It is well documented that predators can benefit plants by reducing herbivore abundance, and much work has investigated how ecological context shapes these tritrophic interactions.  In contrast, less is known about how the top-down effects of predators are modulated by plant traits and resistance to herbivores.  We compared herbivore populations and the impacts of predators among 16 phylogenetically-resolved milkweed species in a field experiment conducted in Ithaca, NY in 2007.  We also identified how predator effects were modulated by nutrient availability.  Replicates of each milkweed species were placed in one of four treatments in a two-by-two factorial manipulation of predators (exclusion vs. control) and nutrient manipulation (fertilization vs. control).  Each plant was inoculated with the aphid Aphis nerii in late June and we monitored aphid populations weekly through early August.
Results/Conclusions

Plant species differed in their resistance to A. nerii.  Both the negative effects of predators and the positive effects of fertilization on aphid abundance differed among milkweed species, but the effects of predators and fertilization acted independently of each other.  A test for phylogenetic signal in fertilization effects, predator effects, and several plaint traits revealed that only plant height was phylogenetically conserved.  In summary, the effects of predators varied among milkweed species in a manner independent of phylogenetic history, and these effects were not changed by host plant quality.