Following herbivory, plants can preferentially allocate newly acquired resources away from attacked sites as an important mechanism conferring tolerance. Although reported previously for both aboveground and belowground herbivores, it remains unclear whether plants can simultaneously allocate resources away from both kinds of herbivore attack, and whether they have interactive effects on plant resource allocation. In the current study, we used dual-isotopic techniques to compare the allocation of newly acquired carbon (C) and nitrogen (N) by the common milkweed Asclepias syriaca following attack by an aboveground herbivore, the monarch caterpillar Danaus plexippus and a belowground herbivore, larvae of the red milkweed beetle Tetraopes tetraophthalmus. Both species induced significant changes in the allocation of C and N in A. syriaca. Specifically, A. syriaca increased allocation of new N to stems at the expense of allocation to damaged tissues (i.e., leaf or root). When under simultaneous attack, the allocation of resources to stems was greater than that induced by either herbivore alone, suggesting that (1) the herbivores have additive effects on allocation patterns by A. syriaca and (2) A. syriaca was able to mitigate the effects of future attack by both herbivore species simultaneously.
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This work was supported by a Block Grant and a Barbour Fellowship from the University of Michigan to L.T. and NSF DEB-0814340 to M.D.H. We gratefully acknowledge Rachel Vannette for assistance in greenhouse arrangements; Huijie Gan, Shan Gao, and Susan Kabat for their help with laboratory work. Comments from Ivette Perfecto, Knute Nadelhoffer, and especially Deborah Goldberg greatly improved an earlier version of the manuscript. We also thank Patricia Micks from the soil ecology lab at the University of Michigan for the chemical analyses. We also appreciate the helpful comments from two anonymous reviewers.
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