Interactions between herbivore pressure and resource availability may result in growth–defense trade-offs in plants (resource availability hypothesis), which promote the potential for ecological diversification and have recently been proposed as a new mechanism of plant invasion (resource–enemy release hypothesis). In the present study, we tested this idea by comparing patterns of maximum growth rate and antiherbivore defense capacity against Agasicles hygrophila (a specialist phytophagous insect on Alternanthera philoxeroides) among six morphs of A. philoxeroides from both native (Argentina) and introduced (US and China) ranges. The results revealed that herbivore resistance consistently exhibited a significant trade-off against the growth rate of A. philoxeroides. Compared with native morphs in Argentina, introduced morphs generally exhibited a higher growth rate and lower defense. The results demonstrate that the combination of flea beetle pressure and resource availability facilitates geographical divergence of A. philoxeroides, and that release from natural specialists and increased resource availability may interact to contribute to its successful invasion.