Geometric analysis of macronutrient selection and balancing in the superworm, Zophobas atratus

The larvae of Zophobas atratus F. (Coleoptera: Tenebrionidae) are a promising source of dietary protein for animal feed. However, there has been no detailed investigation on macronutrient selection and requirement in Z. atratus larvae. Here, we used the geometric framework to explore the behavioural and physiological regulation of protein and carbohydrate in Z. atratus larvae. When provided with pairs of nutritionally imbalanced but complementary foods (choice experiment), Z. atratus larvae self-composed a diet with a protein-to-carbohydrate (P:C) ratio of 1.47:1 or 1.19:1 throughout the experimental period. Across different choice combinations, the intake of carbohydrate was more tightly regulated than that of protein. When confined to one of six foods imbalanced with respect to P:C ratio (no-choice experiment), Z. atratus larvae regulated their protein–carbohydrate intake to form linear intake arrays across nutrient space in a manner similar to those previously described for other nutritional generalists. The negative slope of the linear intake arrays was shallow (−0.67 to −0.54), suggesting that protein was over-consumed by Z. atratus larvae to a greater extent than carbohydrates. The nutritional landscapes fitted for larval performance traits exhibited that the performance of Z. atratus larvae was maximized at the P:C ratio of 2:1 and fell rapidly as the P:C ratio decreased below 1:1. There was evidence for post-ingestive regulation for body nutrient growth through adjusting the utilization efficiencies of ingested protein and carbohydrate. Our results represent the most comprehensive analysis of macronutrient regulation in Z. atratus and have implications for designing an optimal diet for this beneficial insect.