2 MASS. EXPERIMENT STATION BULLETIN 201. 



The factors that facihtate distribution naturally differ somewhat in 

 soluble and insoluble products, dust and spraj^ applications. These 

 attributes comprise a standard for judging insecticides, and apply in 

 ])rinciple to fungicides as well. 



I. STOMACH POISONS FOR BITING INSECTS. 



Nearly all stomach poisons of the present time are compounds of 

 arsenic, and this has led to the general use of the term "arsenicals" for 

 this group of insecticides. Very little work has been done, and still less 

 has been published, in regard to the exact nature of the toxic action of 

 arsenic on the phj'sical structure of insects. The fact that this action 

 takes place and the rapidity of its killing effect upon the insects in question 

 have been practically the only points to which writers have hitherto given 

 their attention. 



The sprays consist of minute particles of the poison, suspended in the 

 water or other vehicle, which are deposited upon the food of the insect and 

 adhere to it upon drying. 



1. Arsenicals. 



There are two forms of arsenicals to be considered : — 



1. The lower or arsenous oxide, or arsenic trioxide (white arsenic) (AS2O3). 



2. The higher or arsenic oxide, or pentoxide (AsiOs). 



When these two oxides are combined with bases, the former yields the 

 so-called (a) Arsenites, and the latter (6) Arsenates. 



Arsenites as a class are noticeably more active poisons than the ar- 

 senates, but are relatively unstable and more likely to cause injury to the 

 plant, and for that reason they have been largely supplanted. 



Depending on the form in which the arsenic may be present, arsenicals 

 are sold on a guaranty in which the amount of arsenic (the active principle) 

 may be stated in the following terms: — 



Percentage of arsenous oxide or arsenic trioxide (AS2O3) or white arsenic. 

 Percentage of arsenic pentoxide (AS2O5). 

 Percentage of elemental arsenic (metallic arsenic). 



The first form is used in guaranties of Paris green, whereas either the 

 second or third form is used in stating guaranties of arsenates. Notwith- 

 standing the fact that the killing power of arsenates and arsenites. varies 

 in rapidity, and possibly in final extent, the percentage of metallic arsenic 

 seems to be the only common denominator by which to compare one 

 arsenical with another. 



(a) Arsenites. 



A number of arsenites have been placed on the market at one time or 

 another. Paris, Schweinfurt, or Emerald green, a well-kno^^^l poisonous 

 pigment, was first used about 1868 (1). ^ This date, therefore, marks the 



1 Numbers in parentheses indicate literature cited, which will be found on pages 35-37. 



