Insecticides From Plants

Louis Feinstein.

More than 2,000 species of plants are said to have some value as insect killers. They belong to 170-odd families. Commercial insecticides of plant origin are found in five families : Nicotine in the Solanaceae family; pyrethrum in Compositae; derris, cube, and timbo in Leguminosae; hellebore in Liliaccae; and anabasine in Chenopodiaceae. Anabasine is also found in Solanaceae.

Who first discovered the insecticidal value of plants is not known. The Romans divided poisons into three groups, animal, plant, and mineral. They used two species of false hellebore in medicines and in rat and mice powders and insecticides. The Chinese discovered the insecticidal value of derris.

Chemists in the Bureau of Entomology and Plant Quarantine since 1927 have conducted research on the principal insecticides of plant origin, such as nicotine, nornicotine, anabasine, rotenone, deguelin and related rotenoids, quassin, and the pyrethrins. They also have worked on more than 450 plants in an effort to discover new sources of these and other insecticides, as well as attractants, repellents, and adjuvants. They have learned that many of the species in the 170 families do not warrant further investigation and that botanical classification is not a dependable guide in the search for insecticidal plants.

Plant insecticides are only a small fraction of the insecticidal material used each year. Yet in the development of new insecticides they deserve careful consideration: Often they are highly effective against many insect enemies that are not successfully controlled by inorganic insecticides. The plant insecticides often are relatively nontoxic to man and other plants. Poisonous spray residues on fruits and vegetables may menace public health. The relative safety of plant insecticides to man helps to maintain their continued use.

In this article I discuss the commercial plant insecticides and other plants that appear promising as insecticides. Included here are plants only of the higher orders (phanerogams). They are listed alphabetically according to plant family and genus. The plants are sufficiently promising to warrant intensive chemical and toxicological studies.

The lower orders of plants (cryptogarns) include the algae, fungi, mosses, ferns, and horsetails. A more complete study of them may also prove to be worth while.

Aesculaceae (Horsechestnut Family). Aesculus californica is called the California buckeye. The horsechestnut is a highly prized street and lawn shrub and tree. The common horsechestnut casts the densest shade of almost any cultivated tree. George H. Vansell and his coworkers in California found that bees feeding on buckeye blossoms became paralyzed and died. Reports of other investigators, however, show that the insecticidal value of species of the horsechestnut family varies.

Annonaceae (Custard-Apple Family). The genus Annona includes some go species of trees and shrubs, mainly in tropical America. S. H. Harper, C. Potter, and E. M. Gillham in England extracted Annona reticulate and A. squamosa seeds and roots with ether. The petroleum ether solution of this extract at o C. precipitated out an insecticidal material that was 5o to 100 times more potent than the original ether extract. Against some insects the concentrate had about the same toxicity as rotenone. More work should be done with the custard-apple.

Apocynaceae (Dogbane Family). Haplophyton cimicidum, the cockroach plant, has been used to combat cockroaches, flies, mosquitoes, fleas, lice, and other insects in Mexico. The dried leaves are toxic to the Mexican fruit fly. The water extract of the stems of plants grown in Arizona is toxic to adult house flies. The crude alkaloid from this plant is effective against most insects. It is as toxic as pyrethrum to the squash bug.

Bora,yinaceae (Borage Family). Heliotropium peruvianum. The borage family contains many well-known garden plants and often is called the heliotrope family. The compound heliotropine was one of the best chemicals tested against the body louse, being apparently nontoxic to the skin and lasting more than 168 hours when used in cocoa butter.

Tournefortia hirsutissima is used as a general insecticide in Haiti.

Cannaceae (Canna Family). Members of this family mostly have tuberous rootstocks, stately, broad leaves, and showy flowers. The leaves and stems of canna plants contain an insecticide that gives results similar to tobacco in greenhouse fumigation.

Celastraceae (Stag-Tree Family). Tripterygium willordil, the thunder-god vine, is a common insecticidal plant in southern China. The poison in it has been found in the root bark. Its chemistry has been investigated by M. Beroza, who reported that wilfordine is a mixture composed mainly of two similar alkaloids, a- and P-wilfordine. Both are insecticidally active ester alkaloids. Powdered fresh small roots are toxic to first-stage larvae of the codling moth, the diamondback moth, and the imported cabbageworm. Alcoholic extracts of the roots are more toxic. Small roots, powdered, are about half as toxic as pyrethrum to the American cockroach. The large and medium roots are nontoxic.

Chenopodiaceae (Goosefoot Family). Anabasis aphylla contains the alkaloid anabasine, closely related to nicotine. It is the only commercial source for the alkaloid. It grows mainly in Russia and is not available in the United States. Anabasis aphylla is related to the American tumbleweed. In this country my coworkers and I extracted anabasine from Nicotiana glauca.

