Natural selection on common milkweed (Asclepias syriaca) by a community of specialized insect herbivores

Hypothesis: Genetic variation in plant defence structures a community of herbivores and ultimately mediates co-evolution. Organisms: Common milkweed (Asclepias syriaca) and five natural insect herbivores (seed bug, leaf mining fly, monarch caterpillar and two beetles). Methods: Quantitative genetic field experiment over 2 years, genetic selection analyses, and measurement of five defensive traits (cardenolides, latex, trichomes, leaf toughness and nitrogen content). Results: All plant traits were genetically variable; directional selection favoured resistance to herbivory, latex production and the nitrogen content of leaves. Trichomes and latex were each negatively genetically correlated with abundances of herbivores, but not with herbivore damage; cardenolides and induced plant resistance were negatively genetically correlated with growth of monarchs. Conclusion: Selection for plant defence was influenced by: (1) genetic correlations in plant susceptibility to multiple herbivores; (2) resistance traits affecting some, but not all, aspects of the insect community; and (3) early season herbivory inducing changes in milkweed influencing later season herbivores.