Allocation of resources away from sites of herbivory under simultaneous attack by aboveground and belowground herbivores in the common milkweed, Asclepias syriaca

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.