INSECTS INJURIOUS TO CONIFERS. 



155 



ment will only equal, or may even be less than, that of a single 

 ordinary season. 



4. Defoliation renders a tree liable to the attacks of other 

 insects, especially of the much-dreaded bark- beetles, which 

 have so often completed the havoc begun in European forests by 

 hordes of caterpillars. I do not intend to discuss here all the 

 various aspects of the perennial and much-vexed question as 

 to the economic importance of these and other wood-feeding 

 insects, and the susceptibility of perfectly healthy trees to their 

 attacks, but reference to it cannot be entirely omitted. 



It may be true that bark-beetles and longicorns never attack a 

 tree in perfect health ; that is the opinion of many entomologists, 

 whose number by no means includes all those whose familiarity 

 with forestry compels them to keep sight of the practical side of 

 the question — men like Eichhoff, the leading authority on these 

 beetles and the head of a large forest-district, Judeich and 

 Nitsche, &c. Yet a disease would not be considered unimportant 

 by the physician because it rarely or never attacked the robust ; 

 if it occurred as the sequela of other complaints, killing patients 

 whose recovery would have been certain, it would demand 

 serious attention. So these beetles, as the health of every tree 

 in a forest cannot be assured, still possess importance even if 

 their attacks are limited to the sickly plant. 



But though no one doubts that they, in common with all 

 insects, prefer an unhealthy plant to a healthy one, they may not 

 always reject the latter. Here in Great Britain there is great 

 difficulty in arriving at a just conclusion, for many injurious 

 species are unknown or are exceedingly rare, and it is danger- 

 ous to argue about the habits of any one kind from analogy 

 with those of another. Though a species breeding in small 

 numbers in a wood with plenty of brood-material may let alone 

 timber which it would attack if it were present from any cause 

 in immense swarms impelled by the necessity of egg-laying and 

 with lack of suitable breeding-places, there is yet evidence that 

 these insects, if dying or injured wood is not at hand, attack 

 sound trees for egg-laying, and if the assault does not succeed, 

 the large number of attempted burrows serve to weaken the tree, 

 which may succumb after several repetitions. "When it is recollected 

 that a square m^tre of bark has been known to contain nearly five 

 thousand larvae of Tomicus typograplms, it will be seen that a 



