Does anthropogenic nitrogen deposition induce phosphorus limitation in herbivorous insects?

Abstract Anthropogenic nitrogen deposition has shifted many ecosystems from nitrogen ( N ) limitation to phosphorus ( P ) limitation. Although well documented in plants, no study to date has explored whether N deposition exacerbates P limitation at higher trophic levels, or focused on the effects of induced plant P limitation on trophic interactions. Insect herbivores exhibit strict N  :  P homeostasis, and should therefore be very sensitive to variations in plant N  :  P stoichiometry and prone to experiencing deposition‐induced P limitation. In the current study, we investigated the effects of N deposition and P availability on a plant‐herbivorous insect system. Using common milkweed ( Asclepias syriaca ) and two of its specialist herbivores, the monarch caterpillar ( Danaus plexippus ) and milkweed aphid ( Aphis asclepiadis ) as our study system, we found that experimental N deposition caused P limitation in milkweed plants, but not in either insect species. However, the mechanisms for the lack of P limitation were different for each insect species. The body tissues of A. asclepiadis always exhibited higher N  :  P ratios than that of the host plant, suggesting that the N demand of this species exceeds P demand, even under high N deposition levels. For D. plexippus , P addition increased the production of latex, which is an important defense negatively affecting D. plexippus growth rate. As a result, we illustrate that P limitation of herbivores is not an inevitable consequence of anthropogenic N deposition in terrestrial systems. Rather, species‐specific demands for nutrients and the defensive responses of plants combine to determine the responses of herbivores to P availability under N deposition.