LAUSR

Biogeographical variation in herbivore communities may drive invasive plants to evolve lower defense against specialist herbivores but higher defense against generalist herbivores (shifting defense hypothesis, SDH). However, empirical tests on this topic have been strongly biased toward examining constitutive resistance and less is known about the evolution of induced resistance and tolerance. We examined constitutive and induced resistance and tolerance of the invasive plant, Alternanthera philoxeroides (alligator weed, Centrospermae: Amaranthaceae) against the specialist herbivore Agasicles hygrophila (Coleoptera, Chrysomelidae) and the generalist herbivore Spodoptera litura (Lepidoptera, Noctuidae), using genotypes from its introduced and native ranges. We found that introduced genotypes, compared to native genotypes, had higher constitutive resistance to the generalist herbivore, but similar constitutive resistance to the specialist herbivore. Furthermore, introduced genotypes, compared to native genotypes, had lower induced resistance to the generalist herbivore, but similar induced resistance to the specialist herbivore. Moreover, although introduced and native genotypes did not differ in their tolerance to the generalist, the introduced genotypes exhibited lower tolerance to the specialist than native ones. Therefore, while individual defense strategy does not indicate a shift from defense against the specialist to defense against the generalist, the results for the different defense strategies jointly support the SDH.