113 
er's direction, of trees of various species — some of which had been 
sprayed and others of which had not — it seems to me clear that 
many of these trees were already in a more or less enfeebled state 
owing to the light and sandy soil and other unfavorable conditions 
affecting their growth, that many treated trees had been further 
greatly weakened by heavy and continuous infestation by the maple 
scale, and that the consequences had been in some cases intensi- 
fied by the action of the kerosene. This is especially suggested by 
the fact that trees seemingly most injured but not actually killed, 
presented the same general appearance — ^^though in a more marked 
degree — as many of those which had not been sprayed at all. Pre- 
cise and extensive experiments, the results of which may be brought 
into strict comparison, are needed to show the strength and the 
amount of the emulsion which it is safe to use under various con- 
ditions and on various kinds of trees. 
In the meantime, in view of the fact that the thorough spraying 
of the top of a tall tree in winter requires a large amount of kero- 
sene emulsion, much of which must fall to the earth, and the further 
well-known fact .that fruit-trees may be severely injured, and even 
killed, by kerosene in the earth about their roots, it will be prudent 
to protect the ground where this insecticide is used by some imper- 
vious or absorbent covering, such as a sheet of canvas or a layer of 
straw, the latter afterwards to be removed. Kerosene will, in fact, 
remain effective in the ground for a surprising time, and this fact 
is the basis of one of its most important uses as a subterranean in- 
secticide.* Thousands of fruit-trees have been destroyed within my 
own knowkdge, by its careful use under the supervision of expert 
operators, where orchards were undergoing treatment for the San 
Jose scale. 
Experiments already referred to, made at Denver, Col., by Mr. 
S. A. Johnson, resulted in a way to indicate that weaker emulsions 
than those used in Chicago may be depended on to destroy the maple 
scale, as shown by the following table. 
Percentage of emulsion 
10 
12 
12X 
17 
20 
25 
33 
50 
November, 1903 
February, 1904 
88 
69 
94 
88 
99 
100 
98 
100 
100 
100 
^ *The most notable instance of this persistence of kerosene in the earth which has come 
to my knowledg'e is reported to me by Professor T. J. Burrill, of the University of Illinois, 
who, in company with Prof. J. C. Blair, of the Horticultural Department, once lightly spong-ed 
the bark of a pear-tree with pure kerosene as an experiment. Nothing unusual was noticed 
the first year, but the second year the tree was seen to be unthrifty, and the third year it was 
dead. When dug- up to learn the cause of its death, the odor of kerosene was still distinct and 
strong- in the earth among its roots, and especially in the bark about the base of the trunk. 
The surplus which had run down the surface of the bark and sunk into the soil had remained 
all this time, acting, no doubt, continuously upon the roots and on the b'ark of the trunk in a 
way to kill the tree. 
