Earlier Reports on Pesticide Activities
The activity of neem against locusts, though not well documented, has been well known to Indian farmers since very early times and some information about it is available in the earlier publications (). It was mentioned that locusts avoided feeding on leaves sprayed with crude extracts of neem and China berry. It was Robert Larson of Vikwood Botanicals, USA, who during his many business trips to India, brought to the notice of American scientific workers the property of neem against insects. This was the era when the use of synthetic pesticides was widespread, and more and more health hazards about them were coming to light, but no alternative was in sight. There was a need for safer and effective biodegradable pest control compounds with greater stability.
The Problems Created by Synthetic Pesticides
It was seen that the continuous and indiscriminate use of synthetic chemicals for the control of insects led to the following problems:
- Environmental pollution, as the chemicals brought about biochemical changes in the various organisms.
- Health hazards due to high residue levels.
- Indiscriminate destruction of insects, without any consideration of their beneficial or harmful nature.
- Poisoning of warm-blooded animals like birds, farm animals, fish and persons coming into direct contact with these.
- Development of resistance by insects.
- Resurgence of certain major and minor pests, which were earlier being dominated by the insects, which were destroyed by the pesticide. With their disappearance there was less competition and new pests appeared.
Pesticides from Plants
Keeping the above point in view, a search for a phytopesticide started; it was seen that about 2500 plants had one or more activities against insects but only neem was found to be a highly effective, non-toxic, and environmentally friendly agent for controlling insects by acting as feeding inhibitor and growth regulator (), and it was projected as the insecticide of the future for protection against field pests (). Thakur et al. () published a bibliography on neem.
Kubo and Klocke (), while looking for limonoids as insect controlling agents, isolated and identified azadirachtin as an antifeedant It was also observed that these limonoids prevented the completion of larval moulting by inhibiting the exuviae after the formation of new cuticle. These compounds did not kill the insects directly but lowered their growth rate and made them more vulnerable to other mortality factors. Jaipal et al. () also noted juvenile hormone-like activities in the bark of neem, and observed that the metamorphosis of the insect was inhibited to varying degrees by these. The use of purified extract of neem was suggested for pest control. Swaminathan () brought forward the potentiality of neem in pest control. Freeman and Andow () described the role of neem as a tree for protection of other plants as an insect feeding deterrent. Jacobson () gave details of its insecticidal activity.
With the above publications, the importance of neem became well known in the scientific world, and it became a topic of discussion at various international conferences. Schmutterer and Ascher () edited the proceedings of a conference which had research papers on the pesticidal activity of neem. Saxena () brought forward the use of neem as an antifeedant in pest management in the tropics and recommended quality control and standardization of its biological properties for introduction on a commercial scale. Kareem et al. () meanwhile observed that with the use of neem oil mixed with custard apple (Anona reticulata) in rice fields, virus incidence was significantly less, and the yield of rice was higher. Singh and Singh () also noted antiviral activity of leaf and bark extract of neem.
Azadirachtin as an Insecticide
Pest control aspects of neem were found to be useful in both developing and industrialized countries by Schmutterer (), who observed that azadirachtin and azadirachtin-containing neem extract acted as an antifeedant growth regulator and sterilant. The mode of action of azadirachtin may be due to interference with the neuroendocrine system controlling ecdysone and juvenile hormone synthesis and to inhibition of ecdysone release from the hormone-producing gland. In addition, azadirachtin causes inhibition of chitin synthesis. Azadirachtin was found to be an unstable compound, whose residual effect lasted for 4–8 days, but degradation may be hastened by ultra-violet light, rainfall and other environmental factors.
