CHAP, xiv.] Insect Enemies 315 



damaged, the injuries usually result in the wilting and death 

 of the seedling, pole, or tree. 



The Numerical Increase of Injurious Insects. 



The reproductive capacity or prolificness of forest insects 

 varies considerably according to the species. But, fortunately, 

 the most prolific varieties are not among the most injurious 

 kinds, as the average rate of reproduction of the latter does not 

 exceed from 100 to 200 ova, according to Ratzeburg. 



Hard winters are not particularly fatal to insects, and on the 

 whole may be said to favour their increase, especially with 

 regard to beetles protected by their tough outer casing of 

 chitin ; for severe cold works greater havoc among the insecti- 

 vorous song-birds than amongst the insects. The most sensi- 

 tive to damp cold weather and raw winds are the naked larvae 

 which have no hairy or protective covering; these are often 

 killed off in great numbers during the processes of changing 

 their skins. Warm, dry, genial weather, ungrubbed stumps 

 remaining in the ground over extensive falls of mature timber, 

 sickly crops of seedlings, saplings, poles, or trees, and sup- 

 pressed or unhealthy individuals with a weakly flow of sap 

 such as may be found in woods that have suffered from wind- 

 fall, snow, or ice-accumulations also want of clearing, thinning, 

 and proper tending of the woodland crops, are all exceedingly 

 apt to encourage the speedy numerical increase of noxious 

 insects, which often multiply with exceeding rapidity in such 

 breeding-centres. Bark-beetles (jBostrychini) and cambial- 

 beetles (Hylesinini] select for ovi-deposition stems that have 

 been thrown or broken by wind, or newly-felled trees, or those 

 already brought into a more or less unhealthy condition by 

 reason of attacks of moth-caterpillars on the foliage ; whilst 

 the large Pine weevil (Hylobius abietis) chooses for its feeding- 

 ground, when available, crowded crops raised from seed or 

 sickly plantations, and at the same time selects the stumps 



