IMPACT OF DIET QUALITY ON DEMOGRAPHIC ATTRIBUTES IN ADULT GRASSHOPPERS AND THE NITROGEN LIMITATION HYPOTHESIS

1. Various formulations of the nutrient stress hypothesis predict that insect herbivore populations will respond positively to increased nut rient quality of host plants, especially dietary nitrogen. Survival an d reproduction by adult females of two grasshopper species [Acrididae; Melanoplus sanguinipes (Fabricius) and Phoetaliotes nebrascensis (Tho mas)] were evaluated in response to defined diets that varied factoria lly in both total nitrogen (1-7%) and total soluble carbohydrate (4.3- 26.7%). These grasshopper species coexist naturally but are typically shifted phenologically so that specific developmental stages normally encounter host plants of different nutritional quality under natural c onditions. 2. Demographic responses by adult females of both species v aried according to diet quality, but not in the same fashion. Diet qua lity affected survival significantly in P. nebrascensis but not in M. sanguinipes. Survival in P. nebrascensis was greatest on diets contain ing the lowest nitrogen concentrations; carbohydrate level had no effe ct. 3. Diet quality influenced reproduction significantly in both spec ies. Egg production rate (eggs/day) in M. sanguinipes exhibited a nega tive linear response to increased carbohydrate, coupled with a signifi cant quadratic response to nitrogen that reached a maximum at an inter mediate level of about 4% total N. A significant quadratic response to total N for pod production rate (indicating the timing of reproductio n) was also observed. Clutch size in M. sanguinipes exhibited a negati ve relationship with total carbohydrate in the diet, but no response t o nitrogen. No interaction was observed between nitrogen and carbohydr ate levels. For P. nebrascensis, response to diet quality was much wea ker, with only a suggestive maximum at 4% total-N for both egg product ion rate (eggs/day) and clutch size (eggs/pod) and a suggestive linear response for pod production rate as carbohydrate level increased. Fem ale body weight did not contribute to any reproductive response as a c ovariate variable. 4. Combined with a similar, previous analysis of de mographic responses by the grass-feeding grasshopper, Ageneotettix deo rum, these results challenge the ability to draw generalizations about host plant nutritional quality and grasshopper demographic responses. These three grasshopper species respond quite differently to defined diets that vary in total nitrogen and carbohydrate levels. Thus, altho ugh host plant quality can contribute significantly to grasshopper pop ulation responses, a uniform explanation is not likely.