| Pest Management |
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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:
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. For Protecting Rice Crop 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. For Protecting Stored Crop 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. For Protecting Crops and Animals 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 in Europe and North America. Neem in extremely useful as an anti-feedent and ovipositional repellent for protection of crops like tobacco, groundnut, cotton and sweet potato from the damages caused by tobacco caterpillar or tobacco cutworm, a serious polyphagous pest of several crops in India. Neem products are quite effective against the larvae of a number of mosquito species which stop feeding and die after treatment. At present developing countries use expensive imported pesticides to control mosquito population. These countries can save a lot of money by using locally available simple neem products which are equally effective. Simple techniques such as throwing crushed neem seeds into pools and ponds in the towns and villages can prevent mosquito breeding. Experiments have shown that neem is also effective against fruit flies. Med fly, one of the most damaging horticulture pests, can be controlled by spraying neem solution under fruit trees. Neem has an advantage over the currently used pesticides. Whereas the conventional pesticides kill fruit flies as well as their internal parasites, neem products on the other hand, leave the biological-control organisms unaffected; they only kill fruit flies. This reduces, in fact, eliminates adverse, unintended effects. Neem is useful against gypsy moth, a pest which is causing severe damage to forests in parts of North America. Laboratory tests have shown that a very low concentration application of neem seed extract formulation, approved by the US Environmental protection Agency, can kill gypsy moths. Blowflies kill a large number of sheep in Australia. Experiments have shown that neem products can be useful in controlling blowflies. This opens up an interesting new line of neem application in animal husbandry. As mentioned above, neem products can influence about 400-500 insect species. So far we have concentrated on the effects of neem products on some of the important insects which cause severe damage to crops and animals. As can be seen from the discussion above, it is now established that neem and its products are highly effective against many pestiferous insects. And Against Non-insect Pests Too Research in recent years has shown that neem is quite effective against non-insect pests also. Thread worms are among the most devastating agriculture pests. These nematodes are very difficult to control. Use of synthetic nematicides is not desirable as they cause toxicological effects. Research has shown that these pests are susceptible to neem products. ‘Certain limonoid fractions extracted from neem kernels are providing active protection/defence against root-knot nematodes’. Water extracts of neem cake are also nematicidal. Neem cake is already being used on commercial basis by cardamom farmers in south India. Fungi attack plants and trees in numerous ways and forms. They cause massive damage to important crops such as wheat, rice and corn. Several tests have demonstrated that neem acts as a fungicide. Should this prove widely applicable, it would have enormous positive effects on agriculture, environment and food supply with highly valuable effects like reducing poverty, increasing production etc. on a global scale. Some tests have shown unusual and promising results neem-leaf extracts failed to kill the fungus Aspergillus flavus but completely stopped it from producing aflatoxin’. This is important because aflatoxin is a powerful carcinogen that is causing increasing concern regarding the world’s food supplies. Pest control, as practiced today in most developing countries relies mainly on the use of imported pesticides. This dependence has to be reduced. Although pesticides are generally profitable on direct crop returns basis, their use often leads to the contamination of terrestrial and aquatic environments, damage to beneficial insects and wild biota, accidental poisoning of humans and livestock, and the twin problems of pest resistance and resurgence. More than 500 arthropods pest species have become resistant to one or more insecticides. Resistance of the cotton bollworm, Helicoverpa armigera, in India and Pakistan, and of the Colorado potato beetle, Leptinotarsa decemlineata, in the USA to all available insecticides, and resistance of the diamondback moth, Plutella xylostella, to all classes of insecticides, including Bacillus thuringiensis, in Hawaii, Malaysia, the Philippines, Taiwan, and Thailand, illustrate