Circular 21 - Insecticides and Subsidiary Materials 6 
applied v/ith mechanical devices called dusters 
(See Circular 20), of which several efficient 
types are available. 
Fumigants are materials which give off 
poisonous gases, fumes, or vapors and for this 
reason are used chiefly to fumigate enclosures 
such as rooms, houses, vaults, storage ware- 
houses and the like vvhere the gas can be con- 
fined with the insects and the products. 
Fumigants are also employed to kill soil and 
wood-boring insects. 
Frequently, because of their pronounced 
odor, taste^ or other qualities, certain sub- 
stances are useful in repelling the attack of 
insects, or relieving the effects of their 
stings or bites. These include certain essen- 
tial oils and their constituents. They are 
generally derived from plants and usually have 
a very pungent odor characteristic of the plant, 
Common examples are camphor, cedar oil, oil of 
citronella, menthol, oil of peppermint, and oil 
of wintergreen. 
In the several Defense Circulars dealing 
with the various insects and pests that may be 
encountered in military camps or posts, defi- 
nite recommendations are given for the control 
of each insect or pest under consideration. 
Specific information is given also on the kind 
of insecticide to employ, the dilution to use, 
eind the manner and time of application. 
This circular deals with the nature of 
the chemicals or insecticides that are used in 
the control of insects discussed therein. 
Since the uses of these materials are given 
only in a generalized way, the circulars aeal- 
ing with specific pests should be consulted 
before treatment is attem"Dted. 
