342 Report of the Department of Entomology of the 



injury to the foliage of elms or to potatoes and cabbage. Willow 

 leaves were only slightly spotted. 



conditions that accompany the injury to apple and grape 



foliage. 



Attention is called to several of the above mentioned observations 

 regarding the injury to foliage. The burning was prevented by lime 

 and was increased by the addition of molasses. It did not occur 

 immediately following the application of the poison, but only after 

 several days; and then apparently it was favored by wet weather. 

 As noted by Hartzell in the experiments on grape foliage, zinc arsenite 

 when combined with lime or bordeaux mixture caused no injury. 

 The same is true in the tests on apple foliage. The lime would 

 function as an absorbent of carbon dioxide and would also unite 

 with any soluble arsenic present, forming calcium arsenite. 



The occurrence of the injury during wet weather points to leach- 

 ing of the arsenic as a contributory cause. But the low water- 

 solubility of zinc arsenite, namely, one per ct. at a dilution of one 

 to one thousand by weight, together with the fact that several 

 days are required before the injury occurs, would indicate the neces- 

 sity of some solvent other than water or that the compound is altered 

 chemically before the burning takes place. The only suggestion in 

 these tests as to the cause for the injury to the apple and grape 

 foliage by zinc arsenite is the ease with which the material enters 

 into solution in dilute carbonic acid. It is generally assumed that 

 due to the process of respiration, carbonic acid is present at times 

 in small amounts on the leaf surface. It is possible that the com- 

 bined action of the wet weather and the solvent properties of the 

 carbonic acid are partly responsible for the damage. 



