16^ THE REPORT OF THE No. 33 



the Curculio. A hymenopterous parasite has been reared from this interesting 

 fly and it is hoped that further investigations of both, as well as the other species 

 mentioned, may yet be carried out. In conclusion, grateful acknowledgment 

 is made of the generous assistance rendered in this study by Mr. Norman Criddle, 

 Entomologist in Charge for Manitoba. 



TWO PROBLEMS IN NATURAL CONTROL 



Norman Criddle, Dominion Entomological Laboratory, 

 Treesbank, Man. 



The study of natural control has always been of great fascination to me. 

 There are so many intricate byways to be traversed and so many inter-relations 

 to be studied before the links begin to form a chain and the chain is joined to 

 othjr chains, thus eventually giving us an insight into the great web of life. 



When we view natural control in this way we see that it goes beyond the 

 realms of any one science — Entomology, ornithology, mammalogy and botany 

 are all involved in its kingdom, which stretches out over the entire world. 



It is not my intention, however, to involve myself in all the intricacies of 

 natural control in this paper. I wish, rather, to touch upon one or two of the 

 more important problems with which I have come in contact during the last few 

 years. 



One of the major problems that has engaged our attention in Manitoba 

 during the present century has been that of controlling the Wheat-stem Sawfly, 

 Cephus ductus Nort. The insect began its career as a pest more than twenty 

 years ago and from that time on it has occasioned enormous losses to farmers of 

 the Prairie Provinces. Its spread was slow at first, but later it made rapid 

 inroads into the grain fields, and to-day it is found over practically the entire 

 wheat-growing areas of Manitoba, over much of Saskatchewan and part of 

 Alberta. It is also a common pest in North Dakota. 



Now Cephas cinctus was originally a grass-stem inhabitant and any stem 

 might accommodate the larvae, providing it was large enough. Consequently, 

 the only limit to the insect's increase was the number of suitable stems and the 

 prevalence of its natural enemies. Of the two I think the latter were the most 

 efficient in keeping it within bounds, but in spite of its enemies the sawfly managed 

 to hold its own without much difficulty and when man began to upset the natural 

 balance, as he is always doing, the sawfly soon took advantage of the new plants 

 provided. Thus it came to pass that the sawfly became a pest, but what had 

 become of its natural enemies? Had natural control, in this instance, lost its 

 effectiveness? Temporarily, yes, but there is reason to hope, not permanently. 

 When the sawfly invaded the wheat and rye fields it succeeded in leaving its 

 parasitical enemies behind and in consequence it spread with far greater rapidity 

 than would otherwise have been possible. For a long time it appeared as 

 though the parasites, so common in grasses, could not follow their hosts into the 

 new food plants; perhaps due to certain cultural methods, or to some other 

 causes imperfectly understood. Gradually, however, odd stems of Cephus- 

 infested grains were found to contain the parasites. They were found chiefly 

 on the edges of fields, at first, just as the first sawflies had been, as if constituting 

 an overflow from the grasses round about. ■ Ultimately one parasite, Micro- 

 bracon cephi Ghn., began to spread farther afield and such was its progress by 



