THE ENEMIES OF ENGLISH WOODLANDS 287 



that their numbers are necessarily greater on a given area. 

 There is a time in the history of every tree, originally 

 healthy, when its susceptibility to parasitic attack arrives 

 at a critical stage, and the balance of power in host or 

 parasite rest with one or the other. When certain trees, or 

 parts of a plantation, are singled out for more than the 

 average share of attention from these pests, there must be a 

 reason for it, although this reason may not be apparent on 

 the surface, and may, to begin with, be more or less of a 

 temporary nature. Trees which are in that condition most 

 favourable to aphis attack are also the trees which would 

 most easily feel the effect of the larch blister, and it is 

 nothing extraordinary to find the one more or less a con- 

 temporary of the other. 



In nurseries, aphis attack may be fought with some 

 prospect of success. Washings or spraying of the various 

 insecticides in general use reduce their number, if they do 

 not altogether destroy them. But when nursery stock is 

 badly infested with this pest, it is an indication that some- 

 thing is wrong somewhere. Dry, hot summers none can 

 remedy, and it is impossible to avoid a certain amount of 

 injury being done in that way, especially on dry soils. But 

 when otherwise healthy larch is affected in an ordinary 

 summer, it indicates that the soil and situation are not quite 

 as they should be, and the best policy is to select a better 

 locality if possible for that particular crop. 



Insects which only attack Unhealthy or Dying Trees. 



This second class of insects is such a numerous one that 

 it would be outside the province of this book to do much 

 more than enumerate them. A few, however, may occasionally 

 do sufficient damage to standing and growing trees to be 

 ranked as " injurious," although the extent of the damage 

 done is rarely large enough to be taken seriously. The 

 majority of the Bostrichidce, or bark beetles, are invariably 

 included under the head of injurious insects, but they seldom 

 deserve the term any more than saprophytic fungi deserve to 

 be termed injurious to growing crops. Insects which attack 



