HEMIPTERA. 169 



conveys them into these burrows, and there watches and protects 

 them. In a short time after the lice have been transferred to the 

 roots of the corn, they mature a winged generation, by means of 

 which adjacent fields may be stocked. These facts indicate the 

 inadvisability of planting corn on a field that was infested by this 

 insect the previous year. The species of Aphis that has attracted 

 the most attention recently is Aphis mdli. This has occurred in im- 

 mense numbers on the young leaves of apple. Usually the injuries 

 of this species are confined to the terminal shoots of a few branches ; 

 but in seasons favorable to its development, the entire tree becomes 

 infested. It is necessary then to resort to the use of alkaline washes. 

 (See Chapter XIV.) 



Myzus. Two species of Myzus are very common. One of these, 

 the Cherry-tree Aphis, Myzus cerasi, infests the leaves and twigs of 

 cherry. Sometimes it becomes so abundant that it completely covers 

 the infested tree. The other species referred to is the Peach-tree 

 Aphis, Myzus per sicce. This lives on the under side of the leaves, 

 causing them to thicken and curl. Its injuries are usually confined 

 to the terminal portion of a few branches. The easiest way to 

 destroy this pest is to dip the infested branches into a pail of soap 

 solution. There is a serious disease of the peach which causes the 

 leaves to curl in a similar manner, and which is known as the curl- 

 leaf. This disease can be distinguished from the injuries of the 

 Peach-tree Aphis by the absence of plant-lice in the curled leaves. 

 It is caused by a fungus known as Exoascus deformans. 



The Hop Aphis, Phorodon Mmuti.This species often swarms in 

 hop-yards to such an extent that it seriously injures the crop. An 

 interesting and important feature in its life-history is that it passes 

 the winter on plum-trees. As soon as the leaves of plum appear in 

 the spring, the first generation of this Aphid hatches from the winter 

 eggs on plum. The first and second generations are wingless, agamic 

 females, which live upon the plum; the third appears early in June, 

 is winged, and migrates to the hops. From this generation there 

 descend several generations of wingless, agamic females. Of these 

 there may be, according to Riley's observations, upon which this 

 account is based, as many as eight, making the last one the eleventh 

 of the season. Late in August and early in September the offspring 

 of the eleventh generation and of the members of the other genera- 

 tions that are still alive become winged. These winged individuals 

 are agamic females which migrate to plum-trees and there produce 

 wingless, sexual females. Late in September, there is produced on 



