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.

Disease ecology across soil boundaries: effects of below-ground fungi on above-ground host–parasite interactions

“…Previous studies have shown that milkweeds with high foliar concentrations of cardenolides can reduce O. elektroscirrha growth and virulence in monarchs [17,18]. Additionally, monarch performance is affected strongly by the nutrient status of its host plants [33]. By measuring and correlating the effects of AMF on host resistance and tolerance with their effects on plant nutrient and cardenolide traits, we show here that AMF have variable and significant effects on host performance across six milkweed species.…”

“…Percentage of colonization was then calculated as the number of colonized sites divided by the total inspected sites. Analysis of foliar cardenolide, N and P concentrations followed [33].…”

Host Diet Affects the Morphology of Monarch Butterfly Parasites

“…Cardenolides may cause these reductions in parasite loads by increasing host immunity to the parasite, as has been seen in other systems (Lee et al, 2008;Povey et al, 2009;Simpson et al, 2015), or by direct inhibition of parasites (Cory and Hoover, 2006). Previous studies have shown that the effects of nutrition and milkweed chemicals on monarch larvae are most influential during early instars (Zalucki et al, 2001;de Roode et al, 2011a;Tao and Hunter, 2012). But further studies are necessary to determine whether these effects are mediated through mid-gut-based immunity or anti-parasite toxicity in the mid-gut lumen.…”

“…One monarch egg was randomly assigned to each milkweed plant and neonates were fed from hatching to the second instar stage with 1 leaf from the third leaf pair. Chemical and nutritional analyses of the larval food were conducted just prior to larval hatching because previous findings have established that milkweed chemical and nutritional effects on caterpillars are most significant during the earliest instars (Zalucki et al, 2001;de Roode et al, 2011a;Tao and Hunter, 2012).…”

“…Asclepias incarnata was chosen because it contains very low cardenolide concentrations (Agrawal et al, 2012;Sternberg et al, 2012;Tao et al, 2015). This switch in larval food, if it occurred, was during the final days of the last larval stage, where milkweed chemical and nutritional effects on larval growth have been shown to be minimal (Zalucki et al, 2001;Tao and Hunter, 2012). Previous work has also shown that milkweed chemical and nutritional effects on parasites are greatest during early stages of monarch development and infection (de Roode et al, 2011a).…”