72 BULLETIN 903, IT. S. DEPARTMENT OF AGRICULTURE. 



In observations with phylloxera? developing on living vines there 

 were secured in a period of three years 13 generations, an average of a 

 little over four generations a year, but had the earliest eggs of each 

 generation been successfully utilized, and had it been possible to 

 start the first of the three seasons with the earliest eggs procurable 

 in the vineyards, there is no doubt that six, and possibly seven, gen- 

 erations could have been developed each year. 



Considering that the hibernant generation occupies a period of 

 half a year, it is apparent that if seven generations are to be pro- 

 duced in a year, the other six must be passed in an average maximum 

 of one month apiece. In summer phylloxera? have been reared from 

 egg deposition to maturity in 21 days, but in April, May, and Oc- 

 tober the cycle rarely falls below 35 days, so that in the six-month 

 period, April 15 to October 15, the average maximum cycle is roughly 

 30 days. Thus, in the vineyard, even on vines that move early in 

 spring, it is probable that more than seven generations rarely take 

 place in 12 months. The period, October 15 to April 15, best rep- 

 resents the cycle of the wintering generation, although these dates 

 are somewhat arbitrary. 



Under vineyard conditions it is always possible to find hibernant 

 phylloxera? as late as the beginning of June. It is also possible 

 to find insects going into hibernation as early as September 20. Since 

 the mature radicicoles deposit eggs for periods exceeding three 

 months, it can be seen readily that the latest eggs of a radicicole 

 hibernant maturing in June may develop larva? which proceed to 

 hibernate. A minimum of one generation a year thus may occur. 

 Observations indicate that this minimum of one generation is not 

 common, even on moribund vines with innutritious roots. 



WANDERING RADICICOLE LARVAE OR " WANDERERS." 



By the term "wanderers" are designated those forms (almost all 

 newly hatched larva?) which forsake the root on which they issued 

 from the egg and seek to reach the surface of the soil or to pass 

 through the soil itself, with the object of finding new food. Those 

 that strive to reach the surface exhibit in their efforts a very marked 

 positive phototropism. It would appear that their first movement 

 is simply one of ascending the root and that as soon as they are 

 brought into the focus of a ray of light they immediately make it 

 their goal, and thus finally ascend to the surface. The initial wan- 

 dering movement comes irrespective of light rays, but as soon as 

 these rays are brought to bear the activity is very pronounced. The 

 conclusion is that the production of individuals destined to wander 

 is due to a combination of influences more than to any single influ- 

 ence — the crowded condition of the phylloxera? in summer, the decay- 

 ing of the roots, especially the fleshy surface rootlets, found on 



