THE HOMOPTERA 



205 



tion to produce vigorous growth will aid in this. The worst injuries are usually 

 where corn is planted to follow corn and therefore where this pest is already 

 present in the field from the preceding year. Any method which will destroy 

 the nests of the ants which care for the lice will also be helpful, and deep plowing 

 and harrowing both in late fall and early spring has 

 proved of value for this purpose. 



Some plant lice attack evergreens and pro- 

 duce rather soft, fleshy galls, generally at the 

 bases of the outer shoots. These appear during 

 the spring months and are of full size by mid- 

 summer. They then dry and crack open, showing 

 little cavities occupied by the plant lice which now 

 leave the galls for other parts, either of the same or 

 some other kind of tree, according to the species 

 concerned. The gall formation interferes with the 

 growth of the tree by preventing wholly or in 

 part, the circulation of the sap in the shoot at the 

 base of which the gall is located, and this results, 



by the death or checking of the growth, in trees which look thin rather 

 than dense, and in some cases they may become worthless as lawn 

 ornaments. In the East the spruce is often seriously injured in this way. 



Many kinds of plant lice often become seriously abundant for periods 

 of 2 or 3 years, then disappear for a time. The Potato Plant louse, the 

 Pea louse, the Beet-root louse, Cherry plant lice and others have all been 

 destructive for a year or two at a time within the last decade, and similar 

 outbreaks of these or others may be expected any year. Wherever it is 

 possible, spraying thoroughly upon the first appearance of the lice, with 



FIG. 198. Oviparous 

 female of the Corn Root 

 Aphis, greatly enlarged. 

 (From U. S. D. A. Bur. 

 Ent. Bull. 85, Part VI.) 



FIG. 199. Aphid parasite (Lysiphlebus testaceipes Cress.) ovipositing in the body of a 

 Spring Grain Aphis. Greatly enlarged. (From U. S. D. A. Bur. Ent. Bull. 110.) 



nicotine sulfate, kerosene emulsion or fish-oil soap should be resorted to 

 as measures of relief. If for any reason this cannot be done and no special 

 method of control seems available, dependence must be placed upon 

 climatic influences and insect enemies to check these pests, and this will 

 occur within 2 or 3 years in nearly every case. 



Among their many enemies is one group of tiny insects which makes a 

 specialty of attacking plant lice. An insect of this group will select a 



