Generalist insect herbivore regulates carbohydrate and lipid intake to increase the efficiency of migratory fuel accumulation

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, although synthesis of body lipids from ingested lipids should be more efficient than from carbohydrates. Locusts have an impressive migratory flight capability that strongly relies on lipid oxidation 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 regulate lipid intake to efficiently fulfill lipid accumulation needs. To study whether locusts can consume a specific carbohydrate to lipid ratio to conserve energy during lipid accumulation, we ran diet choice and developmental performance 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. Nymphs ate non-randomly from the diets and selected a very narrow ratio of ~2:1 carbohydrate:lipid. Development time, survival, mass gain, and lipid growth did not differ between diet treatments. However, as predicted by our hypothesis, nymphs that were reared on carbohydrate-based diet consumed more energy and had higher metabolic rates. Our results suggest that lipid consumption is important, previously ignored “third rail” of the geometric framework for herbivore nutrition. This work was supported by NSF #1942054.