April, ’18] 
BECKER: WOOLLY APHIS 
247 
on trees not immune aphids establish only under peculiarly favorable 
conditions, the experiments with elm cannot be regarded as satis¬ 
factory. 
In these and subsequent experiments young seedling apple trees 
and young seedlings of Cratoegus crus galli were used. Following are 
the results of experiments with apple and Crataegus. 
Table I 
No. Migrants Tested 
No. whose Young Est. 
on Both Hosts 
No. whose Young Est. 
on Apple Only 
No. whose Young Est. 
on Cratsegus Only 
1915. 
12 
4 
1 
4 
1916. 
5 
0 
1 
0 
1917. 
18 
0 
16 
0 
The above does not give us as much data as we would like but these 
tests along with other tests not recorded in this paper show the marked 
immunity of Cratsegus as compared with apple. The difference in 
the results of the three seasons suggests that in addition to an inherent 
immunity, Cratsegus may have a conditional immunity depending 
upon the season. This is further suggested by our experiments with 
Cratsegus seedlings. 
Our spring migrants seemed to be slightly more prolific than Baker’s, 
averaging usually from 7 to 8 per female instead of 6 as reported by 
Baker for Vienna, Va. One individual under our observation de¬ 
posited 19 young. Another deposited 18. 
Experiments with Elm 
In our observations on elm we were impressed at the earliness with 
which the stimulative effect of the aphids affected the leaves. It 
appeared that the aphids must have attacked the leaves almost before 
they had pushed through the bud or at least just as soon as enough was 
through for them to work on. At any rate the attacked leaves pushed 
through and curled at once before the normal buds had pushed out. 
Terminals where embryo colonies were forming were conspicuous on 
the trees before the normal buds had burst. In the accompanying 
plate are pictures of normal and infested buds which were collected 
from the same tree and at the same time, April 6, 1917. 
The difficulty which Baker had in getting this insect to establish on 
elm was witnessed by us. We tried repeatedly to establish second and 
third generation larvae on various potted elms but without success. 
It was noticed on our campus that there was a marked variation in 
the immunity of various elms to this insect. Near our insectary we 
have noticed an elm to be infested every year for the last five years 
