Nutrient-allelochemical interactions: metabolic effects on a generalist insect herbivore

Background/Question/Methods

Levels of herbivory vary widely in nature and are largely the result of differential plant quality. Plant quality is a combination of nutrient quality and quantity as well as levels of plant defenses, all of which vary both spatially and temporally. While the effects of nutrients or plant defenses on herbivores are well studied little is known about the interactions of both factors. In this study I used the ‘Geometric Framework’, a state-space modelling approach that explores how an animal solves the problem of balancing multiple and changing nutrient needs in a multidimensional and variable nutritional environment coupled with respirometry technique.  Grasshoppers (Melanoplus differentialis (Orthoptera : Acrididae)) were reared across the last nymphal stadium on foods with fixed total macronutrient content, but varying in protein:carbohydrate ratio in three different ways (14:28, 21:21, 28:14) as well as alkaloid (gramine) concentration (0, 0.5, 1, or 2% dry weight). Food consumption, nutrient uptake, developmental time, wet mass gain, and survivorship were measured as indicators of insect performances. Additionally, the effects of gramine on insect metabolic rates and respiratory quotient (RQ) across the different diets were recorded using flow-through respirometry.

Results/Conclusions

In the absence of gramine, grasshopper performance was similar over the three types of diets, even though the amount of food eaten varied significatively. Data on nutrient consumption indicated that grasshopper on the carbohydrate-biased food (p14:c28) ate more in order to reach a certain level of protein ingestion. Consequently, over-ingestion of carbohydrate on that diet affected the insect respiratory metabolism as indicated by the high value obtained for the RQ (>1). The effects of the alkaloid on food consumption, nutrient uptake and mortality were dependent on the balance of macronutrients in the food, and were stronger on the grasshoppers fed the carbohydrate-biased diet. Gramine altered performance and metabolism at high concentration for all treatments independent of food nutrient ratio. This study highlights the necessity to consider food nutrient content and metabolic effects when investigating the impacts of plant defenses on herbivores. Further use of this integrative approach may provide important insights into the field of nutritional ecology.