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Crop Losses Caused by Pests and Their Assessment
Crop Losses Caused by Pests and Their Assessment |
Before studying the losses caused by insect pests to crops and methods to assess them, it is important to understand the terms pertaining to the topic. Some of the terms are discussed as under.
Efficiency of crop protection = 100 (Actual yield – yield without crop protection)/ (Attainable yield - yield without crop protection) Methods of estimating the crop losses: Estimation of crop losses caused by the pests is very important in pest management programmes. Estimation of pest damage is use ful in pest management in following ways:
Mechanical protection: The crop is grown in enclosures under protected conditions by using anti-insect nets or cotton cloths in order to keep the pests away. The yield obtained under theses enclosures is compared with that obtained from infested crop grown under similar conditions. This technique has been used with various modifications for estimating the crop losses caused by jassids and whiteflies. The limitation with this method is that the plants generally become week and pale in enclosures due to changes in micro-environment. Further, this technique can not be used on a large scale because it is time consuming and impracticable under field conditions. Chemical protection: In this case the crop is protected from pest damage by applying chemical pesticides. The yield of treated plot is compared with that of untreated which is exposed to natural infestation. This technique has been extensively used and can be employed on a larger area. Here care should be taken that the treated and untreated plots should be as identical as possible in respect of soil type, variety grown, fertilization and other cultural practices. The major drawback in this method is that the crop treated with chemicals may be physiologically affected and hence may vary in yield to some extent. Comparison of yield in different fields: In this case the yield of the crop is calculated per unit area in different fields having different degree of infestation. Correlation between crop yield and level of infestation is worked out to estimate the loss in yield. This technique can be used for estimating crop loss due to different pests over a larger area, however the soil heterogeneity may influence the yield. Comparison of yield of individual plants: In this case the yield of individual plants in the same field is measured and the average yield of healthy plants is compared with the plants showing different degree of infestation and the loss in yield is estimated. The data so obtained can also be used to work out the correlation between yield and infestation level on the bases of the yield of individual plants. This technique has been used with different modifications for the estimation of crop losses in different crops. In this case the soil heterogeneity is greatly reduced, however, plant to plant variation in infestation level may be there. Damage caused by individual insect: Preliminary information is obtained from studies on the biology of the pest. The details regarding the amount of damage caused by different stages of pest are worked out and the amount of loss is calculated. This technique is quite convenient in case of leaf feeding insects. However, it is difficult to use this technique over a large area because it is time consuming. Manipulation of natural enemies: Here the pest is controlled by introducing the natural enemies in to the field and the yield is compared with the plot without natural enemies. This technique is also feasible in a small area. Simulation of damage: In this method the pest injury is simulated by removing or injuring the plant parts. The simulated damage may, however, not always be equivalent to the damage caused by an insect. Insects may inject toxins in to the plant rather than producing injury instantly. Feeding on margins of the leaf may not be equivalent to the tissue removed from the centre of the leaf. Insect feeding is usually extended over a period of time and is rather difficult to incorporate the concept of rate of injury in simulating studies. Furthermore the period of leaf removed may be important, as for example the age, quality and position of the leaf on the plant. In addition the time of simulating damage with respect to the stage of growth is also critical. Simulated studies have been done on spotted boll worm in cotton in India. |
Last modified: Tuesday, 29 November 2011, 10:15 AM