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



confusion, it is assumed that there are five generations annually, 

 since this number is about the average in a vineyard in which the 

 sap moves early, although there might be, under certain conditions, 

 from one to eight or even nine generations within a single year. 

 The hibernant generation having matured in April, the succeeding 

 generation matures about the time the canes have ended their first 

 rapid growth, approximately the end of May. Succeeding genera- 

 tions mature on about the following average dates : Second, July 6 ; 

 third, August 15 ; fourth, September 30 ; the fifth generation hiber- 

 nating. 



A variable percentage of the larvae of generations 2, 3, and 4 

 becomes nymphs, and these later emerge from the ground as winged 

 insects and either fly away or are borne off on the wind. Large 

 numbers of these are caught in spider webs. Many of the newly 

 hatched larvae develop a wandering tendency just after they have 

 issued from the eggshell and seek to emigrate to other vines either 

 through the soil or over the surface of the ground. Large numbers 

 of these migrating larvae are also caught in spider webs on the surface, 

 and while only a small percentage reach their destination, a single 

 individual may start a new infestation. Those of the larvae that suc- 

 ceed in fastening upon a root or rootlet develop as radicicoles. 

 The winged forms normally occur from June to October, and the 

 wandering larvae are found from July to September. 



During July and August, when the adult radicicoles are most 

 prolific, incubation and development proceed most rapidly, and the 

 phylloxera may be said then to have reached its most active stage. 

 It is at this stage that the greatest damage is done to the roots of the 

 vines, although the effects are not generally apparent until the fall 

 and winter following, when the lesions formed during the summer 

 have decayed. 



At the end of September a few of the newly hatched larvae 

 hibernate, and throughout October successive generations become 

 hibernants, so that by the end of the month a large majority of the 

 phylloxera have reached this stage. During November and the 

 first half of December, a few mature radicicoles and growing larvae 

 may be found, but after the middle of December, it is unusual to 

 find any form but the hibernating larva. 



Under conditions of abundant food supply, the period of egg 

 deposition of the radicicoles averages 45 days and may reach a 

 maximum of 110 days. This average is nearly constant throughout 

 the season. The average number of eggs deposited is about 117, 

 but under certain conditions the number may be increased to 486. 

 The daily average number is about 2^+ e £g s 7 an d as many as 23 

 eggs have been deposited in 24 hours by a single phylloxera. 



