EXTERNAL INSECTICIDES L'V) 



oxidizes the sulphite to insoluble sulphate (Reaction 2); 

 and the sulphide to thiosulphate (Reaction 3), with a 

 separation of sulphur in each ease except the second. Ex- 

 posure to the air causes the oxidation changes to take place. 

 The reactions are as follows: 



1. CaS 2 03=CaS0 3 +S 



2. CaS0 3 +0=CaS04 



3. CaS 4 +30=CaS 2 03+2S 



The active constituents as a contact insecticide are the 

 polysulphides which are caustic in nature, destroying the 

 skin of insects. The more sulphides in solution, the more 

 effective the spray; and much variation from the formula 

 given, or boiling too long, or use of impure materials, all 

 lessen the amount of sulphides formed. 



Lime-sulphur is also a fungicide and in addition to the 

 sulphides, the thiosulphate has value for this purpose. 

 When sprayed on the trees and exposed to the air, the 

 thiosulphate and sulphides oxidize as indicated in Reactions 

 1 and 3. The insoluble sulphite and sulphur which are 

 formed are also useful as fungicides. The carbon dioxide 

 of the air also breaks up lime-sulphur as follows: 



CaS 4 +CO2 +H2O = CaC0 3 +H 2 S +3S 



Injury to foliage from the use of this spray is due to the 

 caustic effect of the polysulphides, and occurs for the most 

 part when the sulphides occur in too great concentration 

 or when they penetrate the surface of the leaf through cracks 

 or stomata. Applied too thickly the drops coalesce and run 

 to the edge where by evaporation the solution becomes suffi- 

 ciently concentrated to destroy leaf tissue. Injury, however, 

 takes place only during the early part of the application, 

 for the longer the spray stays on the leaves the more the 

 sulphides are broken up as described above. 



Although there is some evidence that lime-sulphur is an 

 internal as well as an external insecticide, its action in this 

 respect is not sufficiently pronounced for general use, and 

 it is advantageous if some internal insecticide can be mixed 

 with the lime sulphur, so that one spraying, or series of 

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