Population Model for Melanoplus sanguinipes (Orthoptera: Acrididae) and an Analysis of Grasshopper Population Fluctuations in Colorado

A population dynamics model for Melanoplus sanguinipes F. was developed using previously reported data on nymphal development and adult longevity and between-season dynamics (eggs) of M. sanguinipes. Nymphs and adults were modeled using a conventional cohort approach. M. sanguinipes diapause was hypothesized to be a 2-phase process composed of diapause and embryogenesis. Egg hatch occurs only after both phases are completed. To model both phases concurrently, an individual-based modeling paradigm for M. sanguinipes diapause was adopted. The model simulations produced results that are consistent with observations on the relative timing of egg hatch and occurrence of the nymphal and adult stages. Simulations over a 40 yr period, using only maximum-minimum temperatures as input, at each 4 geographically separated sites in eastern Colorado, did not compare favorably with USDA-APHIS Annual Survey data in eastern Colorado. Addition of precipitation-mediated mortality greatly improved model predictions and results were more consistent with known broad-scale population trends. The results of the simulations suggest that, in Colorado, temperature conditions for grasshopper development and reproduction are generally favorable. In most years, there is sufficient heat for development and even in cooler, less favorable years, lowered temperatures do not appear to be greatly limiting. Grasshopper populations in eastern Colorado may be more limited by the lack of suitable forage which is regulated by the amount and seasonal occurrence of precipitation. Future research efforts may yield important insights into grasshopper population fluctuations by focusing on identifying and understanding the most significant mortality factors that act on M. sanguinipes populations.