The action of neem products as pest control agents can be manifested at different levels and in different ways. This is a very important point to be noted since the farmer would be used to the “knock-out’’ effect of chemical pesticides. Neem extracts do not exhibit this type of effect on pests but affect them in several other ways.
Insect growth regulation
Regulation of the insects’ growth is a very interesting property of neem products which is unique in nature, since the products work on juvenile hormones. The insect larva feeds and as it grows, it sheds its old skin. This particular shedding of old skin is the phenomenon of ecdysis or moulting and is governed by an enzyme, ecdysone. When the neem components, especially azadirachtin, enter the body of the larva, the activity of ecdysone is suppressed and the larva fails to moult, remains in the larval stage and ultimately dies. If the concentration of azadirachtin is not high enough, the larva will die only after it has entered the pupal stage. If the concentration is lower still, the adult emerging from the pupa will be 100% malformed, and absolutely sterile.
Feeding deterrent
The most important property of neem is feeding deterrence. When an insect larva sits on a leaf, it will want to feed on it. This particular trigger of feeding is given through the maxillary glands. Peristalsis in the alimentary canal is thus speeded up, and the larva feels hungry and starts feeding on the surface of the leaf. If the leaf is treated with a neem product, because of the presence of azadirachtin, salanin and melandriol, there will be an anti-peristaltic wave in the alimentary canal which produces something similar to a vomiting sensation in the insect. Because of this sensation, the insect does not feed on the neem-treated surface. Its ability to swallow is also blocked.
Oviposition deterrent
Another way in which neem controls pests is by preventing the females from depositing eggs. This property is known as oviposition deterrence, and comes in very handy when the seeds in storage are coated with neem kernel powder and neem oil. The seeds or grains obtained from the market may already
be infested with some insects. Even these grains could be treated with neem seed kernel extract or neem oil. After this treatment, the insects will not feed on them. Further damage to the grains will be halted and the female will be unable to lay its eggs during the egg-laying period of its life cycle. There are also other known modes of action:
Ø The formation of chitin or the hard part covering the insect (exoskeleton) is inhibited;
Ø Mating as well as sexual communication are disrupted;
Ø Larvae and adults of insects are repelled;
Ø Adults are sterilised; and
Ø Larvae and adults are poisoned.
The use of neem products does not give immediate results, unlike chemical insecticides. Some patience is required after the application of neem products. Besides its insecticidal and nematicidal properties, neem is also a promising agent for control of plant diseases. It has also been demonstrated to possess anti-fungal properties. One of the problems with the use of chemical pesticides has been their impact on “non-target” species. Often they have proven harmful to various other species in the ecosystem that could be beneficial. However, neem extracts are devoid of these effects. Neem leaves and seed kernels, when incorporated into potting soil containing earthworms, increased the earthworm population by 25%. Neem products have proven to be remarkably benign to spiders and also other insects such as bees that pollinate crops and trees, ladybug beetles that consume aphids, and wasps which act as parasites on various crop pests. Neem products have to be ingested to be effective. Those insects which feed on plant tissues, therefore, easily succumb. However, natural predators like spiders feed only on other insects while bees feed on nectar. Hence they rarely come in contact with significant concentrations of neem products.
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