April, "13] YOTHERS: OILS ON CITRUS TREES 161 



in the spring shortly before active growth begins, and presumably 

 offers greater obstacles to entrance by oil or quickly replaces destroyed 

 and necessary vital tissues. 



President W. D. Hunter: This paper is now open for discussion. 



[The following has been supplied by Mr. Yothers as a substitute 

 for his remarks. Ed.] 



THE EFFECTS OF OIL INSECTICIDES ON CITRUS TREES 



AND FRUIT S^ 



Bj' W. W. Yothers, Bureau of Entomology 



The effect on citrus trees and fruits of those commercial and home- 

 made insecticides having a mineral oil base is of primary importance. 

 This is especially the case since these are the only kinds of insecticides 

 which have been found up to this time to be effective in controlling 

 the white-flies, and the grower who desires to spray has no alternative. 

 Their effectiveness against scale insects and rust mites also increases 

 the importance of this subject. 



As a general proposition their use is attended with little, if any, 

 danger to the trees or fruit. On the other hand, the killing of the 

 insects results in great benefit to the tree. There are certain chem- 

 icals, however, which usually cause damage when applied to the 

 trees in the form of a spray and there are certain practices which 

 should be followed in the use of oil sprays on citrus trees. 



The use of sulphuric acid in a spray should be avoided. One brand 

 of commercial miscible oil contained 2^ per cent of sulphuric acid 

 according to the analysis of the Bureau of Chemistry. Dozens of 

 tests were made with this insecticide and every one resulted in some 

 form of injury. When used at a strength of 1| per cent of oil or 1 

 part of insecticide to about 55 parts of water in the winter, a large part 

 of the leaves fell and holes were burned in others that remained on 

 the trees. So far as observed no twigs were killed. It is very doubt- 

 ful, however, if this in the least made the following crop of fruit less. 

 When used in April when the oranges were about | to f inch in diam- 

 eter, at 1 per cent of the oil contents the new leaves w^ere injured 

 and many new and old leaves fell. About three fourths of the crop 

 of fruit fell or w^as so badly scarred as to be unsalable. When used 

 in April at | of 1 per cent of the oil contents or about 1 part of insec- 

 ticide to 170 parts of water, it burned holes in the new leaves, and 

 the results even at this weak dilution were so disastrous as to prevent 



1 Published by permission of the Chief of the Bureau of Entomology. 



