Leaf traits of congeneric host plants explain differences in performance of a specialist herbivore

1. Within the host range of herbivorous insects, performance hierarchies are often correlated with relatedness to a primary host plant, as plant traits are phylogenetically conserved. Therefore, it was hypothesised that differences in herbivore performance on closely related plant species are due to resistance traits that vary in magnitude, rather than in the nature of the traits. 2. This hypothesis was tested by manipulating putative resistance traits of three congeneric thistle species (Cirsium arvense, Cirsium palustre, and Cirsium vulgare) and assessing the performance of the oligophagous, leaf-feeding beetle, Cassida rubiginosa. Measurements were done of survival, weight gain, and development time of the beetle on its primary host, C. arvense, and two alternative hosts under low and high nutrient availability, and on shaved and unshaved leaves. 3. Survival of C. rubiginosa was strongly dependent on plant species with final mean survival rates of 47%, 16%, and 8% on C. arvense, C. palustre, and C. vulgare, respectively. Survival was primarily explained by leaf trichome densities, and to a lesser extent by specific leaf area. Leaf flavonoid concentrations did not explain differences in beetle survival, and there were no differences in beetle weight gain or development time of individuals that survived to adulthood. 4. No beetles survived on unshaved (hairy) C. vulgare plants, but manipulating leaf trichome densities of the thistle species by shaving the leaves moderated the plant-specific resistance, and equalised the survival rates. Survival of C. rubiginosa on alternative congeneric hosts was explained by a common physical resistance trait that varied in magnitude.