Puant Lice Insjurtne Fourace AND FRuIt oF THE APPLE 759 
At this period the aphis situation is usually a serious one for the orchard- 
ist, especially if he has not sprayed to control the lice and if the predacious 
and parasitic enemies of plant lice are not present in very considerable 
numbers. The factors determining the numbers and activities of the 
enemies of plant lice are many and are extremely complicated. No experi- 
mental work has been done on this phase of the problem, and up to this 
time only general statements based on field observations occur in the 
literature. With adequate facilities, two factors—temperature and 
moisture — might be studied. It is almost certain that these two factors 
are the controlling ones, but how they act has not been determined. 
The reproductive capacity tables show that during the latter half of 
June the stem mothers probably cease reproducing and most of them die. 
The second and third generations are reproducing at their maximum, and 
the fourth generation of Aphis pomi will reach its maximum about the 
last days of June. It will thus be seen that if the parasitic and predacious 
enemies have not kept pace with their hosts, the severest injury may 
occur during the latter half of June. This is what occurs in an aphis 
year, and the reason for it can readily be seen by consulting the repro- 
ductive capacity tables. During the remainder of the summer, from 
July 1, the numbers of plant lice generally decrease, and this despite the 
fact that young of Aphis pomi are being produced in countless millions. 
The season has now become warm, a condition apparently most favorable 
for parasitic and predacious enemies, and these are able to gain the upper 
hand. 
