1388 



ARBORETUM AND FRUTICETUM. 



'ART III 



course at about right angles from the 1236 



primary channel, on each side of it. jffffi' 



( See fig. 1236.) The true food of the 



insect is the inner bark; and the 



erosion of the soft wood is so slight, 



as to be, perhaps, nearly accidental. 



The course of each individual larva, 



on each side of the primary channel, is 



about parallel to that of the larva next 



to it ; and each forms a channel by its 



feeding that is enlarged as the larva 



increases in size. When each larva has 



finished its course of feeding it stops in 



its progress, turns to a pupa, and then to 



a beetle; after which it gnaws a straight 



hole through the bark, and comes out. 



The beetles begin to come out in 



about the latter end of May of the year 



following that in which the eggs were 



deposited. The sexes afterwards pair, 



and the females, bearing eggs, bore 



through the bark, as before detailed; and so on from generation to generation, 



and year to year. 



The result of the erosions of the female parent, and of the larva, in the inner 

 bark and soft wood, is that of cutting off the vital connexion between these 

 two parts ; and, when the erosions effected in a tree have become numerous, of 

 occasioning its death, by preventing the ascent and descent of the sap. It 

 has been said that the scolytus never attacks a tree in a perfectly healthy 

 state; and, also, that trees suffering under carcinoma (see p. 1385.) are par- 

 ticularly liable to it. In the year 1825, an avenue of elm trees in Camberwell 

 Grove were attacked by this disease, which was supposed to be brought on 

 by the gas which escaped from the pipes laid down along the road being 

 absorbed by the roots ; and which gave rise to a snit in Chancery between the 

 inhabitants and the proprietors of the gas-works. Various persons, considered 

 as competent judges, were employed to ascertain the cause of the decay of the 

 elms ; and their general conclusion was, that the carcinoma had been brought 

 on by old age, excavations for building in an exceedingly dry soil, and an extraor- 

 dinarily dry summer, and that the gas had had no influence in producing the decay 

 of the trees. The trunks of the trees, when examined in 1826, were found 

 infested with an immense number of larvae feeding on the soft inner bark. An 

 interesting account of the Camber well elms will be found in the Gardener's Ma- 

 gazine, vol. i. p. 378. In relation to the capability of the scolytus to effect injury 

 on elm trees, it is stated that 80,000 have been found in a single tree. It 

 has also been remarked that the scolyti seldom destroy the trees they attack 

 the first year that they commence their ravages ; and that they prefer a tree 

 that they have already begun to devour, to a young and vigorous tree. 

 (See the observations of Mr. Spence in p. 1^89.) It is easy to ascer- 

 tain the infested trees, as the bark will be found perforated by small holes, 

 as if made by shot or a brad-awl, in various parts; and little particles 

 of a substance like fine sawdust will be found on the rough surface of the 

 bark, and at the foot of the tree. The scolyti, as Mr. Denson, sen., has 

 observed, never attack dead trees The Scolytus destructor, as an enemy to 

 elm trees, appears first to have attracted the attention of entomologists in 

 England about the year 1824-, by M'Leay's Report to the Treasury upon 

 the state of the elms in St. James's and Hyde Parks. (See this Report in 

 Edin. Phil. Journ., No. xxxi. art. 12.; and see Tilloch's Phil. Mag., Oct. 1823, 

 art. 51.) In the year 1828, a controversy was carried on in a Cambridge 

 newspaper, between Mr. John Denson, sen., the author of A Peasant's Voice 

 to Landowners, &c., and Mr. J. Deck of Cambridge, respecting the cause of 



