Variable Impact of Diverse Insect Herbivores on Dimorphic Datura wrightii

Traits that confer plant resistance to some herbivore species but increase plant susceptibility to other herbivore species are said to carry an "ecological cost" of herbivore resistance. The native perennial, Datura wrightii, is dimorphic for leaf trichome type, and the production of glandular trichomes carries a potential ecological cost over the production of non-glandular trichomes. Glandular trichomes provide resistance to at least six herbivore species, but they also confer susceptibility to Tupiocoris notatus, an abundant mirid bug with special adaptations to glandular trichomes. We estimated herbivore-specific damage to plants of each trichome type and measured plant seed production, a component of fitness, in a common garden over three years. Plant seed production increased with the size and persistence of leaf canopies and occasionally declined with increasing damage caused by some of the defoliating insects. Plant seed production was never reduced by T. notatus damage, however. Even though the ecological cost of glandular trichome production was not apparent in D. wrightii, the trait still was not beneficial. The fitness of plants with glandular trichomes never exceeded that of plants with non-glandular trichomes despite variation in both the composition of the herbivore community and the total level of damage inflicted by herbivores over three years of study. The persistence of the allele coding for glandular trichomes cannot be explained solely on the basis of the herbivore resistance that those trichomes provide.