Because some insecticides may fall into several categories when classified
according to mode of entry (many, for example, are both stomach poisons and contact
poisons), it is more convenient to classify them according to their chemical composition.
While differences exist between insecticides within a category, each category has
characteristics that are applicable, in general terms, to the insecticides in that group.
a. Inorganic Insecticides. Inorganics do not contain carbon, and they often
contain metals or other chemical elements. Most are stomach poisons. Examples
include boric acid, borate, silica aerogel, and sulfur. Others no longer used include
copper acetoarcinate, lead arsenate, mercury, and zinc phosphide.
b. Natural Organic Insecticides. Natural organic insecticides are derivatives
or refined forms of organic compounds occurring naturally. They may be botanicals
(from plants) or products from petroleum and coal tar.
(a) Pyrethrum. Pyrethrum is the term applied to the insecticidal
compounds of the flower heads of Chrysanthemum (Pyrethrum) cinerariaefolium. Four
separate compounds are included, of which Pyrethrum I is the most active. Commercial
pyrethrum extract is a yellow, oily liquid insoluble in water but soluble in a number of
common organic solvents. Pyrethrum is a powerful contact insecticide causing a rapid
paralysis or "knockdown" of the treated insects. Its efficiency is increased by the use of
a synergist such as piperonyl butoxide. Pyrethrum is effective as a direct contact spray
against adult flies, mosquitoes, and other flying household insect. It lacks persistence,
having virtually no residual action. Many insects recover after an initial attack.
Pyrethrum is one of the least toxic insecticides to mammals, having an LD50 value on
the order of 1,500 mg/kg. It breaks down rapidly, posing no environmental hazard.
A synergist is a compound which, when added to an insecticide, will
increase the insecticidal toxicity so that the amount of insecticide needed
can be decreased. Many synergists, such as piperonyl butoxide, MGK 264,
sulfoxide, and sesamin are used in fly and mosquito control, particularly in
aerosol bombs and space sprays.
(b) Nicotine. Nicotine, an alkaloid contained in the tobacco plant, is
obtained from the wastes of the cigar and cigarette manufacturing industries. It is a
dark, viscous liquid that has been used for years as a contact insecticide against soft-
bodied sucking insects. It is highly toxic to most insects, but it is volatile and, therefore,
nonpersistent. It is used as contact spray against sucking insects such as thrips,
aphids, mealy bugs, and scale on vegetation. It is one of the most deadly and rapidly
acting poisons known for higher animals by inhalation or dermal application, especially
through the tongue or eye. Nicotine sulfate, one of the most common formulations, has
an LD50 value of 83 mg/kg. Nicotine is not a standard military item.