COMMUNITY-WIDE IMPACTS OF HERBIVORE-INDUCED PLANT RESPONSES IN MILKWEED (ASCLEPIAS SYRIACA)

Abstract

The effects of early-season herbivory and subsequent induced plant responses have the potential to affect the diversity of herbivorous insect communities. We investigated the seasonal development of the herbivore fauna on common milkweed (Asclepias syriaca) to understand the effect of early-season herbivory by different species on insect growth, natural colonization, and community composition. First, we showed that damage by an early-season stem-feeding weevil (Rhyssomatus lineaticollis) reduced growth of monarch larvae (Danaus plexippus) and leaf beetle larvae (Labidomera clivicollis), suggesting that plant quality is reduced by weevil damage. To better understand the potential for initial herbivore damage to affect subsequent colonization by herbivores in the field, we compared undamaged controls to plants experimentally damaged with one of three herbivores: weevils, monarchs, or leaf beetles. We counted seven species of naturally colonizing herbivores on all plants for the next two months to assess colonization, damage, and insect community richness. Our results showed that initial herbivory by different species altered host plant use by herbivores in two years of experiments. Similarly, induced resistance and suscep- tibility occurred in both years, but due to different initial damaging species on individual plants. Treatment effects also scaled up to alter herbivore community richness. Initial treatments varied in their persistence through the season. For example, in 2001, the influence of initial monarch damage dissipated due to subsequent damage by colonizing herbivores, but the impacts of initial weevil treatment were unaffected. This result suggests that, al- though induced responses to weevil feeding persisted through the season, monarch herbivory was more likely to affect the herbivore community via a cascade of indirect effects. In 2002, plant and insect responses were more specific, depending on the identity of both initial and colonizing herbivore species. Despite year-to-year variation, considerable con- sistency in many responses to our treatments indicates that the identity of the initially colonizing herbivore can affect subsequent plant use and community structure. Given the preponderance of influential early-season herbivores, the effects of induced plant responses similar to those presented here may be widespread and may strongly contribute to the structure of phytophagous insect communities.