Effects of associational resistance and host density on woodland insect herbivores

1. Specialist herbivores often become less abundant per unit of host tissue as host density increases (resource dilution). They usually become less abundant when non-host species are mixed with their host plants (associational resistance). Most studies of these trends have involved herbaceous host plants and have not examined both trends for the same herbivores. 2. Three hypotheses were tested for the response of insect specialists to host plant density: resource concentration, plant apparency and resource dilution. Two hypotheses were tested for the response of herbivores to non-host plants: associational resistance and plant apparency. 3. From 1992 to 2007, I examined the responses of three monophagous insect herbivores to the densities of their host, Pinus edulis, and of two non-hosts, Pinus ponderosa and Juniperus spp. 4. Herbivore loads increased with host density, though the correlations were weak and often variable between generations. These results were consistent with the resource concentration and plant apparency hypotheses, but not with resource dilution. 5. Herbivore loads decreased as non-host density increased, consistent with the associational resistance hypothesis. This and other studies have shown that associational resistance is important in many types of plant communities. 6. The absence of resource dilution on woodland trees contrasted with studies of herbaceous host plants. Further comparisons of woody and herbaceous host plants are needed to elucidate the reasons for this difference.