De, MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
larger airplanes for spraying extensive forests and with the develop- 
ment of newer types of slow-moving aircraft, such as the helicopter, 
for smaller areas and for forest plantations, it is being found practical 
to utilize more generally direct measures of control. 
The manufacture of cheaper and more effective insecticides and the 
use of concentrated spray mixtures would stimulate the practice of 
direct control. One of the newer products, DDT, offers much promise, 
since it is extremely toxic to most insects, both as a stomach poison and 
as a contact insecticide. Because of its residual properties, it is effec- 
tive over a considerable period of time. Before treating large areas 
with dosages of more than 1 pound per acre it is necessary to consider 
its effect on beneficial insects, fish, and wildlife in forested areas. 
Since hundreds of compounds are used as insecticides, the choice of 
any one depends on a number of considerations, such as the type of 
insect to be controlled and the purpose and allowable cost of the appli- 
cation. For convenience, these compounds may be divided into two 
classes, stomach insecticides and contact insecticides. SSome are most 
effective when applied as sprays and some give better results when used 
as dusts. Sometimes combinations or mixtures of insecticides and 
fungicides are used. 
STOMACH INSECTICIDES 
Stomach insecticides are taken through the mouth into the stomach. 
Leaf-feeding Lepidoptera, Coleoptera, and Hymenoptera are insect 
types for which stomach insecticides are used. In applying stomach 
poisons the material is sprayed or dusted on the parts of the plant that 
the insect eats. The most important inorganic stomach insecticides 
are the arsenicals, particularly lead arsenate, but a number of new 
materials are coming into general use. The dosages or concentrations 
vary with the insecticide and with the insect and its stage of develop- 
ment, the intensity of infestation, and many other factors. 
Ore eanic substances, such as the toxic principles of derris root, cube 
root, ‘tobacco, and hellebore, are sometimes used as stomach insecti- 
cides. Derris root and cube root are relatively nontoxic to warm- 
blooded animals and may be used with safety where there is danger 
of poisoning such animals. Some of the newer organic insecticides, 
such as DDT, benzene hexachloride, and chlordane, are very effective 
as stomach insecticides and also as contact insecticides. 
CONTACT INSECTICIDES 
Contact insecticides kill by penetrating into the breathing pores 
or sensory pores, or directly through the body wall of the insect. 
They are used for controlling sucking insects more often than for 
chewing insects. In applying certain ‘contact insecticides it 1s neces- 
sary for the material to hit the body of the insect, but with others, like 
DDT, it suffices to hit only the parts of the plant on which the insect 
is active. Examples of contact insecticides are DDT, benzene hexa- 
chloride, chlordane, nicotine sulfate, free nicotine, pyrethrum powder 
and extract, derris compounds, oil emulsions and miscible oils for 
dormant spraying, thin, miscible, white oils with unsaturated hydro- 
‘arbons removed for summer application, lime-sulfur, and sulfur dust. 
Nicotine sulfate and pyrethrum sprays are particularly useful for 
the control of aphids. Pyrethrum and DDT are more effective in kill- 
