108 THE SAWFLY 



Such- remedies might prove effective when applied to 

 specimen trees, but too childishly futile to attempt in 

 a woodland. When in 1907 the sawfly invaded the 

 extensive larch woods of the Manchester Corporation 

 at Thirlmere, busy brains went to work to devise more 

 rational means of defence. 



The chief natural enemies of the larch sawfly are 

 three in number : (1) birds which feed on caterpillars ; 



(2) a species of ichneumon fly (Mesoleius aulicus), which 

 lays its egg in the pupa or chrysalis of the sawfly ; and 



(3) the short-tailed field-vole {Arvicola agrestis), which 

 hunts out the cocoons from the soft soil under the 

 trees and devours them.^ This little animal, however, 

 is so destructive to field crops and plantations, and so 

 prone to develop from time to time into those visita- 

 tions known as 'vole-plagues,' that it would be the 

 reverse of wisdom to encourage its presence. Non tali 

 auxilio ! wherefore Dr. Gordon Hewitt and Dr. Hickson 

 of Manchester University concentrated their attention 

 on the winged enemies of the fly. 



First, as to the parasitic ichneumon fly. Nothing 

 could be done to propagate it by artificial means. It 

 came in the track of the sawfly, and, finding its game 

 plentiful, multiplied in proportion. The two natural- 

 ists aforesaid established beyond all doubt its efficacy 

 as a check on the pest. Having collected a large 

 number of cocoons, they registered in spring those 

 from which ichneumons and sawfly emerged respec- 

 tively. The result of their investigation was remarkable. 



' There is no doubt that the common shrew (Sorex vulgarii) ia of 

 uee also in this matter, for it is one of our few native Insectivores. 



