenses to better strategize biocontrol programs for invasive plants.