Foraging is fundamental to animal survival and reproduction, and animals often require more than one nutrient to maximize their evolutionary fitness. Here, we test whether caterpillars of Spodoptera exigua (Hübner) balance the intake of multiple nutrients to meet their nutrient requirement. In the choice test, final-instar larvae were offered a choice of two nutritionally complementary diets [1) p42:c0 vs. p0:c42, 2) p42:c0 vs. p7:c35, 3) p35:c7 vs. p0:c42, 4) p35:c7 vs. p7:c35, 5) p35:c7 vs. p5.6:c28, 6) p28:c5.6 vs. p7:c35 and 7) p28:c5.6 vs. p5.6:c28]. Caterpillars tightly regulated their intake of protein (P) and carbohydrate (C) to defend a specific nutrient composition, which was slightly carbohydrate-biased (P:C=1:1.2). In the no-choice test, larvae were restricted to feed on one of 42 diets that varied both in P:C mixtures (1:0, 5:1, 2:1, 1:1, 1:2 and 1:5) and in total nutrient concentration(P+C=67.2%, 58.8%, 50.4%, 42%, 33.6%, 25.2% and 16.8%). Fitness landscapes fitted for key larval fitness variables (e.g., growth rate) over these range of diets revealed that the larval performance was optimized at the regulated position of nutrient intake in this caterpillar.

The aim of this research was to demonstrate whether generalist-feeding caterpillars of Spodoptera exigua (Hübner) regulate their nutrient intake when faced with nutritionally variable food conditions. Six, chemically-defined diets were prepared that differed in the composition of protein and digestible carbohyrate:42% protein with 0% carbohydrate by dry mass(p42:c0), p35:c7, p28:c5.6, p7:c35, p5.6:c28 and p0:42. A total of 288 newly-ecdysed final instar(5th) caterpillars were collected and assigned randomly into 7 food pairing treatments, in which they were allowed to choose between two diet block:one with high P:C ratio and the other with low ratio [1) p42:c0 vs. p0:c42, 2) p42:c0 vs. p7:c35, 3) p35:c7 vs. p0:c42, 4) p35:c7 vs. p7:c35, 5) p35:c7 vs. p5.6:c28, 6) p28:c5.6 vs. p7:c35 and 7) p28:c5.6 vs. p5.6:c28]. Various aspects of food intake and larval performance variables were measured for each insect, including larval survival, stadium duration, pupal mass and body lipid composition. Results showed that the intake of protein and carbohydrate self-composed by caterpillars in all treatments converged to a point in a bivariate nutrient plot and the ratio of protein to carbohydrate averaged over these converging intake points was close to 1:1. This indicates that S. exigua caterpillars have capacity to balance their nutrient intake by defending their species-specific ‘intake target’ despite the differences in amount and proportion of nutrients available in each food choice treatment.