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Using geometry to explore nutrient-allelochemical interactions: a case-study using locusts.
Behmer, Spencer*,1, Murugan, Kadarkarai1, Warner, Kathryn1, Simpson, Stephen1, Raubenheimer, David1, 1 University of Oxford, Oxford, UK
ABSTRACT- Patterns of food selection in herbivores likely represent a trade-off between minimizing the consumption of toxic plant secondary metabolites (PSMs) and balancing nutrient intake. In this study we employ the experimental framework of the geometric approach to explore the detailed nature of this trade-off, using the PSM gramine, an alkaloid found in barley, and 5th-stadium African migratory locusts, Locusta migratoria L. Two separate experiments using synthetic foods were conducted. In the first experiment the effect of different gramine concentrations (0, 0.5, 1, 2 or 4%) on feeding rates and performance was explored. Results indicated that gramine was deterrent starting at 2% but toxic only at 4%. In the second experiment, locusts were given a dish of a high protein, low carbohydrate food and a dish of low protein, high carbohydrate food, both of which contained gramine at 0.5, 1.0 or 2.0%. A third dish containing gramine-free food (one of five different protein-carbohydrate ratios) was also present. Results showed that locusts increased their consumption of the gramine-free foods as the concentration of gramine increased. However, when the gramine concentration was low, its deterrent effect varied depending on the protein-carbohydrate ratio of the gramine-free food and the protein-carbohydrate ratio of the gramine-containing foods. In general, it was strong in protein-rich foods and weak in protein-poor foods. These results are discussed within the context of plant defence theory and models of foraging behaviour.
KEY WORDS: herbivores, nutrients, allelochemicals, insects