Many insects adjust feeding behavior to meet their optimal requirement for multiple nutrients. In the present study, we investigated the behavioral regulatory responses of male and female adults of an omnivorous beetle, Tenebrio molitor (Coleoptera: Tenebrionidae), using choice and no-choice feeding experimental designs. In the choice experiment, newly eclosed adults of both sexes received one of four food-pairing treatments and so were allowed to self-compose their preferred nutrient intake by selecting between two nutritionally-complementary diets (protein-rich vs. carbohydrate-rich diet; 35:7 vs. 7:35, 35:7 vs. 5.6:28, 28:5.6 vs. 7:35, or 28:5.6 vs. 5.6:28, P%:C% by dry mass). Our data showed that both sexes independently regulated their intake of protein and carbohydrate to a 1:1 ratio, indicating that they are capable of adjusting their feeding behavior to optimize their nutrient preference. In the no-choice experiment, adults were forced to eat one of seven single diets that varied in P:C ratio (0:42, 7:35, 14:28, 21:21, 28:14, 35:7, or 42:0). Results showed that both sexes ingested similar amounts of nutrients when the dietary P:C ratio was moderately balanced (14:28, 21:21 and 28:14), but males ingested significantly more than females on those diets that were extremely imbalanced with respect to their P:C ratio (0:42, 7:35, 35:7 and 42:0). This indicates that there exists a sex-specific difference in nutrient intake when the nutritional balance of diets deviates away from the optimal level under restricted feeding condition.

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.