Although thousand-year-old Sanskrit medical writings mention neem's usefulness, the tree's exciting potential for controlling insects has only recently become clear.
Neem's ability to repel insects was first reported in the scientific literature in 1928 and 1929. Two Indian scientists, R.N. Chopra and M.A. Husain, used a 0.001-percent aqueous suspension of ground neem kernels to repel desert locusts. Not until 1962, however, was the real significance demonstrated. That year, in field tests in New Delhi, S. Pradhan ground up neem kernels in water and sprayed the resulting suspension over different crops. He found that, although locusts landed on the plants, they refused to eat anything, sometimes for up to 3 weeks after the treatment. Furthermore, he noted that neem kernels were even more potent than the conventional insecticides then available and that neem's repellency was as important as its toxicity. In neighboring insecticide-treated fields, for instance, the insects also died, but not before consuming the crops.
Neem's insect-growth-regulating (IGR) effects were independently observed in England and Kenya in 1972. In England, L.N.E. Ruscoe, at that time an employee of the ICI Company, tested azadirachtin on insect pests such as cabbage white butterfly (Pieris brassicae) and cotton stainer bug (Dysdercus fasciatus) and noted IGR effects in each case. The azadirachtin was provided by D. Morgan, a Keele University chemist who had been the first to isolate azadirachtin. In Kenya that same year, K. Leuschner, a German graduate student working at the Coffee Research Station in Upper Kiambu, observed that a methanolic neem leaf extract controlled the coffee bug (Antestiopsis orbitalis bechuana) by growth-regulating effects. Most fifth-instar nymphs treated with the extract died during subsequent molts and the few that survived to adulthood had malformed wings and thoraxes.
Neem's fecundity-reducing effects were first recorded by R. Steets (another graduate student) and H. Schmutterer in Germany. Applying methanolic neem-kernel extract and azadirachtin to the Mexican bean beetle (Epilachna varivestis) and the Colorado potato beetle (Leptinotarsa decemlineata) they found that females almost stopped laying eggs. Some females had been completely sterilized, and the effect was irreversible.