160 



THE GYPSY MOTH. 



trees and especially that of peach trees is often destroyed 

 by an ap^jlication which will not injure the apple or many 

 other species. There appears to be a difference among 

 varieties of trees of the same species in their susceptibility 

 to poison, and some indiyiduals of the same variety seem 

 more susceptible to injury than others. This is also tme 

 of shrubs and garden plants. 



Causes of the Burning Effect. 

 We have already stated that the burning of foliage by 

 arsenites appears to be caused by the soluble arsenic in the 

 insecticide ; jet it is known that poisons like Paris green, 

 which are comparatively insoluble in water, will burn the 

 foliage under natural conditions if applied in great strength. 

 This seems to indicate that the arsenic on the leaf is dis- 

 solved more or less by dews and rains, the dehydrating 

 effects of the sun's rays, a moist atmosphere or the 

 exhalations from the leaf, or by all combined. We have 

 noted that although experiments with arsenites perfoiTued 

 in a dry atmosphere in the laboratory produced no burn- 

 ing effect, the same strength of material burned badly out 

 of doors. Professor Fernald's experiments in the insectary 

 indicate that Paris green burns more in warm damp weather 

 than in cool dry weather.* The larger the amount of 

 arsenic deposited on the leaf the greater will be the amount 

 eventually dissolved, and the greater the burning effect. If 

 the liquid is distributed evenly in minute drops over the leaf, 

 there is less danger of burning than if it is distributed in 

 large drops, or if the tree is so copiously showered as to 

 cause the arsenic to settle and accumulate in spots upon the 

 leaf. Wherever such accumulations occur, the burning is 

 the more serious and the injury spreads from the burned 

 spots. Therefore, there will be the least burning from a 

 fine spray, lightly and evenly distributed. 



How to prevent Burning the Foliage. 

 What should be done and what avoided to prevent or lessen 

 the burning of the foliage may be summarized as follows : — 



* Bulletin No. 19, Hatch Experiment Station, page 118, May, 1892. 



