Wednesday, August 6, 2008: 1:50 PM
201 A, Midwest Airlines Center
Richard A. Niesenbaum, Department of Biology, Muhlenberg College, Allentown, PA, Emily H. Mooney, Biology, Massachusetts College of Liberal Arts, Erin Jo Tiedeken, School of Biological Sciences, Trinity College Dublin, Dublin, Ireland and Norris Z. Muth, Biology Department, Juniata College, Huntingdon, PA
Background/Question/Methods Theoretically, induced defenses should be prevalent within low resource environments like the forest understory, and the induced response should be stronger when the herbivore is a generalist rather than a specialist. We examined these phenomena in Lindera benzoin (Spicebush), a common shrub of the eastern deciduous forest. Fifty L. benzoin plants in contrasting light environments were subjected to 5 treatments: control, application of jasmonic acid, clipping, herbivory by the specialist Epimecis hortaria (Tulip tree beauty) and herbivory by the generalist Spodoptera exigua (Beet armyworm). Following treatment, we assessed induced responses using insect bioassays with E. hortaria larvae and by measuring leaf C/N ratio, protein content, and peroxidase activity.
Results/Conclusions
In control plants, larvae gained more weight per mg of fresh biomass consumed in the shade than in the sun. In plants treated with clipping or herbivory, larvae gained less weight per mg of fresh biomass consumed in the shade than in the sun. Leaves from shade plants had more protein and higher C/N ratios than leaves from sun plants, regardless of induction treatment. We found no difference in peroxidase activity between light environments in controls. Peroxidase activity was greater in shade plants treated with clipping or herbivory than in sun plants that had experienced the same treatments; the magnitude of this increase varied between generalist and specialist, with a 32 fold increase in plants exposed to S. exigua and a 9 fold increase in plants exposed to E. hortaria. The E. hortaria larvae did not show a corresponding decrease in consumption or conversion efficiency, suggesting that the specialist has mechanisms to overcome the defenses induced in L. benzoin. As a species adapted the forest understory, L. benzoin may rely on induced defenses in the shade, but take advantage of other defense strategies when gaps or edges increase irradiance.