the complexity of the problem. Shifts in pest status-from minor to major, and resurgence of pests, such as white flies, caused by direct or indirect destruction of pests natural enemies are other unwelcome developments associated with pesticide use. A World Health Organization and United Nations Environmental Programme report (WHO/UNEP 1989) estimated there are 1 million human pesticide poisoning cases each year in the world, with about 20,000 deaths, mostly in developing countries. The problem is rendered even more difficult because few, if any, new compounds are coming to replace old insecticides. The cost of developing and registering new pesticides is staggering almost US$ 60 million, and pesticide manufacturers are unwilling to risk investments on products whose market life could be shortened by development of pest resistance. For ecologically sound, equitable, and ethical pest management, there is a need for control agents that are pest-specific, nontoxic to humans and other biota, biodegradable, less prone to pest resistance and resurgence, and relatively less expensive. Among various options, neem has been identified a source of environmentally "soft" natural pesticides. Crop Pests Neem has had a long history of use primarily against household and storage pests and to some extent against crop pests in the Indian sub-continent. It was a common practice in rural India to mix dried neem leaves with grains meant for storage. Mixing of Neem leave (2-5%) with rice, wheat and other grains is even now practiced in some parts of India and Pakistan. Also, as early as 1930, neem cake was applied to rice and sugarcane fields against stem borers and white ants. Some innovative farmers in Karnataka and Tamil Nadu states in India even today "puddle" green twigs and leaves in rice nursery beds to produce robust seedling and simultaneously ward-off attack by early pests-leafhoppers, planthoppers, and whorl maggots. Controlled experiments confirmed that rice seedlings raised from seed treated with neem kernel extract or cake were vigorous and resistant to rice leafhoppers and planthoppers. Early observations that neem leaves were not attacked by swarming locusts were also confirmed in laboratory studies and attributed to neem's anntifeedant activity against locusts. Pest control potential of Neem The pest control potential of neem in developing countries, however, remained largely untapped due to the advent of DDT and other and other broad-spectrum synthetic insecticides. Also, wide publicity given to slogans such as "the only good bug is a dead bug" and identifying traditional uses of neem as backward, gradually influenced people away from using neem. It is only in the past decade, that the pest control potential of neem, which does not kill pests but affects their behaviour and physiology, has been recognized. Though subtle, neem's effects such as repellence, feeding and oviposition deterrence, growth inhibition, mating disruption, chemo-sterilization etc. are now considered far more desirable than a quick knock-down in integrated pest management programs as they reduce the risk of exposing pests natural enemies to poisoned food or starvation. In spite of high selectivity, neem derivatives affect ca. 400 to 500 species of insects belonging to Blattodea, Caelifera, Coleoptera, Dermaptera, Diptera, Ensifera, Hetroptera, Homoptera, Hymenoptera, Isoptera, Lepidoptera, Phasmida, Phthiraptera, Siphonoptera, and Thysanoptera, on species of ostracod, 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. Pest of Stored Products Postharvest losses are notoriously high in developing countries. Worldwide annual losses in store reach up to 10% of all stored grain, i.e. 13 million tons of grain lost due to insects or 100 million tons to failure to store properly. Dr. R.C. Saxena has recently reviewed the potential of neem against pest of stored products grain legumes, maize, sorghum, wheat rice and paddy, potato tubers. At farm level storage and warehouses, the application of neem derivatives to bags and stored grains has provided protection against insect pests. Powdered neem seed kernel mixed with paddy (1 to 2%) significantly reduced infestation and damage to damage to grain during a 3 month storage period; the effectiveness capacity jute bag (100 x 60 cm) controlled 80% of the population of major insects and checked the damage to wheat up to 6 months. The treatment with untreated control. The neem seed extract treatment was as effective as that of 0.0005% primiphos methyl mixed with the grain. Using this technology in Sind, Pakistan, high benefit-cost ratios were obtained by small, medium, and large-scale farmers. Effectiveness of Neem oil The effectiveness of neem oil alone or in combination with fumigation was evaluated against five major species of stored grain pests infesting rice and