178 PROTECTION AGAINST INSECTS. 



pound of soft soap has been dissolved. It should be con- 

 stantly churned, for ten or more minutes, and is diluted for 

 use by gradually stirring in 11 or more gallons of water. It 

 is especially suitable for suctorial insects, whereas arsenical 

 preparations chiefly serve for biting insects. 



(k) Concluding Remarks. A fuller account of all these 

 methods will be given further on, under the heading of each 

 species. Nature itself can relieve the forest best from insect 

 attacks, for ichneumon-wasps, fungoid diseases, and damp, 

 cold weather kill off myriads of insects and eventually put an 

 end to any abnormal swarms of a destructive species which 

 may occur. 



As a rule, such a swarm lasts three years, but there may be 

 a partial swarm one year before and after this period. 



In Prussia and Saxony, very large sums of money have been 

 spent on the destruction of forest insects ; in Prussia according 

 to the following table : 



1870-73 . ... . . 41,740 



1876-80 .... 98,738 



1884-87 . . . . 28,200 



This shows a large diminution since 1880, from which it 

 may be inferred that insects are diminishing in the Prussian 

 forests. 



In Saxony during the years 1876-77, 55,852 were spent in 

 the destruction of pine-weevils and bark-beetles. 



4. Treatment of Injured Woods. 



Woods injured by insects should be felled only when there 

 are signs that they have been fatally injured. Such signs are : 

 drying-up or wilting of buds, shoots or twigs over the greater 

 part of the crowns of the trees ; development of small leaves 

 or needles, the latter frequently in rosettes; exudation of 

 watery turpentine from the bark ; loosening and subsequent 

 separation of the bark ; appearance of brown or bluish spots 

 on the bast or sapwood ; abundance of insects such as species 

 of Cerambyx, Sir ex, and AnoUum, which live only on dead or 

 dying wood. In deciding on the importance of such signs, we 

 must consider the special circumstances of each case, the 



