INSECTS INJURIOUS TO THE GRAPE 



433 



leaf-galls are very common on American grapes, but are no 

 indication of the presence of the root form, as the roots are rarely 

 injured where the foliage is covered with leaf -galls. On the other 

 hand the European varieties rarely exhibit any leaf-galls, but are 

 very susceptible to the root phylloxera, which multiplies without 

 any external indication of its presence until the vine is seriously 

 injured. The injury to the vine is not due so much to the sap 

 taken from the vine by the myriads of aphids which may inhabit 

 the roots, as to their poisonous effect on the root tissue and its 

 subsequent decay. Wherever the phylloxera attack the roots, 

 small swellings are pro- 

 duced, composed of soft 

 tissue which soon decays. 

 When such a gall is 

 formed at the end of a 

 young root, its growth is 

 stopped, and on larger 

 roots a decay sets in 

 which finally girdles the 

 root and all below the 

 injured point dies. As 

 all the roots become 

 affected the vine stops 

 growing, the leaves be- 

 come sickly and yellow- FIG. 363. Under side of grape leaf showing 

 ish, and the vine dies, and S alls caused b ^ Phylloxera. (After Riley.) 



the phylloxera disappears from the rotting roots, so that the cause 

 of the injury would be obscure were the nature of the injury not 

 known. 



Life History. The life history of the phylloxera is a com- 

 plicated one, involving four different forms of aphids; the leaf- 

 gall form, the root or destructive form, the winged or colonizing 

 form, and the sexual form. The winter eggs are deposited on the 

 rough bark of the old wood in the fall and hatch the following 

 spring. The young aphids settle on the leaves, where the irrita- 

 tion caused by their mouth-parts soon causes a depression around 

 each which forms a gall projecting on the lower side of the leaf. 

 11 In about fifteen days the louse becomes a plump orange-yellow, 

 full-grown, wingless female, and fills its gall with small yellow 



