THE HOMOPTERA 197 



almost any time during the summer, and a wide distribution of the insect 

 is obtained in this way. 



When cold weather approaches in the fall a generation appears, con- 

 sisting of both sexes, and the females of this generation lay fertilized 

 eggs which winter over and hatch the following spring. In some cases 

 this does not happen until the second fall and in a few species at least, 

 sexual individuals have not been discovered and may occur only at long 

 intervals, if at all. 



Many aphids do not feed entirely on one kind of plant but spend a 

 part of the year on one species, and the rest on another. One of the 

 species which is injurious to the apple remains on this tree from fall until 

 May or June when it migrates to grain and spends the summer months 

 there. Another species, living on the elm during the fall, winter and 

 spring, passes to the apple for its summer residence, and a long list of 

 aphids having alternating food plants is now known. 



Plant lice suck the sap from plants and often produce curling or mal- 

 formation and even wilting of the leaves, frequently accompanied by 

 discoloration. Root-attacking forms produce knots and deformities 

 affecting the health of the plant, and young fruit becomes hard at the 

 attacked spots and remains small. The punctures aphids make often 

 enable the spores of fungi and bacteria causing plant diseases to enter the 

 plants, and they may even transfer these from one plant to another. 

 Among the diseases transferred thus are an oat blight, fire blight of the 

 pear and cucurbit wilt. Indirectly by the honey dew in which spores can 

 live for several days, it is probable that the diseases can also be widely 

 distributed through the agency of other insects which visit and feed on 

 honey-dew. In general a year when plant lice are abundant over a 

 large part of the country is certain to result in great injury to plants of all 

 kinds affected by these insects. 



Ants not only gather the honey dew the Aphids produce, but in some 

 cases the relation is closer, particularly with root feeding species. The 

 eggs of the corn-root louse for example, are gathered by ants in the fall 

 and kept in their underground chambers during the winter. In the spring 

 the ants place the insects on the roots of certain weeds but after the corn 

 has begun to grow well, they transfer them to the corn roots where they 

 visit them during the summer to collect honey dew. (See page 203.) 



Plant lice have many enemies which destroy great numbers of them. 

 They are also affected by the weather, cloudy, wet periods being favor- 

 able, though driving rains destroy many. 



In general the best control of plant lice is obtained by the use of 

 nicotine sulfate 40 per cent used at a dilution of from 1 to 800 to 1 to 

 1,000 parts of water. Where this cannot be obtained, kerosene emulsion 

 or fish-oil soap solutions rank next as control. 



