FUMIGATION OF CITRUS PLANTS. 29 



citrus trees is seldom experienced, so extended is the range between 

 initial injury and death. 



It has been shown by the author {21) that the distribution of gas 

 beneath the tent is modified by the method of application; that in 

 pot-generated gas the greatest concentration is toward the top of the 

 tree, whereas in the case of gas generated from liquid hydrocyanic 

 acid the concentration is greatest toward the bottom of the tree. 

 Correspondingly the injury in the case of pot or machine generated 

 gas is most marked toward the top of citrus trees fumigated after 

 this method ; whereas, when liquid hydrocyanic acid is used, the ten- 

 dency is for greater injury toward the bottom. 



THE LENGTH OF EXPOSURE. 



Variations in the length of exposure very naturally modify the 

 effect of the gas on the plant, as early pointed out by Woods and 

 Dorsett {22), and recently clearly presented by Fernald, Tower, and 

 Hooker (-5) in experiments with tomatoes and cucumbers. These 

 latter writers performed experiments in which plants were entirely 

 uninjured when exposed to a certain dosage for 10 minutes but when 

 exposed to the same dosage for 2 hours the plants were killed. The 

 writer has performed many similar experiments with citrus trees and 

 reached the conclusion that for these plants very heavy dosages may 

 be safely used with exposure periods up to 20 or 25 minutes' dura- 

 tion, but where the period of exposure approaches or exceeds 40 

 minutes the injury is decidedly increased. 



Whereas short exposures to hydrocyanic acid have very little dele- 

 terious effect on plants, a correspondingly less destructive action to 

 insects occurs with short exposures than with long exposures. In 

 the commercial fumigation of citrus trees for scale insects the normal 

 exposure ranges from 40 minutes to 1 hour. Results under shorter 

 exposures with the dosages used have not proved entirely satisfac- 

 tory from the standpoint of killing the scales. Experiments by the 

 writer have shown that satisfactory results can be secured with 

 shorter exposures if an increased dosage is used. If, however, the 

 exposure with these increased dosages is greatly extended more 

 injury results. Since commercial outfits consist of from 30 to 100 

 tents, their movements under all conditions within fixed periods of 

 less than 40 minutes is scarcely practicable. At the present time it 

 is not uncommon for large outfits operating on the basis of an hour's 

 exposure to require 1^ hours for shifts or throws with damp tents on 

 large trees. Thus commercial orchard fumigation appears to resolve 

 itself into using dosages which will not injure trees even when the 

 length of exposure slightly exceeds an hour. An outfit consisting of 

 such a few tents that they would be operated unfailingly within short 

 periods could undoubtedly fumigate successfully with greater dosages 

 and shorter exposures than are now common to the practice. 



