Generalist versus Specialist Herbivores on the Invasive Senecio inaequidens and a Native Related Species: What Makes the Difference?

We compared herbivory pressure in the native Jacobaea vulgaris (formely Senecio jacobaea) and the alien invasive S. inaequidens in sites where they co-occur in Belgium. We predicted that the alien species experiences relaxed herbivory pressure by specialist herbivores (enemy release hypothesis ERH) whereas it is still attacked by generalist herbivores. Impacts of two generalist (gastropods and rabbits) and one specialist (the caterpillar Tyria jacobaeae) herbivores were assessed with field observations and exclusion experiments. The generalist herbivores had a higher impact on the bio- mass and survival of the seedlings of the alien S. inaequidens than on the native J. vulgaris. On the contrary, the spe- cialist Tyria jacobaeae attacked exclusively the adults of the native species, supporting one of the main predictions of the ERH. These results are discussed in relation to differences in pyrrolizidine alkaloid profiles between the two species