248 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. II 
and during the same time numbers of elms which are near it have never 
been observed to be infested. 
It appeared from general observation that backwardness of growth 
was associated with susceptibility to the attacks of the insect. To 
verify this, 35 elms were examined at random. They were classified 
according to their size and were rated as advanced, medium and back¬ 
ward, according to the degree of their advancement at the date of the 
observation, April 8, 1917. The results of the observations appear in 
Table II. 
Table II 
Circum. in Inches 
Advanced 
Medium 
Backward 
Infested 
Uninfested 
Infested 
Uninfested 
Infested 
Uninfested 
20 and up. 
1 
3 
0 
0 
0 
0 
10-20. 
1 
1 
1 
1 
2 
0 
5-10. 
1 
3 
2 
2 
2 
2 
Under 5 . 
0 
5 
1 
5 
1 
1 
Total. 
3 
12 
4 
8 
5 
3 
It will be observed that the above seems to bear out this contention. 
In 1915 we observed a small elm, which had been set out the fall be¬ 
fore, to be infested. It was presumably congenial to the insect because 
it was backward during that season. At any rate it has not been ob¬ 
served to be infested since that time. 
Experiments with Crataegus 
Many negative results in attempting to get aphids to establish on 
Crataegus led us to undertake some experiments with a view of deter¬ 
mining to what extent this host was immune to the species. Young 
Crataegus seedlings were used in these tests. Five aphids were ap¬ 
plied at a time and we tried to make 10 transfers, making a total of 50 
aphids, to each seedling before placing it aside as immune. The aphids 
used were secured from infested Crataegus plants. The results of these 
tests appear in Table III. 
It will be observed that the first four seedlings tested took colonies. 
In view of the difficulty of getting colonies to establish later in the 
season it would appear that the hardening of the tissues would be a 
condition which would bring about immunity. The seedlings used in 
experiments Ca-C17 to Ca-C23 inclusive had been used earlier in the 
season in the Aa-C series (Table IV), and in those tests were not 
acceptable to the aphids. They were subsequently cut back and 
watered with manure water until tender shoots were produced. 
Under these conditions two of the seedlings were acceptable to the 
Crataegus aphids. 
