Why does a good thing become too much? Interactions between foliar nutrients and toxins determine performance of an insect herbivore

1In terrestrial ecosystems, nitrogen (N) and phosphorus (P) are generally believed to be the most limiting factors for organisms across different trophic levels. However, accumulating evidence suggests that dietary nutrient concentrations higher than optimal can lead to decreases in consumer growth rate. In the current study, we explored mechanisms underlying the negative effects of high nutrient availability on the performance of a specialist herbivore. Specifically, we investigated the responses of the monarch caterpillar Danaus plexippus to natural and experimental variation in N and P concentrations of three species of milkweed plants (Asclepias syriaca, Asclepias curassavica and Asclepias incarnata) that also varied in their foliar toxin concentrations. We found that high foliar N concentrations in milkweed were associated with decreases in the growth rate of D.plexippus larvae. However, such negative effects of N were only found when larvae were feeding on A.curassavica, which also had high foliar concentrations of cardenolide, a widespread chemical defence in the genus Asclepias. Foliar N concentration was not correlated with cardenolide concentration. Rather, the per unit toxicity of cardenolide was higher as N increased in excess of demand, resulting in deleterious effects of N. Our results suggest that interactions between nutrient concentrations in excess of demand and high dietary toxin concentrations provide an additional mechanism by which high nutrient availability can reduce the performance of consumers.