386 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 10 



solution of the problem. The writer made two trips to the valley 

 and spent several days in company with Mr. Morris visiting the injured 

 orchards, interviewing the owners and securing data upon which to base 

 conclusions. Five samples of lead arsenate were secured from different 

 orchards which had suffered from spray injury. The matter was gone 

 over quite carefully with Mr. Morris; representatives of two important 

 manufacturers of lead arsenate were interviewed ; and the samples were 

 examined by the writer. As a result of the investigation, certain 

 conclusions have been made and will be submitted later on in the paper. 

 In order that the discussion of the case may be better understood, 

 it seems well to present some theories of spray injury, to describe the 

 commercial types of lead arsenate, and to point out the recognized 

 susceptibility of stone fruits to spray injury. 



Theory of Spray Injury 



For injury to result from the application of a spray, it is necessary 

 that the material be absorbed in some way. It seems quite essential 

 that the material should be in solution before absorption can take place. 

 At least, there is no evidence that a solid can enter the tissue of plants 

 or animals to cause lesions of any sort. The modern practice of 

 applying arsenicals to foliage is based upon the proper combination 

 of the poison in a form which is insoluble in water in order to prevent 

 its entering plant tissue and causing damage. It is generally recog- 

 nized that the damage which is sometimes caused by arsenicals is 

 due to the part of the arsenical which is soluble in water and which 

 may thus enter the tissue of the plant to disturb its functions. 



Arsenic has been combined with many substances to find a com- 

 bination which is the least soluble and the least affected by weather 

 conditions, and to find a combination that is the most suitable for use 

 in other ways. Of all the combinations thus far tried, a chemical 

 combination of arsenic with lead seems to most fully meet the above 

 conditions. At present, there are two types of lead arsenate upon the 

 market, each having its own place in spray practice. 



Types of Commercial Lead Arsenate 



Acid Lead Arsenate; Lead Hydrogen Arsenate (often Labeled 

 "Standard"'.) — In an investigation to find the most suitable form in 

 which an arsenical could be applied to foliage, Mr. F. C. Moulton,^ 

 chemist for the Massachusetts Gypsy Moth Commission, selected lead 

 arsenate as the combination most suitable for use in his work. This 

 arsenical offered so many advantages over other arsenicals in use 



iMass. Bd. Agr. Kept., 41, p. 282 (1894). 



