A Perspective of Grasshopper Population Distribution in Saskatchewan and Interrelationship with Weather 1

Abstract

Thirty-two years of annual adult grasshopper survey data collected in Saskatchewan were quantified and used to examine the trends in grasshopper numbers and selected weather variables. Interrelationships between 4 species of grasshoppers, Melanoplus sanguinipes (Fabr.), M. bivittatus (Say), M. packardii (Scudder) and Camnula pellucida (Scudder), heat units, and precipitation were investigated by quantifying the average annual trends and the average distribution patterns. Density ratings of grasshoppers were most highly autocorrelated with the previous years' ratings, the degree of correlation decaying rapidly and becoming nonsignificant after 3 yr. Correlations between precipitation and density ratings were negative for up to a lag of 3 yr. Positive correlations were found between heat units and density ratings and also between heat-precipitation ratios and density ratings. Maximum correlation coefficients tended to occur with the previous years' weather but the response of individual species was different. The distribution of grasshoppers, heat units, and precipitation were related. Higher populations occurred in the area where the average heat accumulation was above 1600°D₅₀ and average precipitation was < 10 in. (250 mm). Probabilities of exceeding the 32-yr mean were calculated for 38 districts in the province for Apr.–July and Aug.–Sept. heat-precipitation ratios, and for combined grasshopper density ratings above 3.0/8.36 m². The correlation between these probabilities was high. A damage index was developed to associate grasshopper density with the heat-precipitation ratio based on variable penetration of grasshoppers into the crop.