Root silica deters below-ground herbivory: An examination of plant mechanical defenses on root herbivores

Background/Question/Methods Plants utilize multiple mechanisms to defend against predation; these include both chemical and physical defenses. Silicon is the second most abundant element in the earth's crust and is known to accumulate within some plants as a crystal called a silica phytolith. It is hypothesized that one function of these crystals is to physically defend against herbivory. Little is known about the functions of silica phytoliths in roots and their relationship with belowground herbivory. In order to determine the efficacy of silica as a belowground herbivore deterrent, roots of Cucumis sativus and Zea mays were fed to first instar Diabrotica balteata larvae. Three treatments of silica (0 mM, 1 mM, and 2 mM in cucumber, and 0 mM, 1 mM and 3 mM in corn) were applied to the plants' nutrient solution for one week prior to the start of the experiment. The no-added silica (0 mM) treatments provided silica-free tissue for larval ingestion, while the other treatments contained up to 3% silica per dry tissue weight. Survival and growth rates of the larvae were calculated for each of the three silica treatments by weighing larvae and measuring head capsule size. Results/Conclusions Results show that the effect of silica on larval fitness was significant (p = <0.0001). Therefore these data support the hypothesis that higher concentrations of plant silica will yield a lower larval survival rate. These data may contribute to the understanding of evolutionary strategies and diversity of root feeding underground animals.