1. Department of Biology, University of Missouri–St. Louis, St. Louis, Missouri 63121, USA
2. Present address: Washington University, Tyson Research Center, PO Box 258, Eureka, Missouri 63025, USA.

Correspondence to: J. T. Lill¹ Correspondence and requests for materials should be addressed to J.T.L. (e-mail: Email: lillj@biology2.wustl.edu).

Abstract

Patterns of association between herbivores and host plants have been thought to reflect the quality of plants as food resources^{1, 2} as influenced by plant nutrient composition³, defences^{4, 5}, and phenology⁶. Host-plant-specific enemies, that is, the third trophic level, might also influence the distribution of herbivores across plant species^{7, 8, 9, 10}. However, studies of the evolution of herbivore host range^{11, 12, 13, 14, 15} have generally not examined the third trophic level, leaving unclear the importance of this factor in the evolution of plant–insect herbivore interactions¹⁶. Analysis of parasitoid rearings by the Canadian Forest Insect Survey shows that parasitism of particular Lepidoptera species is strongly host-plant-dependent, that the pattern of host-plant dependence varies among species of caterpillars, and that some parasitoid species are themselves specialized with respect to tree species. Host-plant-dependent parasitism suggests the possibility of top-down influence on host plant use. Differences in parasitism among particular caterpillar–host plant combinations could select for specialization of host plant ranges within caterpillar communities. Such specialization would ultimately promote the species diversification of Lepidoptera in temperate forests with respect to escape from enemies.