Of about five lakh plant species in the world today, the most useful one is the neem, Azadirachta indica A. Juss, which has been credited as a tree for solving global problems in view of its potential for use in agro-forestry and reforestation, improving soil fertility, excelling pest management and bettering health. Neem tree is supposed to originate in the forests of Karnataka (South India) or the dry island forests of Myanmar. Its natural distribution ranges from the Kanyakumari to Shivalik Hills in India. Now neem trees are found in 80 countries and its estimate is about 91 million trees. South Asian and Sub-Saharan regions constitute the main areas of distribution. Neem trees in India grow under different ecological regimes and very valuable forestry species. Of the several uses of neem, pesticidal value is the most important to ward-off pests and diseases on crops. Today more than 140 active principles have been identified, of which the most potent is azadirachtin. Neem derivatives affect about 500 species of insects. Neem's effects such as repellency, feeding and oviposition deterrence, growth inhibition, mating disruption and chemo-sterilization are more desirable than quick knock-down in integrated pest management programmes as they reduce the risk of exposing pests to natural enemies and leave no or untraceable residues in the environment. In fact, neem pesticides are commonly used to enhance the activity of native organisms in conservation biological control due to its bio-safety. NPV and Bt are highly compatible with neem products. In the case of parasitoids/predators, presampling and timing of application are necessary in order to avoid few ill effects of neem products on them. Neem is a perfect companion with other biopesticides in insect pest management.
S. Raguraman (2009) studied this question.