Consequences of Plant Resistance for Herbivore Survivorship, Growth, and Selection on Egg Size

Plants can vary substantially among species in morphological and physiological traits that influence their resistance to herbivores and their quality for herbivore growth and development. Seeds of the desert trees Cercidium floridum and C. microphyllum vary within and among populations in their resistance to, and suitability for, growth and development of larvae of the seed beetle, Stator limbatus. We examined how this variation influences the magnitude of natural selection on the size of eggs laid by beetles and tested whether female beetles can distinguish between seeds of more vs. less suitable trees (within species). We found that selection favoring large eggs differed substantially between the two tree species (greater on C. floridum) and varied substantially among populations of C. floridum and among trees (within populations) of both C. floridum and C. microphyllum. In general, the magnitude of selection favoring large eggs correlated positively with seed coat resistance between tree species, among populations within species, and among trees within populations. Female S. limbatus are capable of distinguishing seeds of the two host species and respond to species differences in selection by laying larger eggs on seeds of C. floridum than on seeds of C. microphyllum. However, despite large variation in seed suitability within populations of trees, females did not preferentially lay eggs on seeds of less resistant trees, nor did they adjust egg size in response to variation in resistance among trees.