44 BULLETIN 1243, U. S. DEPARTMENT OF AGRICULTURE. 



MAGNESIUM ARSENATE. 



Magnesium arsenate has proved to be the least toxic to the bean 

 plant of all the materials tested, and it is at the same time toxic to 

 the bean beetle. For two seasons no appreciable injury has resulted 

 from its use in 134 treatments in the field. 



As a spray at 2 pounds per acre with caseinate of lime, it has given 

 excellent control. Rain water is not necessary; well water and 

 water from streams do not cause injury to foliage, even though the 

 content of sodium salts is relatively high. When magnesium arsenate 

 was used, increases in yields generally resulted, but slight reductions 

 occurred in a few experiments when bean beetle injury was very 

 light. 



Experiment 6 (Table 11.) gives a good idea of the merits of this 

 material in comparison with zinc arsenite and basic lead arsenate, 

 and the relative merits of the latter in comparison with calcium 

 arsenate are shown in experiment 7 (Table 11). The check plat in 

 experiment 6 was completely destroyed, while the adjacent plat 

 treated with magnesium arsenate remained green and bore a good 

 crop. (PL XII, A, B.) 



Magnesium arsenate is also safe for use as a dust. On account of 

 poor physical properties it should be diluted with from 1 to 5 parts of 

 hydrated lime, depending on the infestation. Further work with 

 this material as a dust is necessary. 



This arsenical, as at first placed on the market, was very high in 

 water-soluble arsenic content, but the method of manufacture has 

 been improved and all the results reported herein were obtained with 

 a high grade of material which did not exceed 0.13 per cent water- 

 soluble arsenic content, expressed as metallic arsenic. The toxicity 

 of this material to the Mexican bean beetle compares very closely 

 with that of calcium arsenate. Caseinate of lime as a spreader, 

 added at the rate of half as much by weight as the arsenical, may be 

 used with this material when applied as a spray. 



BASIC LEAD ARSENATE. 



Basic lead arsenate is probably the safest commercial material 

 for use on bean foliage when mixed with rain water. It does not 

 cause injury to foliage when used as a dust. Its low toxicity to the 

 bean beetle makes it very undesirable. In one instance 90 per cent 

 of the field treated with this material was destroyed, and could 

 hardly be distinguished from the untreated field. Laboratory- 

 prepared material gave similar results. In other instances, under 

 conditions of light infestation, this material gave sufficient protection 

 and yields were notably increased. In one experiment this material 

 was used at the rate of 4 pounds per acre, as a spray, without injury 

 to the bean foliage. Further experiments are necessary to determine 

 the degree of control which can be obtained at this rate of application. 



Basic lead arsenate is the commercial basic lead arsenate such as 

 is used in certain districts of California, and is termed "4, 1, 3, 1-lead- 

 hydroxy arsenate, Pb 4 (PbOH) (As0 4 ) 3 , H 2 0" by McDonnell and 

 Smith. 9 



9 Jour. Amer. Chem. Soc., vol. 38, No. 10, October, 1916, p. 2030. 



