Performance by insect herbivores can be highly influenced by environmental factors such as temperature and food quality. Because both temperature and food quality are often highly variable in the field, insects must adjust to environmental heterogeneity to meet metabolic and nutritional needs. Furthermore, many species have geographic ranges that extend over broad latitudinal gradients, where populations could exhibit very different local responses to similar controlled environmental conditions. In this study, we assess how populations of a generalist grasshopper, Melanoplus femurrubrum, that range along a broad latitudinal gradient respond to temperature and food quality manipulations using “common garden” feeding experiments. We expect populations to be locally adapted to temperature, and expect faster development of individuals originating from the extremes of our temperature gradient.
Controlled feeding trials at three temperatures (25, 31, 37oC) were performed using first generation fifth instar offspring from eggs produced by adults collected from eight grasshopper populations along a north-south (Texas-North Dakota) gradient. Individuals were fed four controlled diets that varied in quality (protein:carbohydrate; 14:28, 21:21, 28:14 and a choice - 14:28 and 28:14). Grasshopper performance (development time, assimilation efficiency, etc.) was examined for a full stadium for each treatment.
Results/Conclusions
Development time of M. femurrubrum varied significantly with population origin, temperature, and food quality. No statistical interactions among these independent variables were detected. Interestingly, individuals from populations more geographically central to the gradient develop more slowly than individuals from higher latitudes. Northern populations are generally constrained by the shorter, cooler growing season, so these populations must develop faster to reproduce before the season ends. Individuals from central portions of our transect experience a longer, more variable growing season and have a longer period to mature and reproduce. Differences in nutrient assimilation efficiency also vary according to temperature, diet, and population origin. Here, a significant site*diet interaction is detected, suggesting that grasshopper populations experiencing different temperature regimes compensate through diet selection and processing to meet nutritional intake needs. Overall, M. femurrubrum individuals along this gradient respond differently to temperature and diet manipulations suggesting local adaptation of these populations.
