Junes, i9i6 Woolly Pear Aphis 353 



With trees under 4 years of age, conditions of injury are different. 

 Heavy infestation of a tree of weak vigor or resistance may result in the 

 death of the tree. Badly stunted growth and the early falling of foliage 

 are characteristic of aphis injury on young trees. Injury and death 

 are due to heavy summer and autumn infestations on the fibrous root- 

 lets and to the inability of the tree to replace the destroyed roots quickly 

 enough to afford plant food for the vegetative portion. Frequently the 

 trees are saved and relief comes from the production in the fall months 

 of a high percentage of migrants which leave behind them for the winter 

 only a small infestation of wingless individuals; and since the aphides 

 increase but slowly in spring, the tree is enabled to send forth new root- 

 lets without danger of having them rapidly destroyed. Sometimes 

 young trees in no wise stunted have been observed to cast their leaves 

 prematurely, and upon examination have been found to be heavily 

 infested with the aphis. It would appear from the absence of stunted 

 growth that these trees did not have, or were not adversely influenced by, 

 an infestation until their summer growth was about completed, and 

 that the simultaneous destruction of feeding rootlets cut off the flow of 

 sap suddenly. The fact that trees were stunted was an indication that 

 the injurious effects of feeding by the aphides were felt earlier in the 

 season. 



In addition to trees noticeably stunted and others prematurely defo- 

 liated are found still others which show no external evidence of infesta- 

 tion and yet upon examination prove to be heavily infested. This 

 phenomenon is frequently noticeable among young trees or in nursery 

 rows, and hints at a power of resistance. 



In orchards and districts where conditions favor large productions of 

 winged forms, or migrants, spring and early summer infestations are small, 

 denoting that few insects passed the winter on the roots. After the month 

 of June, however, such infestations multiply rapidly and become very large 

 by September, the month in which the fall migrants are produced in 

 greatest abundance. After September there remain small wingless colo- 

 nies which increase but little until the summer following. The winged 

 forms are produced in abundance on heavy dry clay soils which crack in 

 summer and autumn. Irrigated orchards produce them in smaller num- 

 bers than those that receive no moisture from ]\Iay to October. On loam, 

 silt, and light-clay soils the winged forms are much less abundantly pro- 

 duced. On such soils the infestation remains largely or wholly wingless 

 the year around, and the conditions are generally unfavorable to such 

 heavy infestations as occur on the heavy clays. The aphides appear to 

 lack freedom of movement, and frequently their colonies are unable to 

 increase perceptibly through summer. Occasionally the wingless infesta- 

 tions are severe the year round; where this is so, in the early part of the 

 year there is caused a considerable stunting of growth and more or less 



