464 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 7 



A large proportion of the lime-sulfur used is for insecticidal purposes. 

 Consequently, examinations made in the valuation of the same should 

 •be quantitative determinations of those properties from which the 

 spray derives its insecticidal value. Actual chemical composition is 

 a secondary matter except in so far as it may be an indication or meas- 

 urement of these properties. For this reason, the determination of 

 specific gravity is perhaps of little value except in a very general way. 

 Samples of lime-sulpur having the same specific gravity may not be 

 alike in chemical composition nor in many other properties. For 

 example, a sample having a low specific gravity may have a greater 

 per cent of polysulfides than a sample having a somewhat higher 

 specific gravity. 



In this discussion of valuation, it is well first to consider the proper- 

 ties which give to lime-sulfur its insecticidal value. The most exhaust- 

 ive investigation available is, perhaps, that of Shafer.^ He showed 

 that with scale insects, like San Jose scale, the calcium polysulfides 

 present in the solution softens the so-called wax about the margin of 

 the insect and, on drying, causes it to stick to the plant. In the cases 

 tried, the insects stuck tightly enough to also cause the death of the 

 young by sealing them under the scale covering of the mother. Shafer's 

 work also strongly indicated that one of the principal, if not the prin- 

 cipal, insecticidal effect of lime-sulfur solution, upon insects of the 

 type mentioned, is its great power of absorbing oxygen, thus causing 

 the treated insects to suffer because of an insufficient supply of this 

 element. Other experiments made by Shafer showed that sulfur 

 dioxide is not "formed in appreciable amounts from sulfur deposited 

 by lime-sulfur except at temperatures much above those found under 

 spraying conditions in the orchard." The liberation of this gas is, 

 evidently, not in amounts large enough to make it of importance in 

 any consideration of the insecticidal properties of the spray. The work 

 done by the department of entomology of this station- indicates clearly 

 that the principal insecticidal constituents are the calcium polysulfides. 

 Experiments tried with calcium thiosulfate on San Jose scale ^ showed 

 this material to have but little, if any insecticidal value. Wellington * 

 arrived at similar conclusions. Shafer's ^ results also indicated that 

 the thiosulfate has a limited insecticidal efficiency. 



It has been known for long, too, that with certain insects, free sulfur 

 has some killing power. It is stated that dry sulfur has been used in 



1 Tech. Bui. No. II, Mich. Agr. Exp. Sta. 



•Unpublished results. 



a Biennial Crop Pest and Horticultural Report, p. 112 (1913). 



♦ Bull. 116, Mass. Agr. Exp. Sta. 



^ Loc. cit. 



