. The Canadian field-naturalist. December, 1932] I78Q 90 The Canadian Field-Naturalist 1800 10 20 30 40 197 6c. Chart showing Sunspot and Grasshopper Cycles. The upper curves represents grass- hoppers, the lower ones Sunspots (Original) hundred miles to the south of us. These foreign swarms nearly always left their eggs in the in- vaded territory and their progeny, hatching the following spring, played far greater havoc with the crops than their progenitors had done before them. As a rule the progeny on acquiring wings, flew elsewhere, and unless fresh swarms arrived the country might be compa
. The Canadian field-naturalist. December, 1932] I78Q 90 The Canadian Field-Naturalist 1800 10 20 30 40 197 6c. Chart showing Sunspot and Grasshopper Cycles. The upper curves represents grass- hoppers, the lower ones Sunspots (Original) hundred miles to the south of us. These foreign swarms nearly always left their eggs in the in- vaded territory and their progeny, hatching the following spring, played far greater havoc with the crops than their progenitors had done before them. As a rule the progeny on acquiring wings, flew elsewhere, and unless fresh swarms arrived the country might be comparatively free from grasshoppers the following year. The point to be emphasized in connection with these invasions is that the invaders probably originated in more arid sections of the continent where increased rainfall might actually be a bene- fit to the grasshoppers by stimulating plant growth and providing water for drinking, where- as extreme aridity might, in these sections, be less favourable to survival. In other words while the minimum rain period might benefit local grasshopper development, in arid areas a con- verse condition might be more favourable. The mean rainfall necessary to increase being the same in both cases although occurring at oppo- site phases in the sunspot cycle. If this were so then it would explain the invasions of Manitoba which took place during supposedly adverse local conditions. There is reason for believing that the great grasshopper period from 1857 to 1876 was main- tained by repeated invasions, rather than through the reproduction of local species and that the 1817-18 outbreak was also due to invaders. If this was so then the almost negligible local correla- tion with sunspot influence at those times is ex- plained. The next group of factors which complicate the rhythm in sunspot-grasshopper periodicity are parasites, predators and diseases. As a rule the first essential to grasshopper increase is an ab- sence of the more important natural
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