356 



Bulletin No. 112. 



^January, 



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 

 <iegree— 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 knowledge, 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. 



♦The most notable instance of this persistence of kerosene in the earth which has come 

 to my knowledge 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 sponged 

 the bark of a pear-tree with pure kerosene as an experiment. I^othing 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 bark of the trunk in a 

 way to kill the tree. 



