170 BIOLOGICAL PROBLEMS 



Mr. Wadsworth, in 1914, no less than 26.9 per cent 

 of the maggots were found to be thus parasitized. 

 But the Aleochara is not the only parasite that 

 destroys the root maggot, as it has been found that 

 to a relatively small extent the pupae are parasitized 

 by two species of Ichneumons, and rather more fre- 

 quently by a species of Cynipid fly. 1 Our knowledge 

 of these parasites may not at present lead to any 

 practical results, as the disk method of protection of 

 the plants seems to be effective in most parts of the 

 country; but where the disk method fails, as it is said 

 to do in some districts, owing to climatic or other con- 

 ditions, the introduction of Aleochara might prove to 

 be an important means of checking the spread of the 

 pest. 



As an example of another method of attack upon 

 a destructive insect, which, I venture to think, has 

 proved to be efficacious, I may refer to the last out- 

 break of the larch saw-fly attack. The first notice that 

 was received of the renewed activity of this destructive 

 insect was in the summer of 1906. The larch trees 

 in the plantations of the Lake District were losing 

 their young shoots, the leaves were turning brown, 

 and the year's growth of wood was evidently being 

 checked. 



As the outbreak was particularly severe on the hill 

 slopes leading down to Thirlmere from which lake 

 Manchester derives its water-supply Dr. Gordon 

 Hewitt was entrusted with an investigation of the 

 insect, and asked to suggest some remedial measures. 

 It was found that an investigation of the natural 

 history of the insect and its natural enemies would 

 take some years to complete, and therefore it was 

 suggested that in the first instance the Corporation 



1 J. F. Wadsworth: Ann. Applied Biology, II, 1915, and /. Econotn. 

 Biology, X, 1915. 



