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



was frequently large. In the case of pieces of vine roots kept in a 

 cellar, abnormal conditions of food, temperature, and humidity fre- 

 quently arose. 



The conditions which affect the relative abundance of migrants 

 are the following : Variety of vine, vigor of vine, humidity, tempera- 

 ture, condition of roots, character of soil. 



Besistant and certain American nonresistant vines normally bear 

 the greatest proportion of migrants. These vines are the descendants 

 of the wild grapevines which formed, and still form, the natural 

 food plant of the phylloxera, and which were immune from serious 

 injury by reason of the fact that there was produced each year a large 

 percentage of migrants, while few or no wingless forms persisted on 

 the vines after the 'winged forms had departed. The wingless 

 radicicole forms during the summer fed only upon the terminal 

 rootlets, and when these decayed the vine was easily able to replace 

 them without suffering injury of any consequence. The resistant 

 vines of to-day, except in instances in which the roots have been 

 supplied with poor or insufficient soil, as is noted below, do not sup- 

 port heavy and continued infestations of wingless phylloxera, and 

 almost all the phylloxeras born in summer and autumn develop 

 wings and become migrants. It may be said here that experimenting 

 with resistant vines grown in pots with soil unchanged for over a 

 year is apt to give misleading results, for as the soil becomes poorer 

 and insufficient for the increasing root system of the vine, fibrous 

 rootlets become scarce, and an abnormal infestation of wingless 

 phylloxeras and a diminishing production of migrant phylloxeras en- 

 sue, thus approaching the conditions normally found on vinif era vines. 

 On vinifera vines and on many American nonresistants, such as 

 Isabella, Catawba, and Champion, the production of winged migrants 

 is never proportionately as large as that which occurs on resistants. 

 Well-nourished resistant vines have been observed to rid themselves 

 entirely of the phylloxeras, the insects all departing as winged forms, 

 and in all cases under normal conditions, if any wingless forms 

 remain after the winged forms have all left, the number is very 

 small. On vinifera vines the total nymphal production has been 

 found to be over 33 per cent of the whole in season, although three- 

 fourths of the individuals produced on fleshy surface rootlets and on 

 nodosities have been observed to develop into migrants, and on suc- 

 culent pieces of severed root cuttings as large a proportion has been 

 reared. 



In the vineyard the larger roots were rarely found to produce a 

 number of migrants in excess of 25 per cent of the whole number of 

 phylloxeras simultaneously developed, and under unfavorable condi- 

 tions extremely few and sometimes no migrants were produced. 



