April, ’18] 
BECKER: WOOLLY APHIS 
251 
The results of our transfer experiments lead us to the conclusion: 
(1) That apterous vivipariae from apple can establish on Crataegus 
though they do not establish on that host as readily as they do on 
apple. 
(2) That apterous vivipariae from Crataegus can establish on apple 
though they apparently do not establish as readily on that host as 
do the apterous vivipariae from apple. 
Northern Spy Immunity 
Aside from experiments undertaken with Northern Spy roots no 
definite experiments were undertaken to see to what extent different 
apple seedlings were immune to the woolly aphis. It might be stated 
however, from general experience, that we noticed a difference in the 
degree of immunity of different apple seedlings. Generally speaking, 
though, we had no difficulty in establishing either the offspring of 
spring migrants or of apterous vivipariae from other apple seedlings 
on this host. 
In these tests pieces of Northern Spy root were placed in tin salve 
boxes and handled in the same manner as described for our experiments 
with the apple root forms. Five aphids were applied to each piece of 
root and when these had died another attempt was made to establish 
a colony by adding five more aphids. The aphids used were usually 
of the first instar and were taken from crab roots. 
We made 161 attempts to establish colonies on Northern Spy roots, 
using a total of 805 aphids. Of the 161 attempts we got colonies to 
establish temporarily in 16 instances. In two of the trials we suc¬ 
ceeded in getting the aphids to establish long enough to produce young 
but the latter failed to establish permanently. In no case did we 
succeed in getting permanent colonies to establish on Northern Spy 
roots. 
As a check against these results we made 51 trials to establish aphids 
on crab roots, using in the tests 255 aphids. Twenty-seven of the 
trials yielded colonies. In most instances these colonies were per¬ 
manent. 
Most of these tests were made by my student assistants, Messrs. 
W. D. Merrill and Wm. M. Lee, and their notes repeatedly call atten¬ 
tion to the sickly condition of the temporary colonies on Northern Spy 
as compared with the thrifty colonies on crab roots. 
From these tests we conclude that Northern Spy is immune to the 
woolly aphis and that, when aphids do establish temporarily, it is due 
to the fact that they are exceptionally hardy rather than the fact that 
the host is congenial. 
