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Phytophagous insects on the ashnola bighorn sheep range Maynard, Richard John

Abstract

There are conflicting statements in the literature concerning the effects of phytophagous insects on rangeland grass yields. This preliminary study was intended to asses the use of range forage by insects, especially grasshoppers. The study areas were two south-facing slopes at about 5500 feet elevation in the Ashnola Resources Management Area in South Central British Columbia. A plant community analysis was made to determine which grasses and forbs were most numerous and which provided most ground-cover in selected areas. The communities under study were characterized by various combinations of four prominent grass species: Bluebunch Wheatgrass (Agropyron spicatum), Junegrass (Koeleria cristata), Kentucky Bluegrass (Poa pratensis), and Columbia Needlegrass (Stipa columbiana). Insect grazing damage to grasses was estimated by random sampling of individual grass blades. The "preferred" (most-utilized) species were Koeleria cristata, Poa pratensis, Poa secunda, and Stipa columbiana, in that order. Agropyron spicatum was not visibly utilized. Grasshopper numbers were estimated in several ways; the most common species were Camnula pellucida, C. xanthippus, and Melanoplus sanguinipes. Grasshoppers were least numerous on the climax Agropyron spicatum community where only 21% of the total grass was of the four preferred species, and where only 32% of the ground was covered by vegetation. Highest numbers of grasshoppers were found on a "disclimax" Poa community, where 90% of the grass was of a preferred species, and where the ground was 62% covered. Experiments to determine the effects of grasshoppers on grass yields used exclosure cages placed on three different communities (Stipa - Agropyron, Poa - Stipa, and Poa) in 1969 and 1970. In all areas the mean grass yields were higher inside the cages, but the differences were not statistically significant. This result is similar to those of other workers who have attempted to demonstrate effects of grasshoppers on rangeland grass yields; while grasshoppers consume a fairly large amount of grass, they appear to have little effect on the total standing yield of grass, as determined by an end-of-season clip of vegetation. Grasshoppers may be attracted to moist areas, for example plant communities dominated by succulent grasses, where they congregate on spots which have been clipped, trampled, grazed by livestock, or otherwise disturbed. In the absence of overgrazing by livestock, even high numbers of grasshoppers probably can do little permanent damage, since their grazing is distributed over a large number of healthy, intact plants. The mature grassland, dominated by Agropyron spicatum, seems to be relatively immune to dramatic developments in insect populations.

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