Diet-mixing in a Generalist Herbivore: Trade-offs Between Nutrient and Allelochemical Regulation
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Despite decades of research, many key aspects related to the physiological processes and mechanisms insect herbivores use to build themselves remain poorly understood, and we especially know very little about how interactions among nutrients and allelochemicals drive insect herbivore growth processes. Understanding the physiological effects of these interactions on generalist herbivores is a critical step to a better understanding and evaluation of the different hypothesis that have been emitted regarding the benefits of polyphagy. I used both lab and field experiments to disentangle the respective effect of protein, carbohydrates and allelochemicals on a generalist herbivore, the grasshopper Melanoplus differentialis. The effect of protein and carbohydrates alone were examined using artificial diets in choice and no-choice experiments. Results were plotted using a fitness landscape approach to evaluate how protein-carbohydrate ratio and/or concentration affected performance and consumption. Growth was best near the self-selected ratio obtained from the choice experiment, most likely due to the fact that the amount of food digested was also higher on that ratio. By contrast, development time was not best near the preferred ratio most likely due to the trade-off existing between size and development time. These results illustrate how nutrient availability can shape an ecological trade-off: growing big or growing fast. When an allelochemical (gramine, an alkaloid commonly found in grass) was introduced to the artificial diets, it had an interactive effect with protein and carbohydrates on performance and consumption and performance were generally improved on diets that contained higher amount of protein. Host plants of two wild populations were determined by gut content analysis and fed in choice and no-choice experiment. First the plant material was dried and ground and its protein, carbohydrate, terpenoid and phenolic content analyzed. Both populations regulated for the same protein-carbohydrate intake. However performance was different, due to variation in plant allelochemical content. This demonstrated for the first time that nutrient regulation, not toxin dilution, is directing food selection behavior in a generalist herbivore. Finally the role of macronutrients was analyzed in a context of cost of detoxification by measuring microsomal p450 production in the presence/absence of gramine. In the presence of choice, nutrient regulation was altered when gramine was present in the protein-biased diet. In the absence of choice, insects performed better on carbohydrate biased diet. I found that gramine elevated the level of microsomal protein in the fat body.
Le Gall, Marion (2014). Diet-mixing in a Generalist Herbivore: Trade-offs Between Nutrient and Allelochemical Regulation. Doctoral dissertation, Texas A & M University. Available electronically from