174 BIOLOGICAL PROBLEMS 



their natural enemies is so small that the damage they 

 do to the trees is inappreciable. It may be important, 

 therefore, in case of any new outbreak of the pest, to 

 note if the most important known parasites are pre- 

 sent or absent, and in the latter case to supply the 

 district with a number of specimens. That this trans- 

 fer can be effected has recently been demonstrated in 

 an interesting experiment. 



When Dr. Gordon Hewitt left Manchester to take 

 up his position as entomologist to the Dominion 

 Government in Canada he found the same saw-fly 

 causing great damage to the larch trees in Manitoba 

 and elsewhere, but on examining the pupae he did not 

 find any specimens of the parasitic ichneumon (Meso- 

 leius tenthredinis) referred to above. A large number 

 of pupae were therefore sent to him from Cumberland, 

 and the ichneumons that emerged from them were 

 liberated in the Canadian forest. It was possible, of 

 course, that the severity of the Canadian winter and 

 many of the other influences that regulate the balance 

 of nature might prove unfavourable to the British 

 immigrant, but Dr. Hewitt was able to report that 

 in 1916 1 the ichneumon parasite had been recovered, 

 and was therefore presumably already acclimatized. 

 We have therefore been able to supply the Dominion 

 with a valuable ally in its battle against the saw-fly, 

 and we may confidently believe that in due time it 

 will prove its value. 



In these matters of economic entomology we seem 

 to have reached only the threshold of knowledge. 

 We have obtained some interesting and a few valuable 

 results, but we really know nothing of importance 

 about the vast struggle for existence that is going on 

 in the insect world, not only in the open air during 

 the summer months, but all through the winter in the 



1 Report of the Dominion Entomologist, 1917, p. 9. 



