26 On an Insect injurious to Fruit-trees. 



I at first supposed that these insects, like many others, ' 

 deposited their eggs only upon parts of the trees previously 

 diseased. Even on this supposition, their injurious effects 

 would be very considerable, as it was clear that they every 

 year greatly enlarged the extent of the canker, not merely 

 by devouring the neighbouring alburnum, but by forming 

 numerous cells in it, which when quitted by the chrysales, 

 are filled with water by every shower, and thus become the 

 source of more speedy and extensive decay. Many of the 

 cankers in the tree above alluded to, have eaten half way 

 through the small trunk and branches, which if not sheltered 

 by a wall, must have been long ago broken off by the wind.* 

 Narrower examination, however, has shewn me, that their 

 attacks are by no means confined to the diseased parts of 

 fruit-trees ; nor directed, as I at first conjectured, against the 

 Apple-tree only. Being more anxious to ascertain the 

 economy of an injurious insect, than desirous of preserving 

 the tree which they chiefly attacked, I took no steps for 

 extirpating them ; and they have, in consequence, seemed to 

 increase every year since I first observed them, an{l last year 

 carried on their operations so extensively, as to threaten 

 more serious injury, in return for my forbearance, than I had 



* This tree is a remarkable example of the effect of partial decortication, as 

 recommended by Dr. Darwin, (Phytologia, page 378), in inducing the pro- 

 duction of flower instead of leaf-buds. Not only the bark, but half the trunk, 

 as above observed, is eaten through in many places ; yet though a new twig is 

 scarcely ever put forth, it never fails to be laden with blossom and fruit Here 

 I may observe that a similar result, as to the increased produce of fruit, and the 

 paler green of the leaves, with that above referred to by Dr. Darwin, I have 

 myself seen on a branch of a Pear-tree, from which nearly a complete cylinder 

 of bark had been gnawed by cattle. It was filled with fruit, while not a Pear 

 was to be seen on the rest of the tree. 



