DAMAGE CAUSED BY ANIMALS. 139 



quently, the generation is frequently only a simple annual one, or 

 else three generations require two years for their development, in 

 which case the second generation hibernates as larvse. 



It must, however, be specially mentioned that any special time 

 of swarming is only noticeable in spring, for during summer the 

 beetles only gradually make their appearance in consequence of 

 the ovi-deposition in the first instance being protracted throughout 

 several weeks. 



Trees that are attacked by any considerable number of beetles 

 soon begin to show signs of the stagnation of the sap, and die 

 rapidly in consequence of the interference with the flow of the 

 latter occasioned by the larval galleries; but the beetles always make 

 their exit from the stems before they are quite dead, so that the 

 felling of such does not enable the forester to stay the current of 

 reproduction. 



This bark-beetle is almost always to be found here and there 

 within Spruce woods; but under favourable circumstances, and when 

 numerous suitable breeding-places are offered to it in consequence 

 of extensive damage by storms or snow, it increases at a calami- 

 tous rate, and has several times committed enormous destruction 

 in Spruce forests. It is only exceptionally that it attacks other 

 species of trees. 



As one of the cases of calamitous devastation of forests may be 

 mentioned the enormous damage caused by bark-beetles, and 

 principally B. typographus, in the Spruce forests of the Bohmischer 

 and the Bayerischer Wald, which immediately followed, and was 

 due to, the very extensive injuries occasioned by a great hurricane 

 in 1870 ; it necessitated the felling of many millions of cubic 

 feet of timber (see note to page 137). 



Similar extensive devastations also occurred in the Spruce 

 forests of the Harz Mountains towards the end of last century. 



The speedy removal of felled timber, the barking of stems 

 that must remain lying in the woods for some time, and diligent 

 inspection of all windfall areas to see that they do not develop 

 into breeding-places for insects, are the best preventive and pro- 

 tective measures ; whilst the felling and barking of stems that 

 have been attacked, the laying down of a sufficient number of 

 decoy-stems in suitable places, and the peeling and burning of the 

 bark from these, constitute the most effective annihilative and 

 exterminative remedies against this dangerous enemy. 



