168 PROTECTION OF WOODLANDS. 



The collection of ova and pupae is also at best only a very 

 disappointing and ineffective sort of measure, as also the killing of 

 the moths during bad weather whilst they are seated at rest low 

 down on the sheltered side of the stem during the time of their , 

 swarming. 



Trenches, either for isolating a crop infested, or for intercepting 

 the migration of the caterpillars from place to place within the 

 area attacked, can only be of use when the latter are either con- 

 fined within small areas, or occur in such numbers as to denude 

 the trees of foliage, and are thus forced to wander about, or 

 migrate to neighbouring areas in search of food. Where young 

 timber crops especially are anything like contiguous to older 

 crops likely to be totally defoliated, they certainly require to be 

 protected by trenches against the migratory caterpillars. For the 

 large caterpillar of this species of moth the trenches require to be 

 depth up to about two feet, and with walls as clean cut and 

 perpendicular as possible. Along the sole of the trenches holes 

 are again dug as traps for the caterpillars ; those caught in them 

 should be crushed or killed by having earth heaped on them. 



The mere removal of moss, or of the layer of dead foliage with 

 which the ground is covered, is of almost no avail so far as any 

 probable removal of the caterpillars along with it is concerned. . 



The numbers of the enemies of this insect which destroy the 

 caterpillars or chrysalides is very small among birds, on account of 

 the hairy growth of the former, and the large protective cocoons 

 in the case of the latter : the cuckoo is the most useful of all the 

 birds in devouring the caterpillars. Mammals, too, even including 

 swine, disdain on this account the caterpillars hibernating on the 

 ground, though, on the other hand, a considerable number of the 

 ova are annihilated by tomtits and other small birds ; but then, 

 unfortunately, the period of life spent in the ovum only lasts for 

 a few weeks. 



It is due in a far higher degree to the so-called useful forest 

 insects, especially ichneumon-flies (Ichneumonidx), that a Li re- 

 number of caterpillars, and even of eggs, is naturally destroyed ; for, 

 particularly when attacks have lasted for any length of time on 

 any very extensive scale, a disproportionately larger number of 

 caterpillars than usual become infested with ichneumon-larvae, 1 



1 The dead caterpillars, thickly covered with the small white cocoons ofMicrogaster 

 globatua, are often visible on the trees from some little distance. 



