DIET CHOICE BY A GRASS-FEEDING GRASSHOPPER BASED ON THE NEED FOR A LIMITING NUTRIENT

1. Phenylalanine is an essential aromatic amino acid needed for cuticle production in immature insects but is of reduced importance in adults. As a consequence, it was hypothesized that nymphal grasshoppers should select diets containing high levels of phenylalanine while adult grasshoppers would demonstrate no selective feeding preference based on the level of phenylalanine in their diets. 2. Using a series of choice test experiments with adults and fifth instar nymphs of the generalist grasshopper, Phoetaliotes nebrascensis, the phagostimulatory property of phenylalanine was evaluated. Three phenylalanine levels were tested against a constant background of sucrose and 17 additional amino acids using levels typical of those found in grasses. 3. Nymphs selected diets high in phenylalanine while adults showed no preference as phenylalanine levels changed. These results suggest that the nutritional requirements of an insect influence dietary selection. 4. The ability of immature insects to select diets high in phenylalanine has direct consequences on both growth and developmental rates. 5. Additionally, these results provide another example of nutritional constraints in insects being relevant to developing foraging theory for herbivores.