APPLE INSECTS 155 



velop on the roots, finally resulting in their decay. Usually 

 most of the injury resulting from the work of this pest is due 

 to the greater numbers of the aphids infesting the roots, the 

 more conspicuous but less numerous colonies above ground, 

 rarely doing much damage, especially on larger trees. In some 

 regions, and especially in Europe, the trees are often seriously 

 injured by a majority of the aphids working above ground. In 

 cases of severe infestation the woolly aphids swarm over the 

 whole tree above and below ground, even attacking the foliage 

 and fruit. The foliage on badly infested trees often presents 

 a yellowish, sickly appearance, and the trees are easily uprooted, 

 as many of the roots have decayed from the work of the pest. 

 Apple trees of all ages and varieties are liable to attack, but 

 usually young trees, especially nursery stock, suffer most. 

 Some varieties, like the Northern Spy, are often more or less 

 exempt from attack. Pear, quince and the mountain ash 

 are also recorded as host-plants ; it also passes a part of its life 

 history on the elm, causing a characteristic curling of the 

 leaves (Fig. 166). The insect may injuriously infest the roots 

 of trees growing in various kinds of soils, and it works destruc- 

 tively over a wide range of latitude. Many thousands of 

 nursery trees are annually either killed by the insect or ren- 

 dered unsalable and destroyed in America. Infested nursery 

 stock is largely responsible for its wide distribution. In most 

 localities large, thrifty orchard trees are not seriously injured 

 by this aphis, but sometimes under favorable conditions it 

 breeds so rapidly that it ranks among the most destructive of 

 the insect enemies of the apple. 



The interesting and rather complicated life history of this 

 woolly aphis is little understood by orchardists, and a few details 

 are still lacking to make it complete. During the summer only 

 the little wingless, agamic female aphids occur on the apple 

 trees. A dozen generations of these may be developed during 

 the summer, each mother aphid bringing forth living young, 



