1. 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.
2. 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.
3. 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.
4. Increased plant species-richness had an additional, but weak, negative influence on loads beyond that due to host-plant abundance.
5. We discuss the implications of resource dilution effects for biodiversity manipulation experiments and for the study of plant–herbivore interactions more generally.