Consequences of plant resistance for herbivore survivorship, growth, and selection on egg size

Plants can vary substantially among species in morphological and physiologi cal traits that influence their resistance to herbivores and their quality for herbivore growth and development. Seeds of the desert trees Cercidium f loridium and C. microphyllum vary within and among populations in their res istance to, and suitability for, growth and development of larvae of the se ed beetle, Stator linibatus. We examined how this variation influences the magnitude of natural selection on the size of eggs laid by beetles and test ed whether female beetles can distinguish between seeds of more vs. less su itable trees (within species).),e found that selection favoring large eggs differed substantially between tile 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 s eed coat resistance between tree species, among populations within species, and among trees within populations. Female S. limbatus are capable of dist inguishing 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 o f C. microphyllum. However, despite large variation in seed suitability wit hin populations of trees, females did not preferentially lay eggs on seeds of less resistant trees, nor did they adjust egg size in response to variat ion in resistance among trees.