18 BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 



second or third year of phylloxeration. Such a vine as has been 

 portrayed generally dies in the fifth or sixth year from the initial 

 attack. 



As has been pointed out above, the decline of a vine is influenced 

 by many conditions, and the hypothetical case given shows the mini- 

 mum longevity of an established susceptible vine after phylloxeration. 

 Under favorable conditions infested vines live much longer, and in 

 extreme cases their length of life seems hardly affected by the con- 

 tinued presence of the insect on their roots, a slight decrease in the 

 size of the crop being the only evidence of injury. 



HOW THE PRESENCE OF PHYLLOXERA IS INDICATED. 



The existence of the phylloxera in a vineyard is indicated by the 

 well-known areas or " oil spots," so termed because o± their man- 

 ner of spreading. A " spot " appears first in the form of one or 

 two vines showing a slight shortening of the canes and a premature 

 seasonal yellowing of the leaves, although the latter symptom may 

 be caused by the red spider (Tetranychus Mmaculatus Harvey), or 

 by alkali in the soil. The year following this indication the vines 

 originally infested exhibit a more noticeably stunted appearance, 

 while other vines surrounding them show slight shortening of canes 

 and premature discoloration of foliage. After this the " spot " in- 

 creases in size, in course of time the vines in its center die, and 

 finally the vineyard may become totally destroyed. The writers have 

 never observed the " spots " to increase as rapidly in California as . 

 they are reported to have done in the vineyards of France after the 

 time the insect first reached that country, when 2,500,000 acres were 

 destroyed in 25 years, and vineyards frequently have been observed 

 in California which had phylloxera " spots " of more than 20 years' 

 standing to have vines still living. 



The " oil spot " generally is circular in shape, but sometimes it 

 assumes other forms. At times it is oval or narrowly elongate, the 

 latter form occurring on hillside vineyards through which water 

 rills run in the spring. In such cases spread of the " spot " is often 

 rapid in a downward direction, indicating that running water is 

 an extra factor in the spread of infestation. The writers have 

 demonstrated by experiment (see " Diffusion of phylloxera," p. 100) 

 that the phylloxerse can be carried in water from one vine to another, 

 and when the rains of March and April occur there are plenty of 

 active phylloxerse on the roots. In other cases the spread of a " spot " 

 follows the direction of the prevailing winds and it appears that this 

 spread is caused by wind agency in the transportation of wandering 

 larvae in summer and autumn. In vineyards where vines are planted 

 rectangularly (i. e., 8 by 12 feet), instead of square, the infestation 



