Two, 1400-ha blocks of rangeland in western South Dakota were treated aerially with malathion liquid spray or carbaryl – bran bait in early July 1986 to determine the immediate and 2nd-year impact of treatments on grasshopper populations. Total grasshopper populations were reduced by 92 and 47% in the malathion and carbaryl – bran bait treatment plots, respectively, within 48 h after treatment and remained at a low level throughout the summer. Populations did not change in the control plots. Populations of the two most abundant species, Ageneotettix deorum (Scudder) and Melanoplus sanguinipes (F.), declined by 65 and 87%, respectively, in the carbaryl – bran bait plots but populations of bran "rejectors" (predominantly Trachyrhachys kiowa [Thomas]) did not change.
Densities of the bran "acceptors" (Melanoplus spp., Phoetaliotes nebrascensis [Thomas], and A. deorum), as a group, did not change significantly in the control plots between the pre-treatment and July 1987 sampling dates. Densities within both sets of treatment plots were significantly lower in the 2nd year of the study than on the pre-treatment sampling date. Although 2nd-year populations of bran acceptors, as a group, did not increase to pre-treatment levels in the treated plots, populations of M. sanguinipes did increase to pre-treatment levels in both sets of treatment plots. Populations of bran rejectors generally remained low in treatment and control plots.
Analysis of covariance of the densities of 2nd-year populations of total grasshoppers and bran rejectors indicated that treatment had no significant effect on populations of these grasshoppers, but the covariable, pre-treatment density, was significantly correlated with 2nd-year densities. Densities of 2nd-year populations of bran acceptors were also significantly correlated with pre-treatment densities.
It was concluded that both the insecticidal spray and bait were effective in controlling most economically important species of rangeland grasshoppers. Although both treatments may have suppressed populations of bran acceptors, as a group, in the 2nd year of the study, neither suppressed populations of M. sanguinipes which increased to pre-treatment levels regardless of treatment. The effect of treatments on 2nd-year populations of bran rejectors could not be determined because populations of this group also declined in control plots.