10 



SOME INSECTS INJURIOUS TO FORESTS. 



but of the .several huiulred cxuniinod there wiis only one larva in a 

 place, Avhich would indicate that the eog-s are not deposited in clusters, 

 but that they 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 ])ark 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 ])oring 

 through the iiuicr bark to the wood, 

 wdiich a healthy condition of the imme- 

 diatel}" surrounding- bark might prevent. 

 It is not improbable that this small area 

 of dead bark may ))e caused by a plant 

 disease, which finds its way to the living 

 plant tissue through the slight wound 

 made by the larvje and which, if this be 

 so, may contribute greatly to the death 

 of l)adly infested trees. 



The young larvoB were found in nearl}^ 

 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 larv« appears to 

 render this quite necessar}^ for their more 

 or less isolated work. It was particularly 

 noted that the remaining unatfected liark 

 of the trees which had sufi'ered most from 

 previous generations of the insect was 

 thickly infested with hibernating larva^, 

 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 imnuinc. 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 hjibits of the larv« also suggest a simple method 

 of destroying them, namely, the cutting and barking of th(^ trees 

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

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

 the larvae. 



Fig. ft.— The locust borer (C'yUaie 

 robinur): Hibernation or larval 

 cells in outer portion of living 

 inner bark. About natural size 

 (originals. 



