In the prehistoric period, the cave man used the smoke of certain plant leaves as protective methods against mosquito bites. When it was discovered that only certain species of Anopheline and Culicine mosquitoes transmit malaria and filariasis and that they are ecologically dependent on specific conditions which are required for their perpetuation and survival, a planned anti-vector disease with control method would be required. However, long before the development of organic insecticides, natural substances derived from plants, the ‘first generation insecticides’ like pyrethrum, retenome and so on were successfully employed in insect control.
But, as a result of the introduction of new insecticides and other causes, the use of pyrethrum, retenome and other natural products was reduced and the discovery of synthetic second generation insecticides- DDT, HCH, Malathion and so on was an outstanding advancement towards the control of mosquitoes and other vector borne diseases in a more effective and economical manner in rural and other areas. But, synthetic organic insecticides although highly efficacious against target species of insects can be detrimental to a variety of animal life including man. In addition to its adverse environmental effects from conventional insecticides, most of the major mosquito vectors and pests have become physiologically resistant to many of these compounds on prolonged application.
Insect pests are a major concern in fields and warehouses. It is estimated that about 35% of crops all over the world are destroyed by them. They cause severe damage to stored grains and processed products by reducing their quantity and nutritional quality making them unfit for human consumption and agricultural purposes. Estimated loss of the world’s supply of stored grains from insect damage ranges from 5-10% of world production. The tropical countries alone suffer a loss of 20% due to unfavourable climatic and storage conditions. It is estimated that every year at least 500 million people in the world suffer from one or the other tropical disease that include malaria, lymphatic filariasis, schistosomiasis, dengue, trypanosomiasis and leishmaniasis of late chikungunya, a serious mosquito borne epidemic has gained momentum in India. These diseases not only cause high levels of morbidity & mortality, but also inflict great economic loss and social disruption on developing countries such as India and China. India alone contributes around 40% of global filariasis burden & the estimated annual economic loss is about 720 crore.
Due to great economic losses caused by stored grain pests, control of infestation in warehouses, factories, ships and mills is of main interest to food manufacturers and distributors. Around the world, residual chemical insecticides are mainly the method of choice for the control of stored grain insects. But, extensive use has made the strains resistant to many of them. In addition they result in environmental contamination and health problems. In overcoming these problems, biopesticides have gained immense importance in recent grain protection technology because of their medicinal, anti-fungal, antibacterial and insecticidal properties. Humans have used plant parts, products and metabolites in pest control since early historic times. Plants are the chemical factories of nature, producing many chemical, some of which have medicinal and pesticidal properties. By using plant parts in early historic times, man has been able to control certain pests with these remedies quite successfully.
Mosquito control is a difficult task due to a variety of factors including the development of insecticides resistant in target population, the high cost of new insecticides and concern over environmental pollution. While it is likely that chemical insecticides will continue to be required for mosquito control, an increased emphasis is being placed all over the world on the development of suitable alternatives to control vector borne diseases. Secondary metabolites are diverse natural products synthesized by the plants for their defence. Several mosquito species belonging to genera Anopheles, Culex and Aedes are vectors for the pathogens of various diseases like malaria, filariasis, Japanese encephalitis (JE), dengue and dengue hemorrhagic fever, yellow fever. Thus, one of the approaches for control of these mosquito borne diseases is the interruption of diseases transmission by killing or preventing mosquitoes to bite human beings.
Herbal products with proven potential as insecticide or repellent can play an important role in the interruption of transmission of mosquito-borne diseases at the larvicidal individual as well as at the community level. Some herbal products such as nicotine tabacum, anabasine & lupinine, the alkaloids extracted from Russian weed Anabasis aphylla, rotenone from Derris eliptica and pyrethrums from Chrysanthemum cinererifolium flowers have been used as natural insecticides even before the discovery, development and use of synthetic organic chemicals with persistent residual action not only overshadowed the use of herbal products against mosquitoes but also become the major weapon for mosquito control. This has necessitated the search for development of environmentally safe, biodegradable, low cost, indigenous method for vector control, which can be used with minimum care by individual and communities in specific situations.
An extensive review of phytochemical from plants has been published against mosquitoes. Members of the plant families- Asteraceae, Cladophoraceae, Miliaceae, Oocytaceae and Rutaceae possess various types of activity against many species of mosquitoes. Some important phytochemical products such as pyrethrum, clerris, quassia, nicotine, hellebore, anabasine, azadirachtin d-limonene, camphor and terpenes have been used as insecticides. These are major groups of insecticides of plant origin that were used in developed countries before the advent of synthetic organic insecticides.
In spite of the wide spread recognition that many plants possess insecticidal properties only a handful of pest control products directly obtained from plants are in use because of number of reasons. Botanical used insecticides presently constitute 1% of the world insecticidal market. With the view to discourage further aggravation of environmental pollution through the use of synthetic insecticides, it is imperative to explore the abundant natural plant resources and replace the intrinsically hazardous chemicals through natural plant products. Since the higher plants to be used as the agents for controlling insect pests and vectors for disease are associated largely with their innate quality of being systematic and relatively easily biodecomposable, hence their judicious exploitation, planned cultivation and appropriate preservation to use at the time of need should be the frontier area of research.
(Author is an Assistant Professor of Zoology, Department of Higher Education Jammu and Kashmir)
