3i 6 Studies in Forestry [CHAP. xiv. 



of newly-felled trees for its breeding-place. Moths also, when 

 laying their eggs, frequently give the preference to crops of 

 backward growth on inferior qualities of soil, which are thus 

 all the more liable to succumb entirely to the ravages of 

 the caterpillars during the following spring. Such favourable 

 breeding- and feeding-places will usually be found to be the 

 centres from which swarms of noxious insects spread to other 

 woodland tracts. Hence the necessity for careful tending even 

 in crops where such outlay does not seem likely to prove 

 directly remunerative. Slovenly, negligent treatment of any 

 one portion of an extensive wooded estate endangers the well- 

 being of all the growing-stock of timber throughout the whole 

 area under forest within a wide circuit. 



All circumstances, therefore, tending to the formation of such 

 breeding-centres are only too favourable to increase in the 

 number of injurious insects ; whilst hot dry summers and long- 

 continued drought, by reducing the woodland growth to a 

 somewhat weakly condition, decidedly add to the prolificness 

 of moths and saw-flies. 



Amongst the influences inimical to the numerical increase 

 of noxious insects may first of all be noted damp, rainy, cold, 

 raw, ungenial weather, while caterpillars are changing their 

 skins, and when beetles and moths are swarming and entering 

 in copula^ although the influence exerted by cold in winter is 

 comparatively slight even in the case of species which hibernate 

 in the larval or the chrysalid stage. But as, before any species 

 can make its appearance in unusual and excessive numbers, 

 the balance of nature must of necessity have been in some 

 way or another disturbed (e. g. by bad breeding-years amongst 

 insectivorous birds and insects which may happen to be 

 its particular enemies) a natural readjustment would always 

 of itself take place, although this might not happen until 

 irreparable damage had been done to the woodland crops. 

 Thus, experience in Germany has shown (i) that bad devasta- 

 tions by moth-caterpillars as a rule last for three years, to which 



