178 MISC. PUBLICATION 273, U. 8. DEPT. OF AGRICULTURE 
would not be adequate for the spraying of trees over 90 feet in 
height or for reaching out into inaccessible forest areas. 
AIRPLANE DUSTING 
Although most defoliating insects, can be easily killed by properly 
apphed insecticides, their control is not a simple one owing to the 
great difficulty of application, (33, 57). Outbreaks of forest defolia- 
tors often cover large areas in inaccessible terrain and affect some 
of the tallest forest ‘trees that cannot possibly be reached with any 
dusting or spraying apparatus operated from the ground. For 
these reasons the only fea- 
sible means of applying 
insecticides to large forest 
areas 1s from the air and 
in recent years the airplane 
has come into use for this 
purpose. Dry dusts are 
best adapted for this pur- 
pose, but in some recent 
orchard work liquid sprays 
have been used. Calcium 
arsenate is the dust most 
frequently used as a stom- 
ach poison for airplane 
dusting against leaf-eating 
insects. 
Airplanes for dusting are 
equipped with a hopper to 
carry up to 1,000 pounds of 
dust (fig. 86). Inside of 
the hopper is an agitator to 
keep the dust stirred up, 
and at the bottom of the 
Figure 86.—Loading hopper and airplane used in hopper IS 4a shding gate 
forest-dusting operations in Washington in 1931. that can be opened and 
closed from the pilot’s seat. 
This opening allows the dust to be discharged into the air stream, 
which catches it and whirls it groundward in a vast cloud. The 
airplane is required to fly very low (fig. 87), and when it is flying 
at a height of 40 feet above the tree tops the dust cloud settles 
over a width of about 150 feet. The airplane fiies back and forth 
letting the strips of dust overlap slightly. The flying must be done 
when wind movement is neghgible and preferably when there is 
dew on the foliage that will help to hold the dust. In some recent 
dusting for the hemlock looper (54) (fig. 87) the dust was applied 
at the rate of 20 pounds per acre, but in heavy forest stands the 
dosage must be increased up to about 60 pounds per acre. The cost 
of the work ranges from $3 to $6 per acre and this expense is justi- 
fiable only for the protection of particularly valuable timber. 
Airplane dusting has certain disadvantages that prevent its general 
adoption as a satisfactory method of controlling forest defoliators. 
It is expensive and very dangerous, since the airplane must fly 
so close to the tree tops. Even under ood conditions. an even distri- 
