Silica deters herbivory in Cucurbits: Potential links to climate change

Background/Question/Methods   As climate shifts, models predict that ranges of tropical insects will shift, changing the plant-herbivore interactions depending on the timing of the plant or insect response to climate change (Parmesan 2006). Thus, plants in those newly colonized areas may experience greater herbivory pressure from a potentially wider range of herbivores than currently present. How well will their defense traits protect them under such a scenario? Understanding the differences in plant defense traits under high herbivore diversity (e.g., in the tropics) versus lower herbivore diversity (e.g., in subtropical and temperate areas) will paint a clearer picture of how plants may respond to increased herbivore diversity (Novotny et al. 2006). Physical plant defenses will be a key deterrent as insect habitats expand into new areas due to increased global temperatures. A first step is to assess effectiveness of physical defenses of tropical plants, such as the accumulation of silica phytolyths. Using juvenile Cucumis sativus (Cucurbitaceae), watered with either a treatment (2 mM silica) solution or a control solution, leaf disks of both treatment and control plants were randomized into petri dishes for choice tests using cucumber beetles (Diabrotica balteata LeConte).

Results/Conclusions   By measuring the amount of leaf area consumed, I found that increased foliar silica, such as that found in tropical cucurbits, significantly (p=0.0254, n=50) decreased herbivore feeding. I also found that the treatment and control plants do not release significantly different volatiles- measured using GC-MS, therefore indicating that it is the physical accumulation of silica bodies itself that deters the herbivory, not a volatile chemical component associated with the silica accumulation. This type of physical defense trait may be well suited to defend plants as they experience potentially higher populations and diversity of herbivorous insects associated with climate change.