206 



JOURNAL OF ECONOMIC ENTOMOLOGY 



[Yol. a 



A NEW CONTACT INSECTICIDE 



By W. M. Scott, Baltimore, Md. 



Lime-sulphur solution is by far the most important contact insecti- 

 cide now in use, and in tonnage it probably equals, if it does not exceed, 

 all the other insecticides, combined. It is unique in that it is both an 

 insecticide and a fungicide and has a wider range of usefulness than 

 any other spray material. Applied to fruit trees in the dormant season 

 it serves to control the San Jose scale, oyster-shell scale and some 

 other insects, as well as the peach leaf-curl disease. In the growing 

 season it is used for the control of apple scab, apple leaf -spot, ''red 

 spider" and several other plant pests. It has largely supplanted the 

 oils and soaps as scale remedies and has partly displaced Bordeaux 

 mixture as a fungicide. 



On the other hand, this material is unpleasant to prepare on the 

 farm, bulky and heavy to transport and difficult to store without loss 

 from leakage a^d evaporation. Owing to these objectionable features 

 there has arisen a desire among fruit-growers for a ''dry lime-sulphur" 

 or a dry material that could be used as a substitute for lime-sulphur 

 solution in the control of insects and diseases. Attempts have been 

 made to produce such a material by reducing lime-sulphur solution 

 to dryness, but the polysulphides of calcium upon drying decompose 

 and become insoluble, thus largely losing their insecticidal value. 

 Calcium thiosulphate, one of the ingredients of lime-sulphur solution, 

 is soluble in water but experiments have shown that it is not an effi- 

 cient insecticide. It therefore becomes necessary to employ some 

 other base as a carrier for the sulphur. The results of experiments 

 conducted by the writer during 1913 and 1914 show that barium is a 

 satisfactory material for this purpose.^ Professor Parrott in his paper, 

 just presented, has shown that a solution of barium and sulphur is 

 fully as effective as lime-sulphur solution in the control of the San Jose 

 scale. 



Barium and calcium belong to the same mineral group and the poly- 

 sulphides of these two bases, being so closely related, might be expected 

 to possess about the same insecticidal and fungicidal properties. One 

 important difference in the two materials is that the polysulphides of 

 barium, or at least one of them, can be produced in the form of soluble 

 crystals while those of calcium cannot. 



Boiling a given quantity of barium sulphide (BaS) in water with all 

 the sulphur that it will take up produces a solution composed chiefly 



^ The chemical work in connection with these experiments was performed by Mr.. 

 C. B. Clark, a chemist of the Thomsen Chemical Company. 



