38 BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 
only a small percentage have been observed that were not hibernants. 
The greater number of the aphids enter hibernation during October 
and the first half of November; that is, a majority of the larvae 
hatching from eggs in this period settle down to hibernate. A few 
of those hatching before October become hibernants. After Decem- 
ber 1 it is very unusual to find eggs. The phylloxeras do not enter 
into hibernation all at one time, and even on a single given grape- 
vine the entering into hibernation is protracted over several weeks 
and often as long as two months. The causes that induce the young 
larvae to hibernate instead of proceeding with their normal growth 
are three: (1) Condition of sap flow, (2) condition of food, (3) 
temperature and humidity. Hibernation in general takes place at 
the time when aerial and radical growth of the vine slacken in the 
fall. If the soil temperature is high, there is a tendency to post- 
pone hibernating until some time after the terminal growths have 
apparently ceased. On decayed and decaying roots the phylloxeras 
hibernate earlier and on nodosities and sound tuberosities later than 
on the surface of a normal root. Eegarding the influence of tem- 
perature, Mayet (15), in discussing the hibernant form, states that 
eggs die when the temperature falls below 10° G. He states further : 
This temperature of 10° C. appears to be the minimum under which the in- 
sects become numb, and above which they go out of their torpor * * * M. 
Maurice Girard proved, experimentally, by means of a freezing mixture, that 
the phylloxera would sustain a temperature of —8° and —10° C. without dying. 
The present writers' observations in the vineyards show that, 
broadly speaking, when the temperature drops to a minimum of 66° 
F. about half the individuals are hibernants, and when the maximum 
in spring has risen to 58° F. about half the individuals have com- 
menced growing. The phylloxerse enter hibernation under a consid- 
erably higher temperature than that which obtains at the time their 
spring growth begins. 
Character of soil has no direct influence on hibernation, but it may 
have an indirect, influence in so far as it may affect the condition of 
the roots. The heavier soils hold the moisture longer than those 
of lighter types, bringing about a more rapid decay of the roots and 
compelling early hibernation. Cold soils also force the insects into 
early hibernation. 
In the vineyards the bulk of the hibernants occur on the lower 
part of the stump and on the basal portions of the main roots. Hiber- 
nants also ascend older vines several inches above the soil surface, 
where they are concealed under layers of bark. Often most of those 
that go above the soil surface perish from cold (16). On the smaller 
rootlets are found small numbers of hibernants, many of them on 
nodosities on which they pass the winter, frequently with a consid- 
erable percentage of mortality. On vines that have been heavily 
