Host plants influence parasitism of forest caterpillars

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Abstract
Patterns of association between herbivores and host plants have been thought to reflect the quality of plants as food resources1,2 as influenced by plant nutrient composition3, defences4,5, and phenology6. Host-plant-specific enemies, that is, the third trophic level, might also influence the distribution of herbivores across plant species7,8,9,10. However, studies of the evolution of herbivore host range11,12,13,14,15 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.