THE GRAPE PHYLLOXERA IN CALIFORNIA. 19 



very frequently spreads along the shorter 8-foot rows, indicating 

 that the insects traverse more easily the shorter than the longer dis- 

 tances. Aerial and subterranean migrations of wandering larvae 

 play an important part in the enlargement of phylloxera " spots." 

 Only an infinitesimal percentage of the thousands of wandering 

 larvae succeed in reaching their goal, but, as they are parthenogenetic 

 radicicoles, a single larva can cause a new infestation or start a 

 new " spot " at quite a distance from the original one, either in the 

 same or in another vineyard. 



The estimation of root injury from external appearance usually 

 can be made with considerable accuracy, and the degree of infesta- 

 tion of a vineyard computed by the number of " spots," their size, 

 and the stunted condition of the vines composing them. 



The diagrams (figs. 2 and 3) indicate a phylloxera " spot" charted, 

 respectively, in the years 1914 and 1915. This " spot " occurred on a 

 heavy black clay soil on a hillside of moderate slope. It appeared 

 that the " spot " started about the year 1907 when the vines were 3 

 years old, and that the first vines died about 1911. Surveys of the 

 " spot " were made October 13, 1914, and November 5, 1915, and the 

 vines were designated in the following manner : Ten was given to 

 vines which showed no external evidences of plrylloxeration ; 9 to 

 those which showed very slight evidence, such as premature yellow- 

 ing of foliage and slight shortening of canes; 8 to those showing 

 more advanced symptoms of plrylloxeration, and so on down to 1, 

 which was given to vines which showed only the most feeble vegeta- 

 tive growth. In order to portray the " spot " more vividly, symbols 

 have been utilized as follows : Healthy vines, H ; vines designated 9 

 and 8, S\ vines designated 7 and 6, /; vines designated 5 and 4, U ; 

 vines designated 3, 2, and 1, D ; vines killed by phylloxera, solid dot. 

 In this vineyard every fourth vine had been replaced by a walnut tree, 

 and these places where vines have been pulled out and not replaced 

 are left blank in the diagrams. 



In the diagrams not all the " spot " is shown, for it has extensions, 

 the principal one being on the north side across a 24-foot avenue and 

 continuing down a swale for some 60 feet. Enough of the " spot " 

 is shown to indicate its general form. Between 1914 and 1915 there 

 occurred an unusually wet winter and the " spot " grew considerably 

 in the 12 months between the surveys. Although the number of dead 

 vines increased only from 43 to 49, and among the badly stunted 

 types not much increase was shown, there was a marked increase in 

 the number of vines showing recent plrylloxeration. 



When more than one variety of vine is included in a " spot," a 

 good index of the resisting power of the several vinifera varieties 

 frequently is observable. Among the dead or moribund vines of 



