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Title: Effects of nymph-overwintering grasshopper density on Ageneotettix deorum survival in a northern mixed grassland

item Branson, David - Dave

Submitted to: Journal of Orthoptera Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2011
Publication Date: 12/27/2011
Citation: Branson, D.H. 2011. Effects of nymph-overwintering grasshopper density on Ageneotettix deorum survival in a northern mixed grassland. Journal of Orthoptera Research. 20(2): 137-139.

Interpretive Summary: Although most abundant pest grasshopper species overwinter as eggs and hatch in early summer, several species hatch in late summer and overwinter as nymphs. These species periodically reach high densities that are an economic problem to ranchers, with approximately 145,000 hectares sprayed for nymph-overwintering grasshoppers in 2003. Researchers have hypothesized that competition from earlier hatching grasshoppers could reduce survival of later developing species and reduce the likelihood or intensity of grasshopper outbreaks. However, the potential impacts of nymph-overwintering grasshopper species on population dynamics of common pest grasshopper species have not been fully examined. A field experiment was conducted in eastern Montana to examine if nymph-overwintering grasshoppers reduced the populations of a common pest grasshopper species that hatches in early summer. These results, combined with those from an earlier experiment, indicate that nymph overwintering grasshoppers are unlikely to have strong effects on later hatching pest species unless densities are extremely high or grass production is low. As a result, high densities of nymph-overwintering grasshoppers are unlikely to significantly reduce grasshopper outbreaks of later hatching pest species.

Technical Abstract: Although most pest grasshopper species in North America hatch in late spring or early summer, some species hatch in late summer and become adults in late spring. It is not well understood how they impact densities of later developing pest grasshopper species. In an earlier study examining temporally separated competition nymphal survival of an egg overwintering species Ageneotettix deorum (Scudder) was reduced only when high densities of adult nymph-overwintering grasshoppers strongly reduced grass biomass. However, early instar A. deorum nymphs overlap phenologically with declining densities of nymph-overwintering grasshoppers. A field experiment was conducted to examine competitive effects from nymph-overwintering grasshoppers on survival A. deorum when phenologies overlapped. Precipitation and grass production during the experiment was well above the long term average. Although the maximum density of nymph-overwintering and egg-overwintering grasshoppers was 100 per m2, interspecific exploitative competition was weak. In years with above average precipitation, competition between nymph-overwintering adult grasshoppers and later developing nymphal grasshoppers is likely to be weak even when densities are high.