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. 
Eesistant 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 
phylloxera? and a diminishing production of migrant phylloxera 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. 
W ell-nourished resistant vines have been observed to rid themselves 
entirely of the phylloxera?, 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 
phylloxera? simultaneously developed, and under unfavorable condi- 
tions extremely few and sometimes no migrants were produced. 
