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.

  • Yield: Yield is a measurable produce of economic importance from a crop. The yield may be evaluated in terms of quantity and/or quality. The distinction between total yield and marketable yield is important if marketability and /or value of the crop is determined by its quality i.e. commercial class or grade.
  • Production: It is the total amount harvested in a country or in a region in a year.
  • Theoretical yield: It is the yield obtained from a crop grown under most favourable conditions. There is no factor limiting the yield potential of the crop, only genetic factor are limiting.
  • Attainable yield: It is the site specific maximum yield that can be obtained under the geographic and ecological conditions at a location using best production techniques to avoid biotic stress. It is determined by factors like climate, latitude, and variety grown.
  • Actual yield: It is the site specific yield obtained when the crop is grown using available cultivation and plant protection practices at a particular place.
  • Crop loss: A crop loss is any reduction in quantity and/or quality of yield and is the equivalent of ‘damage’ Crop loss is measured as the difference between actual yield and attainable yield due to the effect of one or more pests. The term yield loss should be restricted to describe the reduction in yield caused by a single pest.
  • Direct losses: These relate to the decrease in productivity (quantitative) or intrinsic value/ acceptability of the produce (qualitative). Direct losses include killing of flowers, buds, fruits, twigs or whole plant due to the attack of insect-pests. The examples of indirect qualitative losses include light infestation of fruits by scales, puncturing of fruits.
  • Indirect losses: These are primarily of economic interest i.e. decreased purchasing power of farmers due to reduced production. These will lead to decrease in related activities, reduced productivity of agro based industries and forced acceptance of less desirable substitute products.
  • Actual losses: These include the sum total of both direct as well as indirect losses.
  • Potential losses: These refer to the losses likely to be sustained without the adoption of the plant protection measures.
  • Avoidable losses: It is that proportion of the losses that can be saved by adopting proper crop protection measures.
  • Unavoidable losses: It is that proportion of the crop losses that cannot be prevented by using the available crop protection technology.
  • Efficiency of crop protection: It is defined as the percentage of avoidable losses which are actually prevented by the use of crop protection measures.
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:
  • To determine the economic status of a given pest species.
  • For establishing the economic threshold levels and economic injury levels of the pest.
  • To estimate the effectiveness of control measures.
  • For evaluating the crop or a variety for its reaction to the pests.
  • Helping in deciding the allocations for research and extension in plant protection.
  • Helping in assigning the priorities on the bases of relative importance of different pests.
A brief account of the techniques adopted for the assessment of crop losses caused by insect-pests has been given below:

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