Host plant quality affects nutrient regulation and performance of a mixed-feeding insect herbivore

Like many insect herbivores, Melanoplus bivittatus (Say) can maintain relatively constant body carbon (C): nitrogen (N) levels when faced with a range of different nutritional landscapes. We assessed the roles of different regulatory mechanisms (food selection, compensatory feeding, or physiological adjustment) in this generalist grasshopper using natural and artificial diet experiments. Because a pilot study showed that grass: forb consumption can depend on C:N of available host plants, we hypothesize that stoichiometric homeostasis is achieved primarily through food selection in this mixed-feeding species. For the natural diet experiment, leaves of nitrogen fertilized (+) and unfertilized (-) grass (G) and forb (F) were collected at Konza Prairie Biological Station. High (H) or low (L) quality diets (with respect to C:N) with (+) or without (-) plant extracts were used in the artificial diet experiment. In each experiment, 5^th instar M. bivittatus (Say) nymphs were placed in individual cages and provided one of eight 2-diet combinations for the duration of the stadium. Individuals provided with F+ as at least one of the two diets offered had the highest survival, while those given only grass material (G+/G+, G+/G-, G-/G-) experienced 100% mortality. Although the total amount of food consumed did not differ among treatments, significantly more F+ diet was consumed in F+/G+ and F+/G- treatments. In the artificial diet experiment, survival was 90% on the H+/H+ and lowest on the L/L combinations. Total consumption was highest on the H+/L- and lowest on the H-/H-treatments. There were no preferences among diets. For surviving individuals, our results suggest that M. bivittatus employ all three regulating mechanisms to maintain performance when faced with high and low quality resources as would be encountered in a variable field environment.