paddy grains in a warehouse trials conducted in the Philippines. Rice grain treated with 0.05 to 0.1% neem oil or treated with neem oil after fumigation with 'Phostoxin', and stored for 8 months had significantly less Tribolium castaneum adults than in untreated control. Both kinds of neem treatments were as effective as the bag treatment with 'Actellic' at 25ug/cm2 or grain treatment with Actellic at 0.0005%, and suppressed the pest population by 60%. The population build-up also was reduced when either fumigated or non-fumigated rice was stored in bags treated with neem oil at > 1 mg/cm2. Rhizopertha dominica, Sitophilus oryzae, Oryzaephilus surinmensis, and Corcyra cephalonica were similarly affected by neem treatments alone or in combination with prior grain fumigation. Fumigation and Phostoxin were effective only for about 2 months against R. dominica, and for up to 6 months against other pest species, while neem oil treatments were effective up to 8 months. Compared with the pest damage to untreated or fumigated rice, neem oil treatment significantly reduced the damage to rice grain. At 8 months after storage, weevil attacked grains in neem treatments were 50% of those in the fumigated rice and 25% of those in the untreated rice. Neem treatments also reduced the pest populations and damage in paddy. In studies conducted in Kenya, the growth and development of 1st instars of the maize weevil, Sitophilus zeamais, was completely arrested in maize grain treated with neem oil at 0.02%, while the weight loss of treated cobs was less than 1% as compared with a 50% reduction in weight of untreated cobs stored for 6 months. (Kega and Saxena 1996). While neem treatments cannot replace completely chemical pesticides used in stored products preservation, the amounts of pesticides needed could be reduced, thereby decreasing the pesticide load in food grains. With proper timing and innovative methods of application, their use could be integrated in stored products management. Blood-sucking Pests Ascher and Meisner have reviewed the effects of neem on hematophagous insects affecting humans and livestock. Application of a paste made from neem leaves and turmeric in 4:1 proportion to the skin cured 97% of the patients suffering from scabies caused by the mite Sarcoptes scabei in 3-15 d. Monthly sprays of ethanoilic extracts of neem or weekly bathing in azadirachtin-rich aqueous 1:20 'Green Gold' controlled the bush tick, Ixodes holocylus, and the cattle tick, Boophilus microplus in Australia, but were less effective against the brown dog tick, Rhipicephalus sanguineus. In Jamaica, neem kernel extract controlled ticks on cattles and dogs. Neem products repel and affect the development of mosquitoes. Two percent neem oil mixed in coconut oil, when applied to exposed body parts of human volunteers, provided complete protection for 12 h from bites of all anophelines. Kerosene lamps containing 0.01-1% neem oil, lighted in rooms containing human volunteers, reduced mosquito biting activity as well as catches of mosquitoes resting on walls in the rooms; protection was greater against Anopheles than against Culex. Effectiveness of mats with neem oil against mosquitoes has also been demonstrated; the vaporizing repelled mosquitoes for 5-7 h at almost negligible cost. The sandfly, Phleobotumus argentipes, also was totally repelled by neem oil, mixed with coconut or mustard oil, throughout the night under field conditions in India. Application of neem cake @ 500 kg/ha, either alone or mixed with urea, in paddy fields in southern India reduced the number of pupae of Culex tritaeniorhynchus, the vector of Japanese encephalitis, and also resulted in higher grain yield. Pest Resistance to Neem Materials? A few herbivorous insects, including Homoptera, Coleoptera, and Lepidoptera do survive on neem but, largely, it is free from serious pest problems. Although Taylor indicated that insects may possibly adapt to limonoid rather quickly, but Vollinger demonstrated that two genetically different starins of P. xylostella treated with a neem seed extract showed no sign of resistance in feeding and fecundity tests up to 35 generations. In contrast, deltamethrin-treated lines developed resistance factor of 20 in one line and 35 in the other. There was no cross resistance between deltamethrin and neem seed extract in the deltamethrin-resistant lines. Also, the esterase and multi-function oxidase enzyme activity did not change during the 35 generations. The diversity of neem allelochemicals and their combined behavioural and physiological effects on insect pests seem to confer a built-in resistance prevention mechanism in neem. However, wisdom demands that users should refrain from exclusive and extended application of single bioactive materials, such as azadirachtin.
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