Puant Lice InsjurinG FOLIAGE AND FRUIT oF THE APPLE 739 
The fourth generation 
Altho the majority of the third generation under experimental obser- 
vation acquired wings, a goodly number were wingless and were easily 
induced to continue reproducing on the apple. Their young constitute 
the fourth generation on apple. 
The habits and activities of this generation do not-differ in any respect 
from those of the third. The leaves, especially those about the fruit 
clusters, become crowded by the lice, which swarm also on the developing 
fruits. In 1915 they began reaching maturity about June 25, and from 
that date to the middle of July large numbers of winged forms were present 
on the apple trees. All those under experimental conditions. acquired 
wings, and in the field, so far as could be learned by constant observation, 
all of this generation became winged and constituted the main spring 
migration of the years 1915 and 1917. 
As the nymphal stages are practically identical with those of the second 
and third generations, it is not necessary to repeat detailed descriptions. 
The adult corresponds so closely to that of the third generation that the 
description of the winged female of that generation will serve in this case. 
The migratory forms 
At Ithaca the migratory forms of Aphis sorbi may consist of the winged 
females of either the second, the third, or the fourth generation (counting 
the stem mothers as the first generation). At Vienna, Georgia, Baker 
and Turner (1916 b) found that the migratory forms may appear in every 
generation from the third to the eighth on the apple. The writer did 
not succeed in rearing more than four generations on the apple, tho repeated 
attempts were made. However, from field observations and from notes 
in the files of the Department of Entomology at Cornell University, it 
would appear that in some years there are undoubtedly more than four 
generations in certain sections of New York State. As yet there are 
no definite data to prove or disprove such a belief. Brittain (1915b) 
finds that in Nova Scotia the migratory forms are the winged adults of 
he third generation. He reports, however, that in individual rearing 
xperiments, in which the number of plant lice to each seedling was very 
mall, he succeeded in rearing a total of seven generations on the apple: 
N some exper:ments no migrants were produced. Brittain does not state 
