Author
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ZHANG, MINGCHU - UNIVERSITY OF ALASKA |
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FIELDING, DENNIS |
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Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/16/2011 Publication Date: 6/20/2011 Citation: Zhang, M., Fielding, D.J. 2011. Populations of the northern grasshopper, Melanoplus borealis (Orthoptera: Acrididae), in Alaska are rarely food limited. Environmental Entomology. 40(3):541-548. Interpretive Summary: Grasshoppers can cause substantial losses to forage on rangelands and pastures and to field crops, but chemical control of grasshopper pests is rarely justified because of the low per-area value of forages, the extensive areas needed to be treated to protect crops, and because of potential impacts to non-target organisms. More sustainable strategies to manage grasshopper populations require knowledge of the forces that regulate grasshopper populations. In some systems, grasshoppers appear to be food limited in most years, whereas in other systems predators, are more often implicated in population regulation. This experiment was undertaken to determine whether grasshoppers are food-limited in the interior of Alaska. Cages were set up in a fallow field near Delta Junction, Alaska in 2007 and 2008. Fertilizer was added to half the plots to increase plant production, and cages within each plot were stocked with immature of the Northern grasshopper (Melanoplus borealis) equivalent to 0, 20, 36, or 52 grasshoppers per square meter. Grasshoppers in each cage were counted weekly. Near the end of the growing season, surviving female grasshoppers were collected. Femur length was taken as a measure of adult size, and functional ovarioles were counted as a measure of potential fecundity. If the grasshoppers were food limited, I expected to see significant effects of either density or fertilizer on grasshopper survival, size, or fecundity. The fertilizer treatment greatly increased primary production in both years. Neither fertilizer treatment or grasshopper density had any effect on survival, size, or potential fecundity, leading me to conclude that food was not limiting at densities up to 52 grasshoppers per square meter. Technical Abstract: Grasshoppers can cause substantial losses to forage on rangelands and pastures and to field crops, but chemical control of grasshopper pests is rarely justified because of the low per-area value of forages, the extensive areas needed to be treated to protect crops, and because of potential impacts to non-target organisms. More sustainable strategies to manage grasshopper populations require knowledge of the forces that regulate grasshopper populations. In some systems, grasshoppers appear to be food limited in most years, whereas in other systems top down forces, e.g., predators, are more often implicated in population regulation. This experiment was undertaken to determine whether grasshoppers are food-limited in the interior of Alaska. Cages were set up in a fallow field near Delta Junction, Alaska in 2007 and 2008. The experiment was set up as a split-plot design with N fertilizer as an among-plots effect and grasshopper density as the within-plots effect. Fertilizer was added to half the plots to increase primary production, and cages within each plot were stocked with 0, 5, 9, or 13 fourth-instar Melanoplus borealis (equivalent to 0, 20, 36, or 52 grasshoppers m-2). Grasshoppers in each cage were counted weekly. Near the end of the growing season, surviving female grasshoppers (about 40% of the original number) were collected. Femur length was taken as a measure of adult size, and functional ovarioles were counted as a measure of potential fecundity. If the grasshoppers were food limited, I expected to see significant effects of either density or fertilizer on grasshopper survival, size, or fecundity. The fertilizer treatment greatly increased primary production in both years. Neither fertilizer treatment or grasshopper density had any effect on survival, size, or potential fecundity, leading me to conclude that food was not limiting at densities up to 52 grasshoppers m-2. |
