458 FRUIT INSECTS 



The complete life history, as outlined above, occurs only 

 when the phylloxera is living on the species of grapes more 

 closely related to its native food-plant. In Europe and in 

 California the leaf galls are rarely seen. Under these circum- 

 stances it is supposed that the Hce hatching from the so-called 

 winter eggs may migrate at once to the roots. That such is 

 the case, however, does not seem to have been proved by direct 

 observation. 



Means of control. 



The grape phylloxera has been found a most stubborn pest 

 to fight, and immense sums of money have been spent in ex- 

 perimental work in its control. The underground forms can 

 be killed by injections of carbon bisulfide into the ground 

 around the roots, provided the soil is neither too loose, so that 

 the gas escapes too quickly, or so impervious that it does not 

 reach the insects on the roots. The apphcation of carbon 

 bisulfide, however, is too expensive for use on a large scale 

 and it is now rarely employed in commercial vineyards. 



In irrigated regions it is sometimes feasible to destroy the 

 phylloxera by inundating the vineyard for two to three months 

 in the fall or for a longer time in the winter. The phylloxera 

 are more easily killed by water in the summer, but unfortunately 

 there is a great danger of injuring the vines by flooding at that 

 time. The ground should be covered by 1 to 2 feet of water, 

 and the submersion must be continuous for the whole period. 

 Obviously this method of control is only of hmited apphcation. 



The most practicable method of avoiding phylloxera injury, 

 and the one most widely practiced in Europe and in Cahfornia, 

 is the use of resistant stocks. The native wild grapes of the 

 eastern United States, while the original food-plant of the phyl- 

 loxera, and usually badly infested, suffer little, if at all, from its 

 attacks. The most resistant of these wild species which can 

 be used for stocks on which to graft the susceptible varieties 

 are Vitis riparia and Vitis rupestris. In the selection of resistant 



