178 MISC. PUBLICATION 273, U. S. 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 

 applied insecticides, their control is not a simple one owing to the 

 great difficulty of application (33, 81). 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 is from the air and 

 in recent years the airplane 

 has come into use for this 

 purpose. Dry dusts are 

 l3est 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 

 hopper is a sliding gate 

 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 flies back and forth 

 letting the strips of dust overlap slightly. The flying must be done 

 when wind movement is negligible and preferably when there is 

 dew on the foliage that will help to hold the dust. In some recent 

 dusting for the hemlock looper (-5^) (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 good conditions, an even distri- 



Figure 86. — Loading hopper and airplane used in 

 forest-dusting operations in Washington in 1931. 



