Belowground infections of the invasive Phytophthora plurivora pathogen enhance the suitability of red oak leaves to the generalist herbivore Lymantria dispar

1. Globally, vast areas of forest are currently threatened by Lymantria dispar L. and Phytophthora species, which cause widespread declines and cascading ecological impacts. One important aim of evolutionary and ecological studies is to understand their interactions. 2. The present study tests whether Quercus rubra L. trees naturally infected with P. plurivora T. Jung & T.I. Burgess or free of infection are more suitable for L. dispar herbivory, and if relationships between L. dispar performance and herbivory may vary depending on whether trees are infected or free of infection. 3. In choice tests, the consumed area of leaves from trees infected by P. plurivora was four times larger than that from non‐infected trees, probably because the increased values of N, soluble protein, and water content observed in the leaves of infected trees enhanced acceptability. Although larval performance was better in Phytophthora‐infected trees, relationships between larval performance and defoliation did not significantly interact with the health status of trees. 4. The present results suggest that the impact of P. plurivora on natural and managed ecosystems may generate a positive feedback loop for oak decline. The link between the behavioural and physiological responses of L. dispar to infected trees and the population growth in nature deserves further investigation.