Clusiaceae (Balsam Tree Family). Mammea americans is known as mamey, "mamey de Santo Domingo." Harold K. Plank of the Federal Experiment Station at Mayaguez, P. R., believes that this indigenous West Indian tree has greater insecticidal potentialities than any other plant he examined. The active principle in the mature seeds; the most toxic part, is a type of substance somewhat similar in composition and effect to pyrethrins. Plank found that six of the nine parts of the plant were appreciably or highly toxic to one or more insects. The bark has little toxic material.

Cochlospermaceae. Cochlospermum gossypium. Kutira gum increases the effectiveness of nicotine sulfate sprays. The kutira appears to be a synergist to nicotine sulfate in its action against the bean aphid.

Compositae (Thistle or Aster Family). This large family of plants includes thousands of herbs, vines, trees, and shrubs. The dahlia, chrysanthemum, coreopsis, marigold, aster, cosmos, and many other garden flowers are composites. To the dried flowers of Chrysanthemum cinerariaefolium the name pyrethrum is applied. Pyrethrum, a safe and effective insecticide, is widely used in household sprays. Four compounds exist in pyrethrumpyrethrins I and II and cinerins I and II. Pyrethrins are practically nontoxic to warm-blooded animals and can be safely used in the home.

Heliopsis scabra is called oxeye. M. Jacobson, at the Agricultural Research Center, discovered that these plants contain compounds toxic to the house fly. Nearly all the toxic material is extracted by petroleum ether. Jacobson purified the petroleum ether extract and named one of the toxic materials scabrin. W. A. Gersdorff and N. Mitlin, entomologists in the Department of Agriculture, reported that scabrin compares well with pyrethrum in killing value.

Cucurbitaceae (Gourd Family). The cucumber family is often called the gourd, melon, or squash family. Cucurbita pepo commonly is called pumpkin. Freshly cut pumpkin leaves rubbed on cattle and horses reputedly repel flies. Acetone extracts of pumpkin seeds killed mosquito larvae in experiments conducted by A. Hartzell and F. Wilcoxon of Boyce Thompson Institute.

Euphorbiaceae (Spurge Family). Croton tiglium contains croton oil. The plant is cultivated in China, where the seeds are the source of a homemade insecticide. The plant has insecticidal value against aphids. J. R. Spies, a chemist in the Department of Agriculture, reported that an acetone extract of the seeds was more toxic to goldfish than derris extract and that croton resin was more toxic than rotenone.

Ricinus communis, the castor-bean plant, is said to have some insecticidal value. If that is true, the insecticidal principle is present only under certain conditions with respect to variety, cultural practice, and environment. A valuable synergist is prepared from isobutylamine and undecylenic acid, which results from the chemical decomposition by heat of castor oil. By the action of sulfuric acid on castor oil, we get a useful emulsifier for insecticidal oils.

Flacourtiaceae. Ryania speciosa. The active principles of the plant are alkaloids and are effective in the control of the European corn borer. The roots and stems contain the insecticide, which is commercially prepared for use as dusts and sprays.

Fagaceae (Beech Family). Castanea dentata is called the American chestnut. F. W. Metzger and D. H. Grant found that a commercial dyeing and tanning extract of the American chestnut was a good repellent against the Japanese beetle.

Labiatae (Mint Family). Ocimum basilicum is known as common basil or sweet basil. Its oil killed 95 percent of the mosquito larvae tested at a concentration of 5o parts per million, but an extract made from the whole plant killed none. H. D. Hively obtained a patent in 1940 for the use of the plantas an insecticide. It is successful as a contact poison against flies, Colorado potato beetles, and many other insects.

Salvia officinalis, or garden sage. Salvias are grown for their flowers and for their leaves. The leaves of some species are used for seasoning. Hartzell and Wilcoxon found that acetone extracts of the leaves killed 8o percent and extracts of the roots killed 95 percent of the mosquito larvae they tested.

Leguminosae (Pea Family). The pea family is one of the most important group of garden plants in the world. Haematoxylon campechianum is called logwood. Hematoxylon is from the Greek for blood and wood, in allusion to the red wood. Metzger and Grant reported that two commercial extracts were good repellents against the Japanese beetle.

Millettia pachycarpa, fish-poison climber, is worth further investigation. The ground seeds kill several species of insects. Alcoholic extracts of the roots from China paralyze the bean aphid. The plant contains a large amount of saponin and rotenone. The plant acts as a contact and stomach poison when it is mixed with soap.

Mundulea sericea, or M. suberosa, is a promising insecticidal plant. It was discovered in the 1930's. It is a rotenone-yielding species. The plants from India are toxic, but those from various locations in Tanganyika and Zanzibar fall into two main divisions, those with smooth barks, which are toxic, and those with rough, corky barks, which are nontoxic.