INTERACTIONS AMONG GRASSHOPPERS (ORTHOPTERA, ACRIDIDAE) IN INTERMOUNTAIN GRASSLAND OF WESTERN NORTH-AMERICA

A field experiment was conducted in a re-established grassland in nort hern Utah, USA, to examine whether grasshopper species interact strong ly when natural populations reach high densities. During the growing s eason of 1990, plots with average densities of 10-11 grasshoppers per m(2) were treated with carbaryl bran bait (carbaryl plots) or bran bai t without carbaryl (control plots). The grasshoppers fed particularly heavily on alfalfa, driving the percent cover of this species to essen tially zero by mid-August. Application of carbaryl significantly reduc ed the densities of three of the four major grasshopper species presen t (Oedaleonotus enigma, Melanoplus sanguinipes, and M. packardii),and slowed the rate with which these insects consumed the standing crop of alfalfa. In contrast, densities of the fourth major grasshopper speci es present, M. bivittatus, were not reduced by application of carbaryl but instead were greatly elevated. This population response may have arisen in large part as adults of M. bivittatus aggregated in carbaryl plots in response to the reduced rate of defoliation of alfalfa by ot her species. Greater numbers of M. bivittatus nymphs also occurred in 1991 in plots treated with versus without carbaryl in 1990 (no carbary l was applied in 1991), as did greater numbers of O. enigma nymphs (de spite there having been fewer adult females present in these plots the previous year). But the densities of grasshoppers of all species were low in all plots in 1991 (approximately 1 per m(2) by mid-summer), an d no differences in percent cover of alfalfa or other Vegetation were detected in plots treated the previous year with versus without carbar yl. The striking positive response of M. bivittatus to application of carbaryl bran (and associated reductions in population sizes of compet ing grasshopper species) strongly suggests that intense interspecific interactions occurred at the study site in 1990.