INSECTS INJURIOUS TO THE APPLE AND PEAR 515 



or limbs. Their presence in these places is always an indication 

 that others are feeding upon the roots, where they cause gall-like 

 swellings, so that the roots soon become a mass of knots and die 

 in a year or two if the injury continues. When badly attacked 

 a tree becomes sickly, the foliage turns yellow, and if not killed 

 outright by the aphids, it falls an easy prey to borers and other 

 pests. Injury seems to be worse on light soils and not so severe 

 on heavy soils. Whether the insect is a native or a European 

 species is a matter of dispute. In Europe it is called the /'Amer- 

 ican blight," and was described from Germany in 1801. It has 

 now become distributed all over the world on nursery stock, 

 which forms the principal means of its dissemination. 



I., 



FIG. 444. The woolly apple-aphis: at left, apterous viviparous female; 10, 

 fall migrant; 11, over-winter young. (After Gillette and Taylor.) 



Life History. On infested trees aphids will be found in all 

 stages of growth on the roots in early spring. On the trunk, 

 under bits of bark or under the dead bodies of those killed the 

 previous fall, will be found numerous small aphids which have 

 hibernated there, though in the North these may be killed out 

 during severe winters. As the buds begin to open, the aphids 

 on the trunk locate on tender new bark and commence to feed, 

 and many migrate from the roots to the top at about the same 

 time They are not usually detected until they have multiplied 

 sufficiently to make small white patches, on the bark or leaves, 

 which look like mold. During the spring and summer all are wing- 

 less females, not over one-tenth inch long, of a reddish-brown 



