10 



SOME INSECTS INJURIOUS TO FORESTS. 



but of the several hundred examined there was only one larva in a 

 place, which would indicate that the eggs are not deposited in clusters, 

 but that the} T are scattered about in the crevices, so that each larva 

 occupies a separate hibernating cell. The slight wound thus produced 

 in the outer layer of the living bark results in a small dead area sur- 

 rounding the cell. This dead and brown condition was found, on the 

 date mentioned, to have penetrated the thick inner bark to the wood. 



This condition evidently facilitates the 

 operation of the young larva in boring 

 through the inner bark to the wood, 

 which a healthy condition of the imme- 

 diately surrounding bark might prevent. 

 It is not improbable that this small area 

 of dead bark may be caused by a plant 

 disease, which finds its way to the living 

 plant tissue through the slight wound 

 made by the larvae and which, if this be 

 so, may contribute greatly to the death 

 of badly infested trees. 



The young larvae were found in nearly 

 every case in the part of the bark which 

 had not been injured previously, thus 

 indicating that the female deposits her 

 eggs where the bark is perfectly healthy 

 and not in or around the old scars. In- 

 deed, the habit of the larvae appears to 

 render this quite necessary for their more 

 or less isolated work. It was particularly 

 noted that the remaining unaffected bark 

 of the trees which had suffered most from 

 previous generations of the insect was 

 thickly infested with hibernating larvae, 

 while that of near-by large trees which had 

 escaped previous injury contained very 

 few, thus indicating that from some cause 

 there are individual trees which are more 

 or less immune. This fact, which has been 

 so often observed, suggests the importance of experiments in the prop- 

 agation of immune stock by means of seed or root cuttings from 

 immune trees growing among badly infested ones. 



The hibernating habits of the larvae also suggest a simple method 

 of destroying them, namety, the cutting and barking of the trees 

 during the period between the first of November and the first of May. 

 The simple removal of the bark, without burning, is sufficient to kill 

 the larvae. 



OOT£/f 

 B/IAK 



Fig. 5.— The locust borer (C'yllene 

 robiniae): Hibernation or larval 

 cells in outer portion of living 

 inner bark. About natural size 

 (original). 



