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 (2/) 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 14 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. 
