382 ANNUAL REPORT OF THE Off. Doc. 



law of 1910 as well as our own State law, stipulates that Paris green 

 must contain, as its active agent, at least 50 per cent, of arsenious 

 oxid of whicli not more than 3^ per cent, is in water soluble form. 

 As early as 1872 Paris green was used against caterpillars and in 

 1878 it was first applied on apple trees against the codling moth. 

 Soon after this date it became recognized as the most efficient treat- 

 ment for phytophagous insects. 



In 1896 over 2000 tons of this material were consumed in the 

 United States, by far the greater portion of which was used on po- 

 tatoes to kill the Colorado potato beetle. It is still largely used in 

 the potato field applied in the dry state either pure or with a dilutent, 

 by means of bellows early in the morning while the foliage is wet 

 with dew. It does not, however, lend itself so well to use with 

 liquids as it settles to the bottom of the spray tank and violent agi- 

 tation is necessary to keep it in suspension so as to insure an even 

 application. Moreover it does not stick very well to the foliage, be- 

 ing easily washed off by the rain so that repeated applications may 

 be necessary to get the desired results. The high percentage of 

 soluble arsenic in its composition makes it liable to injure the 

 foliage especially when added to solutions containing copper, soap, 

 or ferric chloride. For these reasons its use on fruit trees has been 

 replaced by the safer and more adhesive arsenate of lead, and even 

 on vegetables and field crops, arsenate of lead seems to be preferred 

 by the grower. 



ARSENATE OF LEAD 



It is only in recent years that arsenate of lead has come into use 

 as an insecticide. In 1893 Mr. F. C. Moulton, while working for the 

 Gypsy Moth Commission of Massachusetts, first suggested its use 

 in the extermination of the gypsy moth to replace Paris green, which 

 could not be applied at sufficient strength to kill these caterpillars 

 without seriously injuring the foliage. It was found that lead arse- 

 nate was entirely satisfactory and it has been the standard arsenical 

 insecticide since that time. It possesses important advantages over 

 Paris green, of which are the following: (1) It is not so injurious 

 to the foliage on account of its being less soluble. (2) When sprayed 

 upon leaves and fruits, it forms a thin film which is quite adhesive, 

 often remaining on the plant throughout the entire growing season. 

 (3) It remains in suspension better, requiring less agitation to in- 

 sure even and uniform application. (4) It can be mixed with copper 

 and lime-sulphur solutions without injury to foliage. (5) It forms a 

 white, visible coating where it has been applied. 



Chemically, lead arsenate may be either triplumbic or plumbic 

 hydrogen arsenate. The triplumbic arsenate of lead is prepared by 

 combining normal sodium arsenate with either lead acetate or lead 

 nitrate. If any disodium arsenate is present there is then formed 

 some plumbic hydrogen arsenate. The commercial product usually 

 consists of a mixture of these two forms, the proportion depending on 

 the purety of the raw materials and method of manufacture em- 

 ployed. Arsenate of lead is usually sold in the form of a thick paste 

 but recently the dry powdered form has been put on the market. The 

 National Insecticide Act of 1910 stipulated that arsenate of lead 

 paste must contain at least 12^ per cent, of arsenic acid, of which 



