384 THE MONTHLY BULLETIN. 



THE SELECTION OF PETROLEUM INSECTICIDES. 



By Robert K. Vickert, Superintendent Rex Spray Company, Benieia, Cal. 



There are a number of useful tests that may be applied to insecticides 

 prepared from petroleum in its various forms. These may be divided into 

 three classes: 



(1) Tests to determine the killing power of the oil with reference to the insect 

 for which it is intended. 



(2) Tests to determine what injury the oil may do to plants with which it 

 may come in contact. This will not be considered in this paper as Prof. 

 George P. Gray and Prof. E. Ralph De Ong of the California Experiment Station 

 are expecting to publish on this subject. 



(3) Tests to determine the efficiency of various methods of applying the oil 

 to the pest, emulsions, etc. 



There is much that can be learned by a study of how petroleum kills an 

 insect. The work of Prof. George D. Shafer, published in technical bulletins, 

 Nos. 11 and 21, of the Michigan Agricultural Experiment Station, has changed 

 our conception of how the oil acts on the insect. It has been held for a 

 number of years that death followed the plugging of the spiracles by the viscid 

 oil. That death was not due to suffocation was demonstrated by Mr. Shafer 

 in a series of intricate experiments in which the insects were immersed in pure 

 gases such as hydrogen. He proved that it was some toxic quality in the oil 

 that killed the insect rather than a mere mechanical suffocation due to the 

 stopping of the spiracles. 



Mr. Shafer next proved that it was the vapors of the lighter fractions of 

 the oil that were the chief toxic agent. This is logical since the oil need not 

 come in contact with the insect in order to be effective. 



From these premises he went on to determine what organ, or system of 

 organs, was affected by the vapor of the petroleum. He found unmistakable 

 evidence that the system of enzymes in the body fluid of the insect was very 

 seriously upset and concluded that this disarrangement was probably the 

 cause of death. He further drew the conclusion that it was the inhibition of 

 the reducing enzymes that was the vital factor. The obvious conclusion was 

 that with the reducing enzymes out of action that the insect was literally burnt 

 up by the freeing of the oxidizing enzymes. 



The writer repeated this part of Mr. Shafer's work and came to slightly 

 different conclusions. Good material in the form of silkworm larva? was used 

 and the reactions were speeded up considerably by the use of liquid air. 

 The conclusion was drawn that insects that had been exposed to the effects 

 of petroleum vapor showed a marked increase in the oxidizing enzymes rather 

 than a reduction of the reducing enzymes. In a histologic study it was found 

 that the cenocytes, which are ductless glands located close to the spiracles and 

 in contact with the trachese were very active when subjected to the gases given 

 off by oils. According to Glazer (Biological Bulletin, Vol. XXIII, No. 4, Sept.. 

 1912) these ductless glands are the source of an oxidizing enzyme. The fact 

 that these cells are stimulated to produce more enzymes under the effect of 

 petroleum lends weight to the above opinion. This opinion is not of much 

 value, as under present methods, quantitative work with enzymes, such as 

 those found in the invertebrates, is out of the question. 



From the practical point of view, however, Mr. Shafer's work must be con- 

 sidered in the selection of insecticidal oils. It is evident that the oil must 

 have a due proportion of volatile fractions. On the other hand it is just as 

 important that the light fractions be accompanied by a heavy gravity oil in 

 order to keep the volatile portions from evaporating too rapidly. The best 

 killing oil in some cases may be a blend of a light fraction and a heavy lubri- 

 cating oil. or it may be in other cases an average oil such as kerosene or 

 stove distillate. 



The so-called "penetration" of an oil spray depends on the capillarity of the 

 oil. In the case of the armored scale insects, for instance, the oil with the 

 best penetration would be the one that would creep the farthest under the 

 scale covering. Therefore, it is essential that a spray oil should have those 

 ingredients in it that will give it a high degree of capillarity. Capillarity can 



