THE CANADIAN ENTOMOLOGIST 79 



the husbandman made his appearance. The sowing of cereals 

 has altered matters so that when the grasses fail to produce 

 stems the saw-flies were able to continue their increase by at- 

 tacking wheat and rye, with the result that they are now a serious 

 pest. 



Yet another insect which is checked by lack of moisture is the 

 Hessian Fly. Dry seasons are generally recognized as anti-fly 

 years and in Manitoba the partial second brood is frequently de- 

 stroyed outright by a premature ripening of the grain, due to dry, 

 hot weather conditions in late Juh'. A knowledge of this fact is 

 often of the greatest importance in forecasting as to the probabilities 

 of a Hessian-Fly outbreak. It also comforts us in the thought that 

 weather conditions will seldom remain favourable to the insect for 

 any length of time, though we have the unpleasant knowledge that 

 good crop years are frequently best adapted to Hessian-Fly increase. 

 Moisture is probably still more important over northern lati- 

 tudes, in the form of snow when it provides a thick covering to the 

 objects beneath, protecting them from both frost and sun. It 

 is really astonishing how a blanket of snow will afiford protection 

 even to the tender plants. For instance, potatoes have more than 

 once survived the winter, even in our invigorating Manitoba climate 

 Avhere the temperature, at times, attains a minimum of fifty below 

 zero. Yet expose these same potatoes directly to four or five de- 

 grees of frost and they will present a sorry spectacle next morning. 



Snow is unquestionably a very important factor in preserving 

 insect life in winter time, and there is no doubt that it is largely 

 responsible for the preservation of many forms which would perish 

 without its covering. Perhaps there is no better example of its 

 preserving influence than was shown by the Colorado Potato ' 

 Beetle. This beetle invaded Manitoba a number of years ago, 

 but did not become a serious pest until comparatively recent times. 

 It had, however, gradually extended its range northward. In 1913 

 its ravages were severely felt in Southern Manitoba as far north as 

 latitude 50'^ and to a lesser degree for some distance farther north. 

 In the winter following, that is 1914-15, there was a great scarcity 

 of snow in portions of the province so that some places used wheels 

 throughout the winter. The result of this lack of snow was that 

 the frost penetrated considerably deeper into the ground than 



