Asymmetric plant-mediated cross-effects between a herbivorous insect and a phytopathogenic fungus

1 Cross-effects between a herbivorous insect and a phytopathogenic fungus on their common host plant were examined. Specifically, we addressed the questions whether (i) infection of Chinese cabbage leaves by the fungus Alternaria brassicae affects the development and host selection behaviour of the leaf beetle Phaedon cochleariae and (ii) whether herbivory influences host suitability of Chinese cabbage for A. brassicae. 2 Feeding on fungus-infected leaves prolonged larval development and reduced pupal weight of P. cochleariae. Adult beetles avoided feeding and egg deposition on fungus-infected leaves. In contrast to these local effects, no systemic effect of phytopathogenic infection on the herbivore was detected. 3 Herbivory did not influence fungal growth neither locally nor systemically. 4 Thus, our results demonstrate an asymmetric relationship between herbivore and fungus. Whereas herbivory had no visible impact on fungal growth, fungal infection of the plant induced local resistance against P. cochleariae.