The neem tree is one of the most versatile of India’s plants. Valued for centuries throughout tropical Asia for its multitude of medicinal and other uses, it has recently attracted attention in the United States as an effective botanical insecticide.
The umbrella-shaped neem, a member of the mahogany family (Meliaceae), grows to about 50 feet tall. It is generally evergreen, though in some areas it may be briefly deciduous. Its foot-long leaves are divided into 8 to 18 toothed leaflets which measure 1 to 4 inches long by 1/2 to 1 1/2 inches wide. Fragrant white flowers about 1/2 inch broad in branching groups crowd in the leaf axils. The thin-fleshed, egg-shaped yellow fruits measure about 3/4 inch long and contain a single seed.
The common name neem is derived from nimba, the Sanskrit word for this tree. Botanists know it as Azadirachta indica, the generic name coming from Persian words meaning “free” or “noble tree” and the species name being Latin for “Indian”.
Neem is native to India and much of tropical Asia, and is widely cultivated in the tropics and subtropics, particularly in arid regions. It has been introduced in much of Africa, and it is grown as a shade tree in the Middle East, especially Saudi Arabia. It is planted as a street tree in Haiti and is also cultivated in Guatemala, Cuba, and Nicaragua. It grows in Hawaii and Florida but rarely flowers in the latter state. In areas where temperatures dip below freezing, neem must be grown in a greenhouse or in pots which can be brought indoors during cold weather.
The neem tree thrives in most soils, even saline and alkaline ones, but it does better on dry, poor, rocky soils than on wet ground. The roots can penetrate a hard clay pan, which tends to increase soil fertility and helps to neutralize acidic soils.
Is there a part of the neem tree that hasn’t been found useful? The wood, durable and resistant to insect attacks, has been used for everything from furniture to boat oars, from agricultural implements to drums and carved images. Like its relative mahogany, it takes on a good polish.
The young, tender branches have been widely used in India and other countries as “chewing sticks” to keep the teeth and gums clean and healthy. Commercial toothpastes containing neem extracts are now available in India, Europe, and the United States. Limited clinical trials have shown neem toothpaste to be a potential treatment for gingivitis. Neem branches are also placed in stored grain to repel insects.
The bitter leaves and flowers are eaten as a potherb, and the fruit is also edible. In Indian folk medicine, the leaves are prescribed for many ailments, including intestinal parasites, swollen glands, bruises, sprains, and malaria. Leaf extracts have been shown to have antiviral activity and delay blood clotting (confirming their efficacy as traditional snakebite treatments), and the leaf essential oil has strong antibacterial and antifungal activity. Research on neem’s potential against malaria is now under way in Africa.
The bark of the neem tree yields a red dye which, however, is seldom used. The bark and root bark have been used to treat malaria, jaundice, and intestinal parasites, and to tan goatskins. The fruit has been used to treat urinary disorders and hemorrhoids.
The seeds yield a bitter, garlic-smelling fixed oil (margosa oil) which accounts for about 25 percent of their weight. This is produced commercially in several countries as an ingredient in dentifrices. Like the leaf oil, the seed oil has been shown to be antifungal and antiseptic, and it may also be contraceptive. But it is as an insecticide that the seed oil that has recently gained prominence. Even low concentrations of a constituent, azadirachtin, have been shown to be effective against a wide variety of insects, acting as a repellent and antifeedant and disrupting growth-regulating hormones. Different insects are affected to different degrees by these three modes of action, however. An attractive characteristic of neem insecticides is their very low toxicity to humans, plants, and other animals including certain beneficial insects. (The seed dust can irritate the lungs, but this would not likely be a problem in using a neem-based insecticide, where concentrations are low.) They break down rapidly in sunlight and in the soil. Because the insecticidal components of neem oil have defied economical synthesis, commercial plantations have been established to produce seed extracts for this use.
Neem-based insecticides currently registered by the Environmental Protection Agency (EPA) for use on ornamental plants include Margosan-O (sold only to the nursery trade by W. R. Grace of Baltimore) and the consumer products Bioneem and Azatin. Bioneem, manufactured by Ringer, will be available in American garden centers and hardware stores except in Arizona, California, and New York, where registration is pending. Azatin, manufactured by AgriDyne Technologies, will be available throughout the United States except in California and New York, and in South America. Also pending is EPA registration of Bioneem and a second AgriDyne neem product, Align, for use on food plants.