THEIR RELATION TO PLANTS 



61 



If this trail is along or with the grain, not much harm is 

 done; but in many cases it tends to a girdling or to a 

 cutting across the grain that involves a large part of the 

 circumference. And when there are several such borers 

 at work in one tree, fatal results follow. I have seen 

 entire orchards of pear trees and fields of blackberry 

 killed by borers of this character. 



Sometimes, the larva, instead of making a long 

 narrow gallery, eats out an irregular chamber beneath 



FIG. 21. Flat-head apple borer: a, 

 larva; b, pupa; d, adult. 



FIG. 22. An irregular gallery made 

 by a flat-head borer. 



the bark, and that sort of injury is not nearly so serious, 

 since it does not so much interrupt the circulation of 

 sap. So there is quite a little difference of habit in the 

 selection of the tree to be attacked. Some species 

 never enter living trees and these are, perhaps, in the 

 majority; but some never attack other than healthy 

 tissue. As a rule, if trees are found infested by flat- 

 headed borers, it can be assumed that they were al- 

 ready in rather poor condition before the entry; this 

 new attack marking the first step in nature's attempt 

 to get rid of an organism no longer aggressively healthy. 



