Modeling Simultaneous Selection for Resistance and Tolerance in Goldenrod (Solidago altissima) across a Range of Spittlebug Population Densities

Plant defenses against herbivory include two main strategies: resistance (to minimize the amount of damage) and tolerance (to minimize the fitness impact of that damage). Recent studies have emphasized the need to consider both strategies simultaneously for a fuller understanding of the evolutionary and ecological dynamics of plant defense against herbivores. We used a combination of a garden study, a greenhouse experiment, and mathematical modeling to investigate resistance to and tolerance of spittlebug damage in the goldenrod Solidago altissima. In contrast to traditional expectations, the genetic correlation between resistance and tolerance was highly positive. Selection gradients indicated that directional selection would act to increase tolerance at mean spittlebug densities of more than five nymphs per goldenrod ramet. Selection for resistance was relatively weak and was likely a byproduct of selection acting directly on correlated tolerance traits. We conclude that fitness costs of resistance and tolerance limit their evolution to intermediate levels in goldenrod populations, as the costs appear to outweigh the benefits during years of low spittlebug densities.