BULLETIN 192. INSECTS INJURIOUS TO THE VINE. 107 



five or forty days, and indeed in some soils of the extreme south of 

 France two months has been found necessary. As the insect is most 

 susceptible in midsummer, it was at one time thought that a copious 

 irrigation at that time sufficient to destroy most of the insects without 

 injuring the vines could be effected. At present a flooding in July for 

 not exceeding forty-eight hours is practiced in a few places, but only to 

 supplement winter flooding, or the injection of bisulfid. The insecti- 

 cidal value of the short submersion which the vines will withstand at 

 this time seems to be very slight. Its main value seems to be in 

 prompting a vigorous growth of new rootlets to replace those that have 

 been injured. 



Planting in Sand Method. 



Though no thoroughly satisfactory explanation has been given, the fact 

 is established that in certain very sandy soils vines are uninjured by 

 phylloxera. All sandy soils are unfavorable to the increase of the 

 insect, and vines planted in them die more slowly than in others; but 

 for complete immunity the soil must contain at least sixty per cent of 

 siliceous sand. The looser and more fine-grained the sand, the more 

 resistance it offers to the insect. Sands containing notable quantities 

 of clay, all those in fact which have a tendency to form lumps or 

 " cake," offer less resistance. 



Resistant Vines. 



The most satisfactory method of combating phylloxera is the use of 

 resistant vines, because it is applicable to all conditions and is the most 

 economical in the end. A resistant vine is one which is capable of 

 keeping alive and growing even when phylloxera are living upon its 

 roots. Its resistance depends on two facts: 1st, that the insects do not 

 increase so rapidly on its roots; and, 2d, that the swellings of diseased 

 tissue caused by the punctures of the insects do not extend deeper than 

 the bark of the rootlets and are sloughed off every year, leaving the 

 roots as healthy as before. The wild vines of the Mississippi valley 

 states have evolved in company with the phylloxera, and it is naturally 

 among these that we find the most resistant forms. No vine is per- 

 fectly immune in the sense that phylloxera will not attack it at all; 

 but on the most resistant the damage is so slight as to be impercep- 

 tible. The European vine ( Vitis vinifera L.) is the most susceptible of 

 all, and all the grapes cultivated in California, with a few unimportant 

 exceptions, belong to this species. Between these two extremes we find 

 all degrees of resistance, which is expressed by a series of numbers 

 ranging from 20, indicating the highest possible resistance, to 0, indi- 

 cating the utmost susceptibility. The following table shows the resist- 



