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Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/26/2009 Publication Date: 4/1/2010 Citation: Branson, D.H. 2010. Density-Dependent Effects of an Early Season Insect Herbivore on a Later Developing Insect Herbivore. Environmental Entomology. 39(2): 346-350. Interpretive Summary: Nymph-overwintering grasshoppers periodically reach outbreak densities, but it is unknown if they impact the population dynamics of egg-overwintering grasshopper species, or if their herbivory reduces peak biomass production and causes economic damage. As a result, land managers and ranchers do not have the information needed to determine if grasshopper control is warranted during periods with high densities of nymph-overwintering grasshoppers. Given the potentially important implications, it is important to examine how early season herbivory by nymph-overwintering species influences the population dynamics of pest egg-overwintering grasshopper species. The results indicate that high densities of nymph overwintering grasshoppers would be required to reduce the survival of summer pest grasshopper species. Such high densities of nymph-overwintering grasshoppers would also result in a large reduction in grass biomass available for livestock. As a result, it does not appear feasible to manage populations of summer pest grasshopper species through manipulations of nymph-overwintering grasshopper densities. Technical Abstract: Although most abundant pest species overwinter as eggs and hatch in early summer in central and northern grasslands, several nymph-overwintering species hatch in late summer, overwinter as nymphs, and are adults by late spring. Researchers have hypothesized that that earlier hatching grasshopper species could reduce survival rates of later hatching pest species through competition and reduce the chance of grasshopper outbreaks. Two experiments, utilizing different initial stocking densities, were conducted at a northern mixed-grass prairie site in eastern Montana to examine the effects of early season herbivory by nymph-overwintering grasshoppers on nymphal survival of a later occurring egg-overwintering grasshopper species, Ageneotettix deorum. The experiments were also designed to examine the effects of early season herbivory on peak grass biomass. Temporally separated herbivory by nymph-overwintering grasshoppers did not significantly affect survival of Ageneotiettix deorum nymphs, except when the initial density of 60 nymph-overwintering grasshoppers per m2 led to an 80% reduction in grass biomass. Thus the effect of nymph-overwintering herbivory on survival of A. deorum was evident only under outbreak densities of nymph over-wintering grasshoppers. The effect of nymph-overwintering herbivory on A. deorum survival are best explained by competition from the reduction in grass biomass. The results indicate that high densities of nymph overwintering grasshoppers would be required to significantly reduce the sruvivial rates of nymphal A. deorum grasshoppers. As peak grass biomass would be strongly reduced at such densities, it does not appear feasible to manage populations of A. deorum through manipulations of nymph-overwintering grasshopper densities. |
