156 



the following case may be cited : Mr. W. P. Flint informs me that 

 he has observed that shaded, suppressed white oaks in southern Illi- 

 nois are much more heavily infested by the bark-louse Aspidiotus 

 obscunis Comstock, and by the beetle Phymatodes variiis Say than 

 are the vigorous trees. 



Trees may be injured and killed by animals in many ways, as by 

 defoliating them, boring in the twigs, trunk, or roots, and by the de- 

 struction of the bark and sap-wood of the trunk. Of injuries caused 

 bv insects the work of defoliators of hardwoods is one of the most 

 conspicuous kinds. Repeated defoliation of elms by the elm leaf- 

 beetle Galcrucclla liitcola Miill. will, according to Felt ('05, p. 61), 

 so w^eaken a tree that Tremcx coliimha finds suitable food in its dis- 

 eased and dying substance. With Tremcx present its parasite Thalessa 

 also arrives. The maple borer, Plagionotus spcciosus Say, may also 

 weaken a tree and pave the way for Tremex and Thalessa. A study 

 of the after effects of the prominent defoliators of shade and forest 

 trees, such as the fall web-worm (Hyphantria cunea), the white- 

 marked tussock-moth {Hemerocaiupa leucostigma, Plate XXXI, figs. 

 3. 4 and 5), the bag-worm {Thyridopteryx ephemerarfoniiis), the 

 larch saw-fly {Nematiis erichsonii), the gypsy moth {Portlietria 

 dispar), and the brown-tailed moth (Eiiproctis chrysorrha^a) , would 

 doubtless throw much light upon the details of successions caused by 

 insects. I have not been able to learn that this subject has been studied 

 carefully in this country. Such injuries are clearly not limited to 

 hardwoods, for many similar observations have been made in conif- 

 erous forests. Hewitt ('12, p. 20) has listed some of the beetles 

 which follow the defoliation of larches by the larch saw-fly. Hop- 

 kins ('01, pp. 26-27) found that the spruces of New England were 

 being killed by the bark-beetle Dendroctonus piceaperda Hopkins; 

 that following the damage done came other beetles, such as Polyg- 

 raphus ruftpcnnis Kby., which attacks the weakened tops of the 

 trees, following the attack of its predecessors on the trunk or base; 

 and that also, following Dendroctonus, came Tctr opium cinnamop- 

 icruni Kby., which mines in the dead trees. The vellow pines of the 

 West are killed by the bark-beetle Dendroctonus ponderosa. and this 

 is followed by many kinds of insects which live on the decaying bark 

 and wood, as Hopkins ('02, pp. 10-16) has shown. He also states 

 ('oQ. p. 68) that in the Appalachian Mountains Dendroctonus fronta- 

 lis Zimm. killed a large part of the trees in an area "aggregating over 

 75.000 sc|uare miles." Such examples of multiple attack show the 

 complexity of the causes influencing forest life. When the great 

 amount of influence which insects are able to exert and do exert upon 

 forests is considered, the question is raised as to what may be their 



