The statement of Criddle (1917) that grasshopper outbreaks are associated with periods of dry weather has been repeated by many authors, but a close relationship has never been demonstrated.

Parker (1933) showed that at six localities in the United States grasshopper outbreaks during the period 1915-1933 had been preceded by periods of two to four years when either rainfall had been below normal during May-June or temperature had been above normal during July-September. Similarly Smith (1954) concluded that between 1910 and 1953 outbreaks in Kansas had been preceded by two years of sub-normal rainfall. Parker's charts show that grasshopper outbreaks generally followed periods of hot weather, but the charts of both authors show that only about half the time did outbreaks follow periods of dry weather – a situation that could be largely due to chance.

MacCarthy (1956) calculated the correlation coefficients between weather factors and populations of M. bilituratus (Walker) in five large areas of Saskatchewan during the period 1943-1952. His tables show generally insignificant correlation with total precipitation and only with mean minimum monthlv temperature in June of the previous year was there significant correlation in all five areas.

The present investigation was undertaken in an attempt to determine whether a sufficiently close relationship between grasshopper populations and climate could be found for use in predicting grasshopper outbreaks.