Exotic Weed Invasion Increases the Susceptibility of Native Plants to Attack by a Biocontrol Herbivore

Landscape change has great, yet infrequently measured, potential to influence the susceptibility of natural systems to invasive species impacts. We quantified attack by an invasive biological control weevil (Rhinocyllus conicus) on native thistles in relation to two types of landscape change: agricultural intensification and invasion by an exotic thistle, Carduus nutans, the original target of biological control. Weevil egg load was measured on native thistles in three landscape types: (1) agriculture dominated, (2) grassland dominated with exotic thistles, and, (3) grassland dominated without exotic thistles. We found no difference in egg load on native thistles within grassland landscapes without exotic thistles vs. within agricultural landscapes, suggesting that agricultural intensification per se does not influence levels of weevil attack. However, attack on the native Cirsium undulatum increased significantly (threeto fivefold) with increasing exotic thistle density. Within-patch exotic thistle density explained .50% of the variation in both the intensity and frequency of weevil attack. Since R. conicus feeding dramatically reduces seed production, exotic thistles likely exert a negative indirect effect on native thistles. This study provides some of the first empirical evidence that invasion by an exotic plant can increase attack of native plants by shared insect herbivores.