The Curious Case of Neem
Neem is a member of the mahogany family, Meliaceae. It is today known by the botanic name Azadirachta indica A. Juss. In the past, however, it has been known by several names, and some botanists formerly lumped it together with at least one of its relatives. The result is that the older literature is so confusing that it is sometimes impossible to determine just which species is being discussed.
Neem trees are attractive broad-leaved evergreens that can grow up to 30 m tall and 2.5 m in girth. Their spreading branches form rounded crowns as much as 20 m across. They remain in leaf except during extreme drought, when the leaves may fall off. The short, usually straight trunk has a moderately thick, strongly furrowed bark. The roots penetrate the soil deeply, at least where the site permits, and, particularly when injured, they produce suckers. This suckering tends to be especially prolific in dry localities.
Neem can take considerable abuse. For example, it easily withstands pollarding (repeated lopping at heights above about 1.5 m) and its topped trunk resprouts vigorously. It also freely coppices (repeated lopping at near-ground level). Regrowth from both pollarding and coppicing can be exceptionally fast because it is being served by a root system large enough to feed a full-grown tree.
The small, white, bisexual flowers are borne in axillary clusters. They have a honey like scent and attract many bees. Neem honey is popular, and reportedly contains no trace of azadirachtin.
Previous botanic names were Melia indica and M. azadirachta. The latter name (not to mention neem itself) has sometimes been confused with M. azedarach, a West Asian tree commonly known as Persian lilac, bakain, dharak, or chinaberry. The taxonomy of all these closely related species is so complex that some botanists have recognized as many as 15 species; others, as few as 2.
The fruit is a smooth, ellipsoidal drupe, up to almost 2 cm long. When ripe, it is yellow or greenish yellow and comprises a sweet pulp enclosing a seed. The seed is composed of a shell and a kernel (sometimes two or three kernels), each about half of the seed's weight. It is the kernel that is used most in pest control. (The leaves also contain pesticidal ingredients, but as a rule they are much less effective than those of the seed.)
A neem tree normally begins bearing fruit after 3-5 years, becomes fully productive in 10 years, and from then on can produce up to 50 kg of fruits annually. It may live for more than two centuries.
Neem is thought to have originated in Assam and Burma (where it is common throughout the central dry zone and the Siwalik hills). However, the exact origin is uncertain: some say neem is native to the whole Indian subcontinent; others attribute it to dry forest areas throughout all of South and Southeast Asia, including Pakistan, Sri Lanka, Thailand, Malaysia, and Indonesia. It is in India that the tree is most widely used. It is grown from the southern tip of Kerala to the Himalayan hills, in tropical to subtropical regions, in semiarid to wet tropical regions, and from sea level to about 700 m elevation.
As already noted, neem was introduced to Africa earlier this century. It is now well established in at least 30 countries, particularly those in the regions along the Sahara's southern fringe, where it has become an important provider of both fuel and lumber. Although widely naturalized, it has nowhere become a pest. Indeed, it seems rather well ''domesticated': it appears to thrive in villages and towns.
Over the last century or so, the tree has also been established in Fiji, Mauritius, the Caribbean, and many countries of Central and South America.-In some cases it was probably introduced by indentured laborers, who remembered its value from their days of living in India's villages. In other cases it has been introduced by foresters. In the continental United States, small plantings are prospering in southern Florida, and exploratory plots have been established in southern California and Arizona.
The tree is easily propagatedboth sexually and vegetatively. It can be planted using seeds, seedlings, saplings, root suckers, or tissue culture. However, it is normally grown from seed, either planted directly on the site or transplanted as seedlings from a nursery.
The seeds are fairly easy to prepare. The fruit drops from the trees by itself; the pulp, when wet, can be removed by rubbing against a coarse surface; and (after washing with water) the clean, white seeds are obtained. In certain nationsTogo and Senegal, for examplepeople leave the cleaning to the fruit bats and birds, who feed on the sweet pulp and then spit out the seeds under the trees.
It is reputed that neem seeds are not viable for long. It is generally considered that after 2-6 months in storage they will no longer germinate. However, some recent observations of seeds that had been stored in France indicated that seeds without endocarp had an acceptable germinative capacity (42 percent) after more than 5 years.
The tree is said to grow 'almost anywhere' in the lowland tropics. However, it generally performs best in areas with annual rainfalls of 400-1,200 mm. It thrives under the hottest conditions, where maximum shade temperature may soar past 50C, but it will not withstand freezing or extended cold. It does well at elevations from sea level to perhaps 1,000 m near the equator. The taproot (at least in young specimens) may be as much as twice the height of the tree.
