Regulation of sterol nutrition in the fall armyworm, Spodoptera frugiperda

Plant sterols constitute the sole source of sterol nutrition for insect herbivores, yet the sterol content of plants is complex and variable, and insect sterol nutrient regulation remains to be elucidated. In this study, we investigated cholesterol regulation in Spodoptera frugiperda using a geometric framework for nutrition. In the choice experiment, we provided larvae with eight food pairings. The insects did not adjust their feeding on the pair of complementary diets to achieve a specific cholesterol intake target. In the no-choice experiment, seven foods with different cholesterol concentrations were administered to explore sterol regulation. The larvae did not show a tendency to reduce or increase their intake of non-cholesterol nutrients to reach a certain cholesterol intake. We then measured the relative expression of the NPC1b gene in the intestine and the cholesterol content of the feces. As the cholesterol content in the food increased, NPC1b gene expression gradually decreased and the larvae excreted more cholesterol in the feces. Meanwhile, the cholesterol content in the larvae remains largely consistent, indicating that the larvae has a strong control on the body sterol content. Altogether, our results suggest that the larvae regulate cholesterol homeostasis by balancing cholesterol uptake and excretion, and cholesterol level can be kept within a physiologically appropriate range.