338 The Large Poplar Longhom 



Economic Importance of S. carcharias. 



In the areas examined by myself, only vigorously growing healthy, 

 trees between the ages of five and twenty years have been chosen for 

 attack by S. carcharias; the species is therefore of considerable economic 

 importance in our forestry. This importance lies not so much in the fact 

 that the insect is attacking a species of poplar, namely, Populus tremula 

 Linn., which is chiefly an ornamental one, but in the fact that in the 

 absence of this host, or on a sudden increase of its numbers, as in the case 

 of many other injurious insects, other valuable timber-producing poplar 

 species would be endangered. 



The destructive work of S. carcharias, both in the adult and larval 

 stages, is partly of a physiological and partly technical nature. The 

 adults, through their habit of eating out portions of the centre of the 

 leaves reduce the leaf -surf ace of the tree, and in cases where the midribs 

 of the leaves have been cut, the food supply is interrupted (see Fig. 21). 

 Then there is a second kind of damage done by the adult, namely, the 

 cutting of the egg-incisions on the basal portions of the stems (see 

 Fig. 22). This is the more serious kind of adult damage, for, as a result 

 of this gnawing of the egg- bites, the outer bark or bast layers and cam- 

 bium may be cut. The insertion of eggs through these bites adds further 

 to the injury of these layers. Later these incisions are the origin of the 

 large deep fissures on the surface of infested stems. Further, the female 

 beetles have the habit of cutting a large number of nicks on the basal 

 portions of stems, of a similar appearance to egg-bites, but no eggs are 

 inserted into them. Such incisions afford suitable openings or wounds 

 for the entrance of spores of parasitic fungi. In any case should the 

 cambium layer be destroyed in the cutting of these incisions, and this 

 is very frequently the case in stems of small diameter, the injury gives 

 rise to defects in the growth of the stem. 



By far the greatest damage, however, is done by the larva. First 

 of all, the larvae on hatching tunnel along the surface of the sapwood 

 in a horizontal direction and as a result the inner bast layers, cambium 

 and outer sapwood may be badly injured. Where there are several 

 larvae at work, the stem can be completely girdled by the union of these 

 horizontal tunnels and the flow of sap interrupted. On young stems, say, 

 about five to seven years of age, the presence of only a few larval tunnels 

 on a stem is sufficient to prevent the flow of sap. In several cases that 

 came under my notice, the union of the horizontal portions of only two 

 larval galleries completely ringed the stems. 



