Insects and Weeds

Natural Pesticides

Martin Jacobson, research chemist, Insect Chemical Ecology Laboratory, Agricultural Environmental Quality Institute, Beltsville Agricultural Research Center, Agricultural Research Service.

From the time of the early Romans until 1900, only three plant-derived insecticides pyrethrum, hellebore, and nicotine have had widespread use. The discovery of rotenone and several plant-derived insecticides followed in rapid succession. Advances in chemistry and improved screening techniques have led to the discovery of many plant-derived insect toxicants, repellents, attractants, feeding deterrents, growth inhibitors, and sterilants.

Some of these compounds, produced by the plants as defenses against pests and pathogens, may be developed commercially from and or semiarid plants established as new crops. They would expand the range of available products for insect pest control as safe substitutes for some synthetic insecticides that are ecologically disruptive and for others to which insects have developed resistance.

Six Promising Plants

Six plants appear to be particularly promising. Not only are they good sources of insecticides and related chemicals, but they are adapted to areas that are marginal for production of traditional food and fiber crops and represent potential multiproduct crops for the United States. They are calamus (Acorns calamus L.), basil (Ocimum basilicum L.), big sagebrush (Artemisia tridentate Nutt.), chilcuan [Heliopsis longipes (A. Gray) Blake], mamey (Mammea americans L.), and neem. (Azadirachta indica A. Juss.). Although all six could be commercially viable, the neem tree is by far the most useful and likely to succeed. Much applied research on agronomy, commercial processing, and marketing is needed before commercial production of these species as sources of insecticides would be possible in the United States.

Calamus. This plant (also known as sweetflag) is a member of the family Araceae. It is a semiaquatic robust perennial that can also grow on dry land. Calamus is 5 to 6 feet tall, has a horizontal rootstock, and grows at altitudes from 3,000 to 6,000 feet. The plant grows wild in the United States from Florida to Texas and in Idaho, and in the various provinces of Ontario and Nova Scotia in Canada. It is propagated by division in the spring or autumn.

The large rhizomes are repellent or toxic to clothes moths, house flies, fleas, lice, mosquitoes, and many stored-grain insects. Beta-asarone, the component primarily responsible for these pesticidal effects, is highly effective against the rice weevil, which is probably the most damaging insect pest of stored grains in grain elevators. Beta-asarone would probably be very useful as a fumigant for protecting grain-filled storage areas without leaving residues on the grain after the areas are ventilated. Other potential uses of calamus compounds are for the treatment of tuberculosis, as a germicide, and in perfumery.

Basil. Sweet basil is a smooth or slightly pubescent aromatic herb with white or slightly purplish flowers. It was experimentally cultivated from seed by USDA in Virginia for several years in the 1930's. The seed was imported from France, and the whole fresh flowering herb was used for distilling the oil. The plants grow rapidly on clay soil improved by mulching with manure and reach a height of 13 to 16 feet. Good yields of the oil may be produced in many parts of the United States and sold at reasonable prices.

Neem-based insecticide is sprayed on a chrysanthemum to test its efficiency against leafminers.

In addition to its widespread use as a spice, sweet basil, which is a member of the family Lamiaceae, is recommended for use against gastric disorders, malarial fevers, and skin diseases, and for insect control. The oil distilled from the fresh flowers or the entire plant is used extensively as a flavor in confectionery, baked goods, condiments, and spiced meats, and as an aroma in certain perfume compounds. It is also an effective repellant and larvicide for mites, aphids, and mosquitoes; a growth inhibitor for milkweed bugs; and a fungicide. Although not all the repellent compounds have been identified, cineole, linalool, and methyl chavicol are implicated. Two compounds, designated as "juvocimene I" and "juvocimene II," are responsible for the growth inhibition of milkweed bugs.

Big Sagebrush. A member of the family Asteraceae, big sagebrush is a rapidly growing multibranched aromatic perennial that is the dominant plant of the Great American Desert. It has been in the West since 1881 as a fodder plant for range cattle and occurs wild in parts of Utah, Colorado, northern Nevada, and northern Arizona, where it reaches a height of 10 feet. The plant thrives in light, well-drained, dry, stony soils.

The leaves and shoots are placed in granaries to protect stored cereals from weevils and other pests, and the water in which they are steeped is used to kill or repel insect larvae, fleas, and locusts. Several species of ticks are killed rapidly by exposure to the vapors of the plant. An extract of the branch ends is exceptionally effective in preventing the Colorado potato beetle from feeding. (The beetle's resistance to insecticides in potato-growing areas is an increasingly serious problem, and this resistance probably will become widespread.) Various parts of the plant are used to destroy or expel parasitic worms. One compound responsible for preventing feeding has been identified as deacetoxymatricarin. Like many other species of Artemisia, big sagebrush contains absinthin, which is avoided by insects.