Silica as a plant defense against herbivorous insects

Background/Question/Methods

The stationary lifestyle of plants creates a need for both physical and chemical defenses, especially where rates of herbivory are high.  We experimentally tested the effectiveness of silica as an anti-herbivore defense in laboratory studies and in parallel field experiments across a latitudinal gradient. We posed two questions: (1) What is the effect of foliar silica on insect herbivore preference, growth and survival and (2) does increased foliar silica result in lower herbivore density and diversity and decreased herbivory rates? We chose two species of Cucurbitaceae, cucumber (Cucumis sativa) and pumpkin (Cucurbita pepo), as model species because their foliar silica concentration could be experimentally manipulated by supplying different amount of silica fertilizer. In the laboratory, we used a generalist herbivore, the banded cucumber beetle (Diabrotica balteata) to measure the herbivory rates on cucumber and examine the effects of foliar silica concentrations on growth and survivorship of the beetles.  In the field, we set up a 70-day, common garden study at three sites -- Panama, Florida and Wisconsin -- that differed in herbivore density and diversity. Herbivore density, diversity and feeding guild and herbivory rates were recorded daily for all plants.   

Results/Conclusions

In the laboratory, cucumber beetles significantly (p=0.0254, df=44) preferred non-silica treated plants relative to silica-treated plants.  However, growth and survivorship of beetles was not affected by silica content of their diet (p=0.2663, df=24). In field trials, a higher density and diversity of herbivores were observed on plants plants at higher latitudes, contrary to the common notion of higher herbivore diversity and abundance in the tropics.  A preliminary analysis shows that in Panama, < 200 arthropod herbivores from 13 families were recorded, whereas in Florida, 576 herbivores from 17 families were recorded and at the high latitude site, Wisconsin, 1020 herbivores from 30 families were recorded.   Preliminary analyses of herbivory rates show higher herbivory at the higher latitudes — again, contrary to our expectation.  Overall, high foliar silica concentrations resulted in lower herbivory, implying that accumulating silica is beneficial for plant defense.