D3199: The third macronutrient rail for herbivores: Migratory locusts regulate their ratio of carbohydrate to lipid consumption

It is well documented that herbivorous insects regulate their macronutrient intake toward a specific ratio of protein to carbohydrates; however, whether herbivorous insects regulate their lipid intake stoichiometrically with protein or carbohydrate consumption is unknown. Theoretically, lipids and carbohydrates might be interchangeable as an energy source, requiring no such regulation, although synthesis of body lipids from ingested lipids should be more efficient than from carbohydrates. Locusts and grasshoppers, which have been extensively used as herbivore models in nutritional ecology studies, have an impressive migratory flight capability that strongly relies on oxidation of lipid stores that are accumulated in the late-juvenile and early adult stage. While late-instar locusts can synthesize lipids de-novo from carbohydrates, we hypothesized that they might stoichiometrically regulate lipid intake to efficiently fulfill lipid accumulation needs. In order to study whether locusts regulate a specific carbohydrate to lipid intake ratio, we ran a series of diet choice experiments using 5^th instar migratory locust nymphs, Locusta migratoria. We designed artificial diets varying in carbohydrate to lipid energy ratios, all with constant energy and protein content. To attempt to control for variation in taste, we ran tests with three different vegetable oils: canola, sunflower, and grapeseed. In all three experiments, nymphs ate non-randomly from the diets and selected a very narrow ratio of ~2:1 carbohydrate:lipid. Our results suggest that lipids can be an important, previously ignored “third rail” of the geometric framework for herbivore nutrition. This work was supported by NSF # 1942054.