Box 16.3. Neem
The neem tree, Azadirachta indica (family Meliaceae), is native to tropical Asia but has been planted widely in the warmer parts of Africa, Central and South America, and Australia. It is renowned, especially in India and some areas of Africa, for its anti-insect properties. For example, pressed leaves are put in books to keep insects away, and bags of dried leaves are placed in cupboards to deter moths and cockroaches. Extracts of neem seed kernels and leaves act as repellents, antifeedants, and/or growth disruptants. The kernels (brown colored and shown here below the entire seeds, after Schmutterer 1990) are the most important source of the active compounds that affect insects, although leaves (also illustrated here, after Corner 1952) are a secondary source. The main active compound in kernels is azadirachtin (AZ), a limonoid, but a range of other active compounds also are present. Various aqueous and alcoholic extracts of kernels, neem oil, and pure AZ have been tested for their effects on many insects. These neem derivatives can repel, prevent settling and/or inhibit oviposition, inhibit or reduce food intake, interfere with the regulation of growth (as discussed in section 16.4.2), as well as reduce the fecundity, longevity, and vigor of adults. In lepidopteran species, AZ seems to reduce the feeding activity of oligophagous species more than polyphagous ones. The antifeedant (phagodeterrent) action of neem apparently has a gustatory (regulated by sensilla on the mouthparts) as well as a non-gustatory component, as injected or topically applied neem derivatives can reduce feeding even though the mouthparts are not affected directly.
Neem-based products appear effective under field conditions against a broad spectrum of pests, including phytophagous insects of most orders (such as Hemiptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera), stored-product pests, certain pests of livestock, and even some mosquito vectors of human disease. Fortunately, honey bees and many predators of insect pests, such as spiders and coccinellid beetles, are less susceptible to neem, making it very suitable for IPM. Furthermore, neem derivatives are non-toxic to warm-blooded vertebrates. Unfortunately, the complex structures of limonoids such as AZ (illustrated here, after Schmutterer 1990) preclude their economical chemical synthesis, but they are readily available from plant sources. The abundance of neem trees in many developing countries means that resource-poor farmers can have access to non-toxic insecticides for controlling crop and stored-product pests.