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



only a small percentage have been observed that were not hibernants. 

 The greater number of the aphicls enter hibernation during October 

 and the first half of November; that is, a majority of the larvse 

 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 phylloxera? 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. Regarding the influence of tem- 

 perature, Mayet (15), in discussing the hibernant form, states that 

 eggs die when the temperature falls below 10° C. 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 phylloxera? 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 



