﻿52 BULLETIN 1147, U. S. DEPARTMENT OF AGRICULTURE. 



soluble arsenical is used in combination with a fungicide than when 

 an insoluble arsenical is used. Accordingly it may be possible to 

 explain chemically the increased activity or efficiency when sodium 

 sulphid is used with arsenicals. 



Evidence seems to show that it is not always true that an insecti- 

 cide containing a high percentage of arsenic is more toxic than one 

 containing less arsenic, tor the reason that toxicity depends not alone 

 upon the amount of arsenic present, but also upon its form of combi- 

 nation. The insecticidal role played by the base itself is small and 

 sometimes nonexistent. 



When lime or Bordeaux mixture was combined with the arsenicals 

 the toxicity of the arsenicals was reduced. The fact that the addi- 

 tion of lime or Bordeaux mixture to the arsenicals reduced the 

 toxicity of these insecticides to insects may be explained in two ways: 

 (a) Leaves sprayed with the arsenicals combined with lime or 

 Bordeaux usually contained less arsenic than those similarly sprayed 

 with the arsenicals alone; (b) the toxicity was greater in the tests 

 with honeybees fed honey containing the arsenicals alone than in 

 tests in which bees ate honey containing the arsenicals with lime or 

 Bordeaux mixture. These results support the theory that the cal- 

 cium present prevents or counteracts the formation of soluble or 

 more toxic arsenic compounds. 



Based on the reported results, it would appear that if all seven 

 species of insects used had been tested under similar conditions, 

 their susceptibility to an acid lead arsenate would probably be in 

 the following order, beginning with the insect most susceptible: 

 Honeybees, silkworms, grasshoppers, potato-beetle larvae, tent 

 caterpillars, webworms (H. textor) , and web worms (H. cunea) . 



The arsenious oxid (" white arsenic") samples were crystalline; 

 the other commercial arsenicals generally lacked crystal outline and 

 were probably for the most part amorphous. The calcium arsenates 

 used contained some small " octahedral" crystals, but were largely 

 composed of apparently amorphous material. The arsenious oxid 

 samples gave variable results in the toxicity studies and wide varia- 

 tions were found in the results when calcium arsenates were used. 

 On the other hand, the amorphous acid lead arsenates and the 

 amorphous Paris green samples gave uniform toxicity data. The 

 data show a relation between the uniformity of the products and 

 uniformity of toxicity. Where the products were not uniform 

 variations in toxicity were found. 



Commercial arsenicals used for spraying or dusting purposes are 

 usually judged chemically on the basis of the total arsenious or arsenic 

 oxid contents and on the percentages of the total amount of these 

 oxids which go into solution under certain conditions. The per- 

 centage of base present is also determined. Soluble arsenic oxids 

 or arsenic rendered soluble after the application of arsenicals will 

 burn foliage, the extent of the injury depending mainly on the 

 amount of soluble oxid present or formed m the spray or solution 

 applied. The results here reported indicate that it is the soluble 

 arsenic or the arsenic rendered soluble by the insects that causes 

 death. The rapidity with which arsenicals are made soluble in the 

 bodies of insects seems to be the most important factor in connection 

 with their toxicity. What happens to the soluble arsenic inside the 

 insects is not known, except that part of it passes through the in- 



