Effects of plant defense chemicals on insect performance depend on plant nutrient levels

Background/Question/Methods

From a bottom-up perspective, performance of insect herbivores is regulated by levels of nutrients and defense chemicals in plants. Compared to the extensive work on the effects of plant nutritional and chemical quality on insect performance, investigation of their interactive effects is surprisingly scarce. In fact, plant nutrients and defense chemicals are never independent. Plant nutritional status can affect the effective concentrations of secondary metabolites, alter the immuno-responses, and change the physiological status of insect herbivores, which can all affect the effectiveness of defense chemicals. Our goal is to investigate the effects of plant nutrients on the efficacy of chemical defenses and explore potential mechanisms for such context dependency. We selected common milkweed Asclepias syriaca and its specialist herbivore, the monarch caterpillar Danaus plexippus as our study system. The plants were cross fertilized with 3 levels of nitrogen (N) and 3 levels of phosphorus (P). We took opposite leaves from each plant and added one of four levels of digitoxin (commercially available cardenolide) to one leaf and used the other as control. The pairs of leaves were fed to pairs of caterpillars, and their differences in consumption, assimilation efficiency and growth rates were calculated as measurements of toxicity.   

Results/Conclusions

Growth rates of D. plexippus were negatively correlated with the level of digitoxin and plant carbon (C) : N and C : P ratios. More interestingly, there were significant interactions between digitoxin and plant stoichiometry in affecting D. plexippus growth rates. Namely, magnitudes of decreases in growth rate of D. plexippus caused by digitoxin were greater when plant nutrient levels were low (C : N and C : P were high). This happened because consumptions of A. syriaca by D. plexippus were inhibited by digitoxin at greater extents when the plants were of lower nutritional quality. The negative effects of digitoxin on D. plexippus assimilation efficiency, however, were independent on plant stoichiometry. Our results provided the first evidence in specialist insect herbivores that efficacy of plant defense chemicals is dependent on plant nutrient levels. Plant toxin is more potent when plant nutritional quality is low due to a greater decrease in plant palatability, but not because of their interactive effects on insect assimilation.