Neem is renowned for good growth on dry, infertile sites. It performs better than most trees where soils are sterile, stony, and shallow, or where there is a hardpan near the surface. The tree also grows well on some acid soils. Indeed, it is said that the fallen neem leaves, which are slightly alkaline (pH 8.2), are good for neutralizing acidity in the soil. On the other hand, neem cannot stand 'wet feet', and quickly dies if the site becomes waterlogged.
Neem often grows rapidly. It can be cut for timber after just 5-7 years. Maximum yields reported from northern Nigeria (Samaru) amounted to 169 m3 of fuelwood per hectare after a rotation of 8 years. Yields in Ghana were recorded between 108 and 137 m3 per hectare in the same time.
Weeds seldom affect growth. Except in the case of very young plants, neem can dominate almost all competitors. In fact, the trees themselves may become 'weeds.' They spread widely under favorable site conditions, since the seeds are distributed by birds, bats, and baboons. For the same reason, natural regeneration under old trees is
Neem fruits. The olive like fruits can occur in large numbers on the tree. Animals like the fleshy outer part, but the seed in the center is the source of the ingredients people use most.
Neem is renowned for its robust growth and resilience to harsh conditions, but, like all living things, it has various shortcomings, some of which are discussed below.
By and large, most neem trees are reputed to be remarkably pest free; however, in Nigeria 14 insect species and 1 parasitic plant have been recorded as pests. 3 Few of the attacks were serious, and the trees almost invariably recovered, although their growth and branching may have been affected.
However, in recent years a more serious threat has emerged. In some parts of Africa (mainly in the Lake Chad Basin), a scale insect (Aonidiella orientalis) has become a serious pest. This and other scale insects sometimes infest neem trees in central and south India. They feed on sap, and although they do little harm to mature trees, they may kill young ones. Now that one type has been detected in Africa, the impact could be severe.
Other insect pests include the following: The scale insect Pinnaspis strachani (very common in Asia, Africa, and Latin America); Leaf-cutting ants Acromyrmex spp. (common defoliators of young neem trees in Central and South America); The tortricid moth Adoxophyes aurata (attacks leaves in Asia including Papua New Guinea); The bug Helopeltis theivora (considered a serious neem pest in southern India); and The pyralid moth Hypsipyla sp. (attacks neem shoots in Australia). Even though neem timber is renowned for termite resistance, termites sometimes damage, or even kill, the living trees. They usually attack only sickly specimens, however.
Despite the fact that the leaves contain fungicidal and antibacterial ingredients, certain microbes may attack different parts of the tree, including the following: Roots (root rot, Ganoderma lucidum, for instance); Stems and twigs (the blight Corticium salmonicolor, for example); Leaves (a leaf spot, Cercospora subsessilis; powdery mildew, Oidium sp., and the bacterial blight Pseudomonas azadirachtae);5 and Seedlings (several blights, rots, and wilts including Sclerotium, Rhizoctonia, and Fusarium). A canker disease that discolors the wood and seems to coincide with a sudden absorption of water after long droughts has also been observed. It has been suggested that a drastic lowering of the groundwater level around Lake Chadwhich nearly dried out during a drought in the Sahelwas the main reason for the outbreak. This is perhaps true; scale insects are usually 'secondary' pests that multiply best on plants that are already damaged by other pests or other adverse environmental factors.
A lack of zinc or potassium drastically reduces growth. Trees affected by zinc deficiency show chlorosis of the leaf tips and leaf margins, their shoots exude much resin, and their older leaves fall off. Those with potassium deficiency show leaf tip and marginal chlorosis and die back (necrosis).
Fire kills neem seedlings outright. However, mature trees almost always regrow, especially if the dead parts are quickly cut away. Seedlings regenerating beneath stands of neem are sensitive to sudden exposure to intense sunlight. Thus, clear-felling neem trees normally produces a massive seedling kill, especially if the seedlings are small.9 In some localities rats and porcupines kill young trees by gnawing the bark around the base. Even when not causing any physical damage, rodents can be pests: wherever they are numerous, the fruits may disappear before the farmer can harvest them.
Neem, with its intensely bitter foliage, is not a preferred browse, but if nothing else is available goats and camels will eat it. In fact, in Asia goats and camels have been known to browse young neem trees so severely in times of scarcity that the plants died.10 In Africa neem is generally ignored by livestock (which makes the tree easy to establish even within villages and courtyards). The reason that livestock treat neem differently in Asia and Africa is unknown at present. It may be differences in the tree specimens, or in the animals' preferences or past experiences.
Indeed, leaves could be used as a reserve fodder for camels, sheep, and goats. Despite their repellent bitterness, the leaves have a low fiber content and a high nutritional value (15 percent protein), comparable to that of leguminous leaves. However, there are also records of toxic effects of neem leaves on goats (Sudan).