CHAP, xiv.] Insect Enemies 313 



to older crops only when they have greatly increased in number ; 

 whilst on the other hand the Spruce moth (Liparis monacha) 

 and the Pine moth ( Gastropacha pint] attack the older woods 

 first, and then on rapidly increasing proceed to devastate the 

 younger crops also. 



And though the rough classification, slightly injurious, 

 noticeably injurious, and very injurious, is extremely indefinite 

 and elastic (for Liparis monacha, one of the most destructive 

 of moths, is often for decades a comparatively rare insect), yet 

 sylvicultural experience abroad has shown that the Spruce 

 bark-beetle ( Tomicus typographies^, the large brown Pine weevil 

 (Hylobius abietis\ the cockchafer (Melolontha vulgaris], the 

 Spruce moth (Liparis monacha), and the large Pine moth 

 {Gastropacha pirn) must certainly be classified as very injurious 

 species ; whilst it would not be at all incorrect to include in such 

 a category also the Pine Beauty (Trachea piniperda\ the Bor- 

 dered White moth (Fidonia piniaria\ the Pine Sawfly (Lophyrus 

 pint), the small brown, white-spotted weevil (Pissodes notatus\ 

 and others which, under ordinary circumstances, are usually 

 regarded as merely noticeably injurious. The debateable 

 ground between this latter class and those that would have to 

 be termed slightly injurious is also very vague and indefinite. 

 It depends entirely on circumstances ; and whenever the only 

 safe motto relative to injurious insects Principiis obsta 

 is neglected, any comparatively rare or innocuous species may 

 ultimately develop into a downright scourge, especially where 

 large pure forests of Pine or Spruce are their feeding-grounds. 

 The monophagous species then become polyphagous, and 

 with still greater increase pantophagous, devouring everything 

 within their reach. 



The places in which the greatest number and variety of insects 

 will usually be found are the warm plains and uplands, espe- 

 cially those with sandy soil. Southern exposures, frost-holes, 

 and crops on inferior soil or of poor growth from any cause, are 

 usually the places where the danger of a rapid increase in the 



