Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2008
Publication Date: 6/1/2008
Citation: Branson, D.H. 2008. Influence of a large late summer precipitation event on food limitation and grasshopper population dynamics in a northern Great Plains grassland. Environmental Entomology. 37(3):686-695. Interpretive Summary: Although grasshoppers make a number of positive contributions to grassland health, periodic outbreaks cause significant economic damage to the livestock grazing industry in the western U.S. Despite this, very little is known about the generation of grasshopper outbreaks. In this study, the origin of a severe grasshopper outbreak was traced to the availability of high quality grass biomass following a late summer rain. In years with lower late summer precipitation, similar grasshopper densities would be expected to lead to stronger and more consistent density dependent effects. Although the weather conditions that led to the population explosion are uncommon, the results are indicative of a direct link between weather variation, resource quality and grasshopper population dynamics leading to a large increase in grasshopper population densities. The results also indicate that infrequent pulse precipitation events can directly impact grasshopper outbreaks. More knowledge on grasshopper population dynamics is needed to aid in the development of grasshopper management approaches that manipulate ecological interactions to reduce grasshopper outbreaks.
Technical Abstract: Little is known about the ecological processes that generate grasshopper outbreaks, and the complex ecological interactions between grasshoppers, weather conditions and plants that cause fluctuations in grasshopper populations remain poorly understood. The effects of initial and increasing grasshopper densities on grasshopper population dynamics, reproductive correlates and rangeland vegetation were examined at a northern mixed-grass prairie site highly dominated by western wheatgrass though manipulations of grasshopper densities inside 10 m2 cages. High quality grass growth occurred following a large late summer precipitation event of 9 cm. Reduced proportional survival was apparent in the two higher density treatments prior to the rain, indicative of food limited density dependent mortality. However, the late summer rainfall event mediated the effects of high grasshopper densities on demographic characteristics due to the growth of young higher quality vegetation in late summer. This led to weak effects of competition on survival and reproduction at the end of the experiment even in the increased density treatment where densities were above 30 per m2. A severe grasshopper outbreak occurred in the following year, with nymphal densities reaching 130 per m2. As a result, the origin of a severe grasshopper outbreak can be traced to the availability of high quality grass biomass following late summer rain. In years with lower late summer precipitation, stronger and more consistent density dependent effects would be expected with similar grasshopper densities.