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POTENTIAL OF SOME MEDICINAL PLANT EXTRACTS AGAINST STORED GRAINS INSECT PEST OF WHEAT




  • Wheat is the staple food of people of Pakistan and is grown over more than 8 million hectares with annual production of more than 20 million tons. Being a leading food grain of Pakistan, it occupies a central position in agricultural policies. It contributes 12.7% to the value added in agriculture and 2.6% to GDP. It was grown over about 8.414 million hectares during 2007-08 with a production of about 21.75 million tons. During the years 2008-09, Pakistan has harvested bumper wheat crop and the Govt. has not been able to purchase entire quantity of wheat, the growers offered for sale. Usually about 25-30% of wheat produced is procured by the government and stored for 6 to 12 months, in order to meet the food requirements of urban population of the country. Remaining 70-75% of the produce is retained by the farmers in the rural areas for meeting their food, feed and seed requirements, and also by the traders for storage on small scale levels.

    Luckily, in the field, wheat crop has no serious insect pest problem, which may warrant extensive control measures, however, during storage, most of the post-harvest damages are due to insect pests. The quantitative and qualitative losses to wheat occur due to huge number of insect pests. In Pakistan this important food grain has been reported to suffer post production losses to the tune of 15.3%. According to FAO estimate, 10-25% of world’s harvest is damaged annually by insects and rodents.

    Storage losses have been reported to vary significantly; 6% per annum, 3.6-25.5%, 4-10% due to insect pests, 3.5%, 2-6%,  3.24% and 1.23% weight loss of wheat due to insect pests. In qualitative losses the total soluble sugar, reducing sugar, non reducing sugarand starch contents of wheat, maize and sorghum grains were affected adversely at 25%,50% and 75% insects infestation caused by Trogoderma grainarium and Rhyzopertha dominica separately or by mixed population. Rhyzopertha dominica significantlyreduced available carbohydrates at 50% and 75% infestation. The insect species which infest wheat during storage, Rhyzopertha dominica (Fab.) is the most important and major insect pest.

    The red flour beetle is a tenebrionid beetle. It is a worldwide stored product pest. Red flour beetles attack stored grain products (flour, cereals, pasta, biscuits, beans, nuts, etc.) causing loss and damage. They may cause an allergic response but are not known to spread disease and cause no damage to structures or furniture. The red flour beetle is of Indo-Australian origin and less able to survive outdoors than the closely related species Tribolium confusum. It has, as a consequence, a more southern distribution, though both species are worldwide in heated premises. The adults are long-lived and may live for more than three years. The red flour beetle is a common and most destructive pest of stored products and is cosmopolitan in distribution. Both the adults and grubs cause serious damage to all kinds of grains including flour and dried fruits. This pest generally found in granaries, mills, warehouse, and stored grains. Neither larvae nor adults could generally damage sound grains but they could feed on those grains only which had already been damaged by other pests. In tropical countries like Bangladesh, the climate and storage conditions are favorable for insect growth and development. Among the coleopterous insect pest in storage grain pest are reported to be of Tribolium castaneum (Herbst), Rizopertha dominica (Fab), Trogoderma granarium (Everts) are the most important. The major populations of stored grain pest are reported to be of Tribolium granarium and Rizzopertha dominica other species came across in small numbers are Sitophilus oryzae and Trogoderma granariu.  

    These insect pests are mainly controlled by synthetic chemicals but because of their several ill effects and development of resistance against phosphine there is an urgent need to develop environment friendly alternatives having potential to replace highly toxic chemicals. Furthermore, there is an increasing dichotomy between the demands of the first world for quality food uncontaminated by insecticidal residues, and desperate need of third world population to maintain level of food security. These treatments due to their residual effects are toxic and continuous applications of such chemicals lead to environmental pollution and health hazards, besides developing resistance in organism.

    These problems have highlighted the need for development of new types of selective insect control alternatives. Plants may provide potential alternatives to currently used insect control agents because they constitute a rich source of bioactive chemicals. These are often active against a limited number of insect species including specific target insects, biodegradable to non-toxic products and potentially suitable for use in integrated pest management, they could lead to the development of new classes of safer insect-control agents.

    Plant products like essential oils, possess a broad spectrum of pest control properties and have been widely investigated for their larvicidal, toxic, repellent,ovicidal, antifeedant and anti-ovipositor effects. These plant derivatives are safe to man and other non target organisms and friendly to the environment. At least 46 families of flowering plants have insecticidal activities. Plant extracts are considered to be non-pollutant, less toxic and easily bio-degradable. Certain plants have already been reported to possess repellent action against stored grain pests.

    The use of plant materials as traditional protectants of stored products is an old practice used all over the world. The protection of stored products generally involves mixing grains with plant-based protectants. In addition to high toxicity to insects, many natural products are also repellent or attractive. Plants may provide potential alternatives to currently used insect-control agents because they constitute a rich source of bioactive chemicals. Plants derived insect-control agents could be developed into products suitable for integrated pest management because many of them are selective to pests, have no or few harmful effects on non-target organism and the environment, act in many ways on various types of pest complex and may be applied to the plants in the same way as other agricultural chemicals. The plants studied during the last 20 years have inferred that neem Azadirachta indica extracts and its compounds have proved best for the control of insect pests and pathogens as well. Neem provides better control of the larvae at their different developmental stages particularly soon after emergence from eggs, while the mid stage instars and late instars are affected less. Neem has no adverse effect on the predators, including lady beetles, lacewings, spiders and predatory bugs. The insecticidal activities of neem against different insect pests have also been evaluated. The leaf extract of neem as the most effective repellent, against Tribolium castaneum on groundnut seeds. The safety assessments for various neemderived preparations and compared the outcome with the ingestion of residues, on the food, treated with neem preparations.

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