SYMPTOMS OF DISEASE. 23 



are soon destroyed by insect larvae or by squirrels. 

 Careful observation will frequently detect at least some 

 remains of these stools. On incense cedar, for instance, 

 the soft and juicy fruiting bodies are readily destroyed 

 by the larva? of certain insects, many of which then bore 

 into the dead outer bark of the tree. These are eagerly 

 sought after by woodpeckers, which chop them out of 

 the bark. The space formerly occupied by the fruit- 

 ing body then appears as a cup-shaped depression, 

 which seems to be pierced by innumerable fine shot 

 holes, the burrows of the insect larvae. Insect work on 

 pines is indicated by pitch tubes on the bark, which 

 are characteristic of the attack of Dendroctonus beetles. 

 In such cases the bark should be peeled off here and 

 there to verify the presence of insect work. 



Longitudinal rows of small holes in the wood are 

 caused either by woodpeckers or by the roots of mis- 

 tletoes. The latter kind are particularly conspicuous 

 in the sapwood under the bark. Woodpeckers very 

 frequently bore horizontal rows of holes in the bark, 

 which often heal over, leaving, as an outward sign, 

 peculiar protruding ridges. 



The weakening of a tree by one injurious agent very 

 often opens the road to another, which would not have 

 been able to attack the sound tree. Cankers on the 

 trunks of white fir, caused by Razoumofskya, are very 

 commonly connected with stringy brown rot, caused by 

 E chinodontium tinctorum (p. 50). Certain insects 

 almost invariably follow injury by fire. Insects and 

 parasitic fungi or mistletoe very often work in this way 

 for each other and in collaboration. 



