sets of test conditions. Like LD50 values, LC50 values are approximate values obtained
on the basis of laboratory tests.
Section II. INSECTICIDES
CLASSIFICATION ACCORDING TO MODE OF ENTRY
There are three types of insecticides when classified according to the mode of
entry into the insect's (arthropod's) body.
a. Oral. Oral poisons must be swallowed in order to kill the insect. They are
used against insects with chewing, sponging, or lapping mouthparts. These insecticides
are usually applied in the form of dusts or sprays to vegetation or other natural foods
eaten by the target insect. The insect consumes the insecticide when it eats the foliage
or when it cleans appendages to which the insecticide has adhered through contact with
treated surfaces. Stomach poisons may also be mixed with baits that are more
attractive to the insects than natural foods. A satisfactory stomach poison must be
quick acting, inexpensive, and available in large quantities. It must be palatable to the
target insects, or they will avoid it. The insecticides used as stomach poisons are
chiefly the inorganic chemicals (para 1-6a) and some of the chlorinated hydrocarbons
b. Dermal. Dermal poisons kill insects by contacting and entering the body
either directly through the body wall and into the blood, through the mouthparts and into
the digestive system, or through the respiratory system. These insecticides are used
primarily against insects with sucking mouthparts, which would not eat normally applied
stomach poisons. However, they are also effective as stomach poisons if eaten by
insects with chewing mouthparts. Contact poisons may be applied directly to insects'
bodies as sprays or dusts, or they may be applied for residual action on surfaces with
which the target insects will come in contact. Contact insecticides in common use
include the natural organic (para 1-6b) as well as the synthetic organic (para 1-6c)
c. Respiratory. Respiratory chemicals are volatile chemicals that kill by
entering a pest through the respiratory system. They are used in gaseous form or as
solids or liquids which rapidly vaporize forming poisonous gases. They are particularly
appropriate for killing insects in stored products where the gas will penetrate cracks,
crevices, and tightly packed material. They are extremely toxic to all animal life;
therefore, they are also effective as rodenticides. Their extreme toxicity makes
fumigants particularly hazardous to use; accordingly, their use is restricted to trained
and certified personnel. They are discussed in Section IV.