Insect herbivory and vertebrate grazing impact food limitation and grasshopper populations during a severe outbreak

Authors: Branson, David - Dave; Haferkamp, Marshall

Submitted to: Ecological Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2014
Publication Date: 2/1/2014
Publication URL: http://handle.nal.usda.gov/10113/58796
Citation: Branson, D.H., Haferkamp, M.R. 2014. Insect herbivory and vertebrate grazing impact food limitation and grasshopper populations during a severe outbreak. Ecological Entomology. 39(1):371–381.

Interpretive Summary: Grasshoppers are often the dominant herbivore in western U.S. grasslands. Grasshopper outbreaks also cause significant economic hardship for ranchers and can result in competition with livestock for rangeland plants. Grasshopper populations tend to increase with grazing intensity in the northern Great Plains, but our understanding of why specific outbreaks occur is still lacking. Little research has examined how the timing of livestock grazing might negatively impact grasshopper populations. An experiment was conducted at the USDA, ARS, Livestock and Range Research Lab in eastern Montana, during a naturally occurring grasshopper outbreak where densities of grasshoppers reached 130 per m2. We manipulated grasshopper densities and sheep grazing inside cages to examine their relative importance on grasshopper populations. Both sheep herbivory and grasshopper herbivory led to food limitation and reduced future year grasshopper populations, but the larger effects were from grasshoppers. Grasshopper populations crashed over six fold in the year following the experiment, due to low levels of reproduction. The results indicate that grasshoppers can have stronger effects than livestock grazing in determining future grasshopper populations during outbreak periods, although livestock grazing can have important effects at lower grasshopper densities. The results can be used to better predict the onset and decline of future grasshopper outbreaks. These results indicate that severe outbreaks can rapidly subside when food is limiting, potentially reducing the economic justification for control programs which have non-target effects.

Technical Abstract: Interspecific competition between distantly related herbivores, as well as between large vertebrate herbivores and phytophagous insects, has received little attention. Livestock grazing is the dominant land use in western North American grasslands, where phytophagous insects can be the dominant herbivore. Large scale studies have examined interactions between livestock grazing and grasshoppers, but manipulations of grazing and grasshopper densities to clearly examine competition are lacking. In a novel design, we manipulated grasshopper population densities and the timing and frequency of sheep grazing inside 10m2 caged mesocosms to examine their impact on grasshopper population dynamics and vegetation. We utilized a replicated, factorial experimental design during a naturally occurring grasshopper outbreak. Field densities of grasshoppers reached 130 per m2 in 2000. Grasshoppers removed much of the available vegetation biomass during the experiment, as green grass biomass was eight times higher in vegetation control cages than in field density cages. As a result of high densities, exploitative competition from grasshopper herbivory was dominant in all treatments. Proportional survival and number of grasshoppers surviving were lower in field relative to reduced density cages. Higher grasshopper population densities and late season and multiple grazing both accelerated the population decline resulting from food limitation. Strong density dependence was evident in reproduction, as fewer grasshoppers hatched in field density than in reduced density treatments in 2001. Few detailed experiments have been conducted during grasshopper outbreaks. The results indicate that phytophagous insect herbivory can have stronger effects than livestock grazing on population dynamics during an outbreak, but livestock grazing and grasshopper herbivory interacted at the lower density. Additional research examining interactions between livestock grazing and grasshoppers is needed at lower grasshopper densities.