Neem seeds and leaves contain many compounds which are useful for pest control. Unlike chemical insecticides, neem compounds work on the insect’s hormonal system, not on the digestive or nervous system and therefore doe not lead to development of resistance in future generations. These compounds belong to a general class of natural products called ‘limonoids’.
The liminoids present in neem make it a harmless and effective insecticides, pesticide, nematicide, fungicide etc. The most significant liminoids found in neem with proven ability to block insect growth are: azadirachtin, salanin, meliantriol and nimbin. Azadirachtin is currently considered as neem’s main agent for controlling insects. ‘It appears to cause 90% of the effect on most pests. It does not kill insects – at least not immediately – instead it both repels and disrupts their growth and reproduction. Research over the past years has shown that it is the most potent growth regulator and feeding deterrent ever assayed. It will repel or reduce the feeding of many species of pest insects as well as some nematodes. In fact, it is so potent that a mere trace of its presence prevents some insects from even touching plants.’
Certain hormones are necessary for growth and development of insects. These hormones control the process of metamorphosis as the insects pass from larva to pupa to adult. Azadirachtin blocks those parts of the insect’s brain that produce these vital hormones. As a result, insects are unable to molt. It is through these subtle hormonal effects that this important compound of neem breaks the life cycle of insects. The insect populations decline drastically as they become unable to reproduce.
Meliantriol and salannin act as powerful antifeedants. Nimbin as well as nimbidin (another neem component) have antiviral property.
But, for all the uncertainty over details, various neem extracts are known to act as various insects in the following ways:
- Disrupting or inhibiting the development of eggs, larvae or pupae.
- Blocking the molting of larvae or nymphs
- Disrupting mating and sexual communication
- Repelling larvae and adults
- Deterring females from laying eggs
- Sterilizing adults
- Poisoning larvae and adults
- Deterring feeding
- Blocking the ability to “swallow” (that is, reducing the motility of the gut)
- Sending metamorphosis awry at various stages
- Inhibiting the formation of chitin.
All these effects listed above are not equally strong or certain. Blocking the larvae from molting is considered to be neem’s most important quality which can be used to eliminate many pest species. Neem products are harmless to most insect eaters, humans and other mammals, except certain marine life like crabs, lobsters, fishes and tadpoles.
In spite of high selectivity, neem derivatives affect ca. 400 to 500 species of insects belonging to Blattodea, Caelifers, Dermaptera, Diptera, Ensifera, Hetroptera, Hymenoptera, Isoptera, Lepidoptera, Phasmida, Phthiraptera, Siphonoptera and Thysanoptera, one species of ostracad, several species of mites, and nematodes and even noxious snails and fungi, including aflatoxin-producing Aspergillus flavus. Results of field trials in some major food crops in tropical countries will illustrate the value of neem based pest management for enhancing agricultural productivity in Asia and Africa.
Neem’s effects on some of the major pests
Locusts (winged insects) are a great menace to crops and trees in Africa and Asia. The effects of ingredients and seed kernels of the neem tree on locusts and grasshoppers were studied in laboratory conditions, semi-field and field trials in Africa, Asia and Europe. There was very strong phagorepellent effect of neem oil on the desert locust and on the red locust. The same applied to the variegated grasshopper. The results showed that neem oil and other products (aqueous seed kernel extracts, neem seed powder) can be applied against some important locusts and grasshopper species in farmer’s fields with success.
Neem oil enriched with azadirachtin prevents locusts from developing into their migratory swarms that are so destructive to vegetation. Even doses equal to a mere 2.5 liters per hectare are enough to prevent formation of plagues of locusts. “Although alive, they become solitary, lethargic, almost motionless and thus extremely susceptible to predators such as birds.”
Grasshopper nymphs are affected by neem in a similar way. By applying neem products to soil or by using seeds soaked with neem products can protect some crops from locusts for a week to a month.
Neem seed extract has been shown to retard the growth of several cockroach species. It kills the young cockroaches and inhibits the adults from laying eggs.
De-oiled Neem cake (the residual remaining after the oil has been pressed out of the seeds) and neem oil are quite effective against rice pests. Five applications of a 25% oil emulsion sprayed with an ultra low-volume applicator can protect rice crops against brown plant hoppers. Neem products greatly reduce the tungo virus transmission efficiency of green leaf hopper in rice.
Neem is quite effective against armyworm, one of the most devastating pests of food crops in the western hemisphere. Azadirachtin in extremely low concentrations – a mere 10 mg per hectare – inhibits the pests.
Neem extract is useful against leaf miner, a serious pest in parts of North America. Neem seed extract works as well as available commercial synthetic pesticides. It has been approved by the US Environmental Protection Agency for use on leaf miners.
Experiments have shown that neem products are quite effective against European corn borer, a deadly pest which causes massive damage to corn and other crops.
For Protecting Stored Grains
One of the traditional uses of neem in Asia has been for controlling pests of stored products. Farmers usually mix neem leaves with grain before keeping it in storage for several months. Neem leaves, oil or extracts acts as repellent against several insects such as weevils, flour beetles, bean-seed beetles and potato moths. Treatment of jute sack by neem oil or azadirachtin-rich-products prevents the penetration of pest like weevils and flour beetles. Neem oil destroys bean-seed beetles (bruchids) – a variety of insects mostly attacking legumes – at the egg-stage itself. A mixture of neem leaves with clay and cow-dung develops pest resistant property so it can be used to make bins for storage of grain.