Nutrient-allelochemical interactions and generalist insect herbivores: A fitness landscape approach

Background/Question/Methods

Two hypotheses have been proposed to explain diet mixing by generalist herbivores.  The first suggests that mixing provides a mechanism for better matching nutritional needs; the second suggests mixing is a mechanism for reducing detoxification costs, by keeping the intake of any single plant allelochemical low.  Both hypotheses have received support, but importantly these two hypotheses are not mutually exclusive.  To properly understand the benefit of diet mixing, a better appreciation for how nutrients and allelochemicals interact to influence insect herbivore feeding behavior and performance is needed.  In this study we use an integrative nutritional approach (designed within the context of the ‘Geometric Framework’) to study nutrient-allelochemical interactions. Protein (p) and digestible carbohydrates (c) are two nutrient groups important for insect herbivores, which can also be highly variable in plants.  In this study we reared last instar generalist grasshoppers (Melanoplus differentialis (Orthoptera: Acrididae)) on foods with different protein-carbohydrate ratios and amounts (9 treatments).  We then used Response Surface methods to generate fitness landscape plots for a number of variables (developmental time, mass gain, mortality, lipid content) across our defined nutrient landscape.  Using this data as a reference point, we then added a plant allelochemical (gramine, an alkaloid) to each of our diet at 2 different concentrations (0.5 or 2% dry weight).

Results/Conclusions

Our first analysis, which examined insect response to nutritional variation in the absence of gramine over a range of treatments, showed performance that gradually drops off as foods becoming increasingly unbalanced, and as total nutrient concentrations decline. When gramine was added to these same diets, the fitness landscape plots shifted dramatically, especially as the concentration of gramine added to the food increased. Our results highlight the dynamic nature of the interaction occurring between food nutrient and plant defensive compounds, and demonstrate the value of employing a nutritional-based approach to better understand insect-plant interactions.