Resource dilution effects on specialist insect herbivores in a grassland biodiversity experiment

Summary The resource concentration hypothesis predicts that specialist insect herbivores attain higher loads (density per unit mass of the host‐plant species) when their food plants grow in high‐density patches in pure stands. We tested the resource concentration hypothesis for nine specialist insect herbivore species sampled from a field experiment where plant diversity had been manipulated experimentally, generating gradients of host‐plant abundance. The specialist insects responded to varying host‐plant abundance in two contrasting ways: as expected, specialist herbivore species were more likely to be present when their host‐plant species were abundant; however, counter to predictions, in plots where specialists were present we found strong negative linear relationships between herbivore loads and host‐plant abundances ‐ a ‘resource dilution’ rather than concentration effect. Increased plant species‐richness had an additional, but weak, negative influence on loads beyond that due to host‐plant abundance. We discuss the implications of resource dilution effects for biodiversity manipulation experiments and for the study of plant–herbivore interactions more generally.