388 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 10 



dissolved by chemicals of an alkaline nature which are commonly found 

 in many spray materials. This property distinguishes the acid type 

 of lead arsenate from the basic. The latter is not easily affected by 

 alkaline chemicals and is a much more stable chemical compound 

 under neutral or alkaline conditions. It is not easily made soluble 

 by ordinary influences. A theoiy was advanced b}^ Volck that the 

 acid type of lead arsenate was decomposed by the small amounts of 

 ammonia (alkaline) which are sometimes present in the atmosphere, 

 producing a soluble form of arsenic. Under the conditions prevailing 

 in the Pajaro Valley, there was present on the foliage enough moisture 

 to dissolve the soluble arsenic, thus formed, and make possible its 

 absorption by the foliage. The basic lead arsenate, on the other hand, 

 is absolutely insoluble in ammonia and soluble arsenic cannot be 

 formed in this manner. Whether this theory is correct or not has not 

 been fully confirmed by other experimenters, but the fact remains, 

 however, that the basic type is the safer arsenical to use upon foliage 

 in the humid coast regions. The basic type is practically the only 

 arsenical which may be safely mixed with any of the other spray 

 materials as a combination spray. This has been fully tested out 

 in the Insecticide and Fungicide Laboratory and has been found to 

 be much more suitable when alkalies of any sort are mixed with it. 

 The acid type is a stronger poison, however, and its action upon the 

 insects is much more rapid. The basic type contains much less arsenic 

 to the pound and a much longer time is required to poison insects. 



The stronger and quicker acting acid lead arsenate is therefore 

 the one to be naturally selected for use upon foliage which is not 

 peculiarly susceptible to spray injury and where the weather conditions 

 are not apt to cause its decomposition, that is, in the more arid regions 

 away from the coast. The basic type should be used whenever an 

 arsenical is to be mixed with any other material (with one or two 

 exceptions) as a combination spray. The basic type is the one to be 

 recommended for use in the humid coast regions and upon all stone 

 fruits or any others which are especially susceptible to spray injury. 



Susceptibility of Stone Fruits to Spray Injury 



It is a well-known fact that the foUage of all stone fruits is peculiarly 

 susceptible to injury from many kinds of sprays. This fact has been 

 noted in respect to the use of arsenicals and the California Agricultural 

 Experiment Station has advised that the basic type of lead arsenate 

 should be the only arsenical applied to stone fruits. The wisdom of 

 this advice has been doubted by a great many and the cause of the 

 doubt is not difficult to find. If the weather is favorable, the acid 

 type of lead arsenate usually may be applied with impunity. In 