Jacobson (), under phytochemical pesticides, covered various aspects of this tree in the book edited by him, laying stress on its insecticidal and insect repellent properties. In this book, Ascher and Meisner () discussed the effect of neem on insects attacking various crops and also on beneficial insects like the honey bee. Warthen () compiled the literature on the pesticide activity of neem for the years 1979–1989. All the organisms, including arthropods, molluscs and nematodes, were covered. In a symposium on the insecticides of plant origin (), Saxena () described insecticide from neem, and Remboldt () gave an account of the structure and mode of action of azadirachtin. In another symposium, Klocke and Barnby () discussed azadirchtin as an antifeedant. Powell (), in yet another symposium on higher plants as a source of new insecticide compounds gave a detailed account of azadirachtin. Rovesti and Deseo () further discussed the potentiality of neem in pest control. Arnason and Philogene (), in memoirs of the entomological society of Canada, gave an account of plant-derived substances in insect control. Isman et al () studied variations in the azadirachtin content of twelve commercial samples of neem by their growth inhibition, antifeedant and moulting disruptive activity and concluded that the bioactivity of neem oil was dependent on its azadirachtin content. The possibility of a neem-based insecticide for Canada was discussed. Maramorosch () reviewed the current status of research, while Remboldt and Raychaudhuri () gave further details of the growth-inhibiting properties of azadirachtin. Champagne et al () described the biological activity of limonoids from neem and the other members of the Rutales family. Mordue and Blackwell () presented an update on azadirachtin. The potential and limitations of neem pesticide were reviewed by Soon and Bottrell (). The authors outlined the use of neem to control pests and its effect on nontarget organisms like the honey bee, earthworms, aquatic life, man and other warmblooded animals. In a workshop (), the production of neem ingredients, pheromones, and their effect on phytophagus insect pests, fresh water snails and pathogenic fungi were discussed. Remboldt () gave a further account of azadirachtin and its mode of action.
In India, Bambarkar () presented a table showing the activity of neem against twenty species of insect pests, ten nematodes and several fungi, while Subramanyam () reviewed literature on the growth-disruptive effect of neem on insects. Singh and Kataria () evaluated neem against insects, nematodes and fungi. Gujar () reviewed briefly the latest developments and suggested a need for standardization of formulations by using biological standards. An interesting observation was made by Gupta (), who isolated antifeedant microorganisms from the leaves and endosperm of neem, active only in sunlight. Tewari (), in his book Monograph on Neem, devoted a chapter to pest mangement, stressing its use in the practice of forestry. Saxena () suggested it as a source of natural insecticide under integrated pest management, Nagasampagai et al. () discussed bioefficacy, toxicity and compatibility of commercial products and Singh () bioactivity against insect pests. Randhawa and Parmar () edited the book Neem Research and Development. Sidhu () discussed the role of neem in pest management in forestry.
The above exhaustive studies confirmed the application of neem in the fight against pests. It was found to act on eggs, larvae/nymph and adults.
Mode of Action of Azadirachtin
The various studies showed that the mode of action () may be as follows:
- Antifeedant through mouth.
(a) Primary: it inhibits the activity of sensory receptors of mouth parts, distorts normal probing feeding and intake of food.
(b) Ingestion of active ingredients through food leads to starvation and death.
- Dermal action: it enters through the cuticle of the insects and inhibits chitin synthesis, thus causing desiccation and death.
- Repellent effect: due to change in the locomotor and settling behavior of insects, in some cases mating as well as sexual communication is disrupted.
- Growth-disruptive effect: by inhibition of the normal growth of the insect by interfering in the moulting cycle. It suppresses the activity of ecdysone so the larva does not moult, but remains at the young stage and dies.
- Effect on survival and reproduction by oviposition deterrent action: when the female comes to an egg-laying period of her life cycle, the egg laying is prevented.
- Effect on endocrine system: neem preparations are accumulated in the neurosecretory system and, by penetrating the blood brain barrier, are concentrated in the corpus cardiacum, resulting in reduced turnover of neurosecretory proteins.
Neem does not have an immediate knock-down effect like most of the synthetic chemicals and thus it is effective against those insects that have now become resistant to chemicals. It was also found effective against those pests that live concealed and well protected in the plant parts. Neem is not universal in its effect, which varies from insect to insect, lepidoptera being more sensitive to it, as compared to others.
Selections from the book: “Neem: The Divine Tree Azadirachta indica”. Edited by H.S.Puri. Series “Medicinal and aromatic plants – industrial profiles”. 1999.