Similar insects infest wheat during storage in the public sector sheds and the farm level. However, the population dynamics of different insect species varies with the factors affecting storage.
Rice weevil (Sitophilus orzae L.) is the dominant pest of stored wheat causing grain damage from 2-5 percentage. Most damage is caused during Monsoon season plus the couple of months following monsoon. It feeds internally, reducing the weight and degrading the quality of the grain. For instance the grain may become humid, hot, and unfit for human consumption.
Lesser grain borer (Rhyzopertha dominica) is also a destructive pest causing damage throughout the country. Adults and larvae feed inside the grain. This reduces the weight and degrades the quality. The lesser grain borer is most abundant in humid climates and whenever the moisture content of wheat is high.
Khapra beetle (Trogoderma granarium) is a widespread but sporadic pest. It causes extensive damage in conditions of high humidity and high moisture content. Red flour beetle (Tribolium castaneum) and Rice moth (Sitotroga cerealella) also cause significant damage to wheat.
Pakistani farmers attempt to control insects using sun drying, applications of available insecticide, phosphine producing compounds (e.g., Phostoxin), elemental mercury and neem, a natural material of plant origin The use of pesticides is more common in the irrigated areas where 13 percentage of farmers use insecticides and fumigants and 41 percentage treat the grain with mercury. Although some degree of control seems to have been achieved, most chemical treatments are unsatisfactory and can be dangerous to health. Moreover, the widespread and uncontrolled use of pesticides waste scarce resources when treatments are ineffective. The exposure of insect pests to sub-lethal doses may promote resistant strains of pest species.
The amount of grain lost to rodents provides further evidence of the need to control field infestations of rodents. The rat damage to wheat in upland valleys of both wet and dry mountains, where Bandicota bengalensis is a serious pest, has been estimated at 6.0 percentage in post harvest system. Some reports indicate that loss due to rats have been projected as high as 4-6 percentage and as low as one percentage. The private MICAS Associates in 1976 estimated 2.3 percentage wheat loss from rats at the farm level. Studies conducted by the FSM Project of USAID indicated that the rodent infestations at the village level and in the town market measure less than five percentage.
The use of tracking dusts of zinc phosphide (5 and 10 percentage), racumin (0.75 percentage) and liquid warfarin (0.025 percentage) was highly effective in reducing the populations by 80-90 percentage. With the use of these compounds 10-20 percentage greater yields can be achieved with a 50-fold return on the cost of investment.
4.2.1 Traditional Pest Control Methods
In south Asia the following are the most important methods practised at farm level during wheat storage:
Sun drying: The sun drying is the single most popular method of moisture reduction and pest control. Luckily the temperature during and after harvest of wheat provide for the initial kill of insects and reduce moisture in the grain. This helps to delay infestation of insects and formation of mould. The effectiveness of this method for small and large farmers alike is equally good. Small farmers are more efficient in drying their grain in small-scale storage. After 2 or 3 months of storage, they kill insects which might have developed during this period and eliminate any insect problem once they have carried out sun drying in August/September.
Use of Mercury: The use of mercury is a local tradition in South Asia for insect prevention in storage particularly in the Punjab provinces of Pakistan and India and nearby districts. Despite its potential hazards farmers have adopted the practise. No studies have been conducted to demonstrate the toxic effects of mercury on human and animal health.
Use of Neem: The neem tree (Azardirachta indica) is native to the Indo-Pakistan sub-continent and grows abundantly in this region. Neem trees are plentiful in South Asia and certain other developing countries where farmers are aware of its properties. In a diagnostic survey, it is reported that food grain is mostly stored in gunny bags in which farmers sometimes mix dried neem leaves. Those who store wheat in mud bins, rub fresh neem leaves on the inside walls of the bins. In the districts of Nawabshah and Khairpur, in Pakistan Palli is commonly used for storage. Some farmers plaster its walls and top with mud having crushed neem leaves. In Rahim Yar Khan District, neem extract is sprinkled on the wheat straw packed at the bottom of Palli before placing the grain. In other areas, farmers treat storage bags with neem extracts before putting in the grain. Farmers presently utilise neem, mixing whole neem leaves with grain in gunny bags or in earthen containers. They also use ground neem leaf paste mixed with mud used for making mud bins. Empty gunny bags are soaked overnight in water containing 2-10 percentage neem leaves on a weight/volume basis, and the grain is stored in these bags after drying them. Most farmers rate the first method to be superior.
Considerable research has been undertaken on the properties of neem as grain protectant. However, most of this research has not been adopted for practical application on larger scale. The water extract of neem leaves was highly repellant to major stored grain insect pests. The neem seeds compared with the leaves, flowers, and fruits exhibited the maximum potency. Tests show that flour beetle, fed on neem seed extract treated at the rate of 800 ppm, failed to produce viable progeny; and their feeding was greatly reduced. Based on experiments, it is reported that 20 percentage of water extract of neem leaves can block the penetration of insects into treated bags (paper or cloth) for at least 6 months during storage.
4.2.2 Chemical Pest Control
The majority of farmers in developing countries belong to the subsistence group and often cannot afford the costly modern grain protectants. Fumigation with toxic gases is most effective in airtight structures and is only economical if carried out on a commercial scale. Even if properly applied, the fumigated grains are still liable to frequent re-infestation by insect pests. This technology is not yet applicable to the farm level in Pakistan because storage structures are not airtight and located inside or near residential areas where fumigation may be dangerous.
The admixture of insecticide dusts with grain can provide protection against insects, but pose a danger from their persistent harmful residues. Breeding of resistant strains of insects cannot be explicitly prevented nor can the high cost of environmental pollution be ignored. Moreover, application of insecticides requires sophisticated techniques and complicated calculations, which farmers cannot easily comprehend.
However, there are no traditional methods adopted for pest management, in the public sheds. Local market dealers or agents procure wheat directly or from the government. Wheat is transported using private trucks to the food department sheds, which can be privately hired storage facilities.
The storage loss studies and the socio-economic surveys provided the justification for a pilot-scale program of loss reduction. Since insects were the major cause of storage loss, the loss reduction activities focused on finding ways of successfully fumigating farm grain stores. Alternative methods of insect control, such as the admixture of insecticides with grain could not be considered since appropriate formulas are not available in Pakistan. The design of the local metal bin was modified in consultation with PARC's agricultural engineers to assist the manufacturers and farmers to produce a much stronger bin, more suitable for fumigation. The fumigation of small quantities of grain in bags was also tested in villages. The bags are enclosed in a polythene envelope, which is sealed after introducing the phosphine-producing compounds. If the polythene sheet is left in place after treatment, the risk of cross-infestation is significantly reduced.