DAMAGE CAUSED BY ANIMALS. 123 



I broken branches as the points for rapping on. Where the soil is 

 I covered with a growth of heather, whortleberry, or similar weeds, 

 i it is advisable to spread out sheets below the crowns operated on, 

 I otherwise valuable time has to be spent, only more or less success- 

 I fully, in searching after the fallen caterpillars. 



Ditches for intercepting the migration of caterpillars are of 



I special utility when the attacks of the insects are comparatively 



I severe, but are concentrated within a small area. The ditches 



I! may either be for the express purpose of isolating the infested 



I areas by preventing the caterpillars from wandering across into 



i the neighbouring portions of the crop (when, of course, they can 



I only be serviceable so long as the leaf canopy overhead is siinul- 



I taneously interrupted), or else they may be dug throughout the 



I infested area with the express view of catching the caterpillars 



I .when moving about from place to place. These find very consider- 



I able difficulty in ascending the smooth steep sides of the ditches, 



and in creeping about in search of an easy means of exit, they get 



caught in holes made for this purpose along the bottom of the 



ditch, when they can be killed by throwing earth over them. The 



depth to which the ditches must be dug depends on the size of 



the caterpillars ; they are about two feet deep when operations are 



being conducted against the large Pine moth (Gastropacha pini). 



I This method is especially recommendable when the caterpillars 



are naturally prone to migrate, and when small areas are totally 



| ! denuded of their foliage. 

 Eings or girdles of tar were formerly considered to be means only 

 applicable to areas of limited extent ; but they now find application 

 i on the most extensive scale, particularly in combating the attacks 

 1 of dangerous moths like the Pine and the Spruce moths, since it 

 has been found possible to prepare a special kind of viscous tar 

 or patent glue, which retains its stickiness, and other qualities 

 obnoxious to the insect, sometimes for months, without becoming 

 hard through exposure to the atmosphere. By means of these 

 tar-girdles, the caterpillars of the Pine moth are prevented from 

 ascending the stems in spring after their period of hibernation on 

 the ground, and the ascent of young caterpillars of the Spruce 

 moth is barred, when they come out from eggs deposited below the 

 ring. It also hinders the re-ascent of all caterpillars, whether 

 they have been shaken down from the crowns, or have spun them- 

 selves down to the ground on cobweb-like threads, as in the case 



