NATURAL INSECTICIDES

The war against insects is a constant battl--no matter how hard we try to eradicate a pesty species with chemicals (insecticides), the species is never conquered. The nonaffected (resistant) individuals within a population are able to breed and thereby produce a new generation that is more resistant to the insecticide that was being used. Consequently, the dosage and frequency of application for that insecticide must be increased, or else something different must be used.

Insecticides have been used for centuries to fight unwanted pests. There are several natural (plant) insecticides that have been widely used, although compared with modern synthetics the plant substances are relatively weak. One benefit of a plant insecticide is that many of them are biodegradable. More than 1500 species of plants have been reported to have insecticidal value, and many more exist, but two products, rotenone and pyrethrin, have been economically important.

Rotenone.

Before World War I, agricultural nations were ignorant about the plants that contain rotenone. Rotenone was a mysterious and unidentified fish poison (barbasco) of the deep forests of South America, where natives collected roots of a viney shrub, Lonchocarpus nicou, and threw the crushed roots into small streams and pools. The chemical in the root stunned the fish and caused them to float to the surface, where the fish were easily collected. Humans were not poisoned by consuming rotenone, which is only toxic in very large doses. Use of rotenone as a fish poison became widespread in the 20th century, and it was immortalized in 1954 when a boat captain, played by the great character actor Nestor Paiva, just happened to have rotenone on board, which was used to stupefy the monster in Creature from the Black Lagoon.

The South American rotenone-bearing, leguminous plant is not known now from the wild. Nowadays, Lonchocarpus is cultivated by hand labor in tropical regions of Brazil and Peru. In the Far East, particularly in Java and Sumatra, a closely related legume, Derris elliptica, which also contains rotenone, was used as an arrow poison. Derris, which has been grown commercially in Puerto Rico, has lower yields of rotenone than does Lonchocarpus. Rotenone also occurs in the legume genus Tephrosia.

Rotenone is found in resin ducts, which occur in the phloem and xylem. The root is dried to 20% moisture content and then shipped to buyer countries. Rotenone is a terpene; it was applied as a spray on fruits and row crops, even several times before harvesttime, because the chemical residues do not linger. It is a potentially lethal toxin for aphids, cockroaches, houseflies, corn borers, Mexican bean beetles, and mosquitoes.

Pyrethrin.

Pyrethrin comes from a perennial daisy of the genus Chrysanthemum (not the cultivated mum, however). The biologically active chemicals are esters, which occur in the flower heads. Action of pyrethrin is as a contact poison, which paralyzes the insect victim, usually within 90 seconds. Normally the poison is administered in emulsion or dust form; as a dust it can be used with diatomaceous earth. The primary physiological reaction of the toxin is that ventral vagus ganglion shows vacuolization. Acute toxicity in mammals is relatively low, because the pyrethrin esters are converted in the stomach into nontoxic compounds.

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