The organic phosphates hexaethyl tetraphosphate (HETP), tetraethyl pyrophosphate (TEPP), and diethyl p-nitrophenyl thiophosphate (parathion) are highly toxic to animals. In insects and in warm-blooded animals, they poison the cholinesterase.

A chemical called acetylcholine is formed in certain nerves and aids in the transmission of nerve impulses. If it is not destroyed immediately after it has served its purpose, it will continue to cause impulses to move along the nerve. The enzyme cholinesterase is always at hand to destroy the acetylcholine. The organic phosphate insecticides poison the enzyme, thus allowing the acetylcholine to accumulate, and cause uncoordinated nervous activity through the whole animal. The results are tremors, convulsions, muscle paralysis, and finally death. It is possible that the organic phosphates poison insects in other ways, but the action we described is the major one now known.

Another organic phosphorus compound that shows much promise for control of some insects and mites is schradan (octamethyl pyrophosphoramide). Many plants absorb it from the soil. Insects and mites that feed on the plant sap are poisoned. Schradan seems to have little effect on the cholinesterase system of insects; it is not particularly toxic when it is sprayed on them. But the fact that the sap of plants that have taken it up is highly poisonous to cholinesterase indicates that the mode of action is the same as that of the other phosphates only, however, after it has been changed in some manner by plant tissue. Animal liver cells also increase the anticholinesterase activity of schradan.

Of the cyanides used in controlling insects, hydrocyanic acid, or prussic acid, is a liquid that evaporates rapidly; calcium cyanide is a solid that gives off hydrogen cyanide gas more slowly. Both are classed as fumigants because the killing action is due to gaseous hydrogen cyanide.

Hydrogen cyanide is extremely toxic and acts quickly on all animals. In warm-blooded animals it poisons the enzymes that enable cells to use the oxygen supplied to them. As all living cells require a constant supply of oxygen, the failure of the supply results in the rapid and widespread poisoning of tissues that is characteristic of cyanide. The poisoning action of cyanide on insects is probably the same, for the enzymes involved are common to practically all living cells.

Methyl bromide, also used as a fumigant, is less toxic than hydrogen cyanide, and its poisoning action is much slower.

The mode of action of methyl bromide on insects has not been studied. Research with vertebrates has yielded two opposing theories. One states that methyl bromide is changed in the animal to methyl alcohol and a harmless bromine salt. The methyl alcohol then poisons the animal. Another theory proposes that the methyl bromide is not changed in the animal but poisons as methyl bromide. Whatever the mode of action may be in vertebrates, it will probably be similar in insects, for the effects of methyl bromide seem to be common to all animals.

Ten years ago we had a dozen or so insecticides and knew little about their modes of action. Today we have several dozen new ones and know nothing of how they act. Entomologists are gradually turning from trial-and-error ways of discovering new insecticides, however. These are being replaced by research on the fundamental aspects of poisoning action. Eventually we will be able to predict whether a chemical will be poisonous and to what insects. Then we can make insecticides to suit our needs.

JOHN J. PRATT, JR., is an entomologist in the Bureau of Entomology and Plant Quarantine. He has degrees from the University of Massachusetts, North Carolina State College, and Cornell University. During the war he served with the Army and the United States Public Health Service, and joined the Bureau of Entomology and Plant Quarantine in 1948. Dr. Pratt conducts research on the physiology of insects.

FRANK H. BABERS, a biochemist in the Bureau of Entomology and Plant Quarantine, has charge of research on insect physiology and the mode of action of insecticides.

Flea beetle.