352 Journal of Agricultural Research voi. vi, no. io 



appears to attack the roots of all types of pears, and it is especially 

 injurious to the French wild stock so largely used in California as a 

 stock for the Bartlett. Quince roots are fed upon, but much less freely, 

 and the quince may be credited with a considerable degree of immunity. 

 The Kieffer stock is attacked, but it is possible that Japanese stock may 

 show immunity to a satisfactory degree. Observations to date indicate 

 that both these stocks are more resistant than that from France. It 

 should be said that the individual plants of the wild stock from France 

 vary greatly, and there appears to be among the plants some variation in 

 intrinsic vigor or in power to resist the woolly aphis. However, the 

 majority of the imported seedlings show no satisfactory evidence of a 

 power of resistance, and a different stock is very desirable. 



The insect works especially upon the smaller fibrous rootlets and may 

 be encountered on any such rootlets within the topmost 3 feet of soil and 

 perhaps deeper. Infestations are usually heavier on the rootlets near 

 the trunk, but frequently the aphides are as abundant 10 or 12 feet from 

 the stem. In a badly infested orchard the soil on being overturned 

 may in places be found to be white with the wool and skins of the insects. 

 The aphides attack less frequently larger roots up to X inch in diameter 

 and sometimes settle on still larger roots or on the main stem where 

 abrasions have set up a callus growth. They often colonize the under- 

 ground portions of sucker growth, feeding on the succulent stalks. After 

 the insects have forsaken a rootlet, fungi sometimes appear and com- 

 plete its destruction. 



This method of feeding upon the fibrous rootlets is somewhat analo- 

 gous to the habits of the grape phylloxera {Phylloxera vitijoliac Fitch) 

 on the resistant types of grapevines in that chiefly the smaller rootlets 

 are attacked. It is directly opposed to the habits of the woolly apple 

 aphis and of the grape phylloxera on nonresistant types of vines, for 

 both these insects feed upon the larger roots and cause the formation of 

 tuberlike lesions. The woolly pear aphis rarely forms any perceptible 

 lesions, but it destroys great numbers of young rootlets, especially in 

 late summer and autumn. In old trees this sometimes results in a 

 dwarfing of growth and in a generally unthrifty appearance and condi- 

 tion. The majority of old infested trees do not show evident injury 

 ascribable to the aphis, although it is presumable that they are suf- 

 fering to some extent. They remain thrifty on account of their intrinsic 

 vigor. In many instances where old trees were showing injury, extra 

 cultivation of the soil and better irrigation practice resulted in the 

 establishment of thrifty conditions, even though this method did not 

 appear to reduce the numbers of the aphis. The effect on the crop is 

 hard to estimate and can not be satisfactorily specified, but in general 

 it is such as may result from the diversion of the flow of sap in the tree. 



