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.
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. 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.
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.
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.