The yields of crops are governed by a variety of factors including pests. It is not always possible to manipulate the factors that determine crop yield. The assessment of crop losses due to insect pests is of significance from the planning point of view, as it is helpful in prediction of crop production and planning for higher production in the future. Thus, information on crop losses would serve as a guide for research programmes in crop improvement.
The loss suffered by a crop is a function of the pest population, behaviour of the pest and the crop plants. Damage to the plant occurs because of the effect of injury by the insect. A simple damage to the plants by an insect may or may not lead to crop loss. The reduction in quantity/quality of the produce is the crop loss. The loss in quality may affect the appearance of the crop produce, its nutritive value or it may result in the produce being rendered unfit for use.
Insect pests damage crop plants either by feeding or during the process of oviposition. Some of the insect pest species are host specific and they feed on plants of a single species and are termed monophagous. Others attack plant species belonging to the same family and are known as oligophagous.
Others are capable of infesting plant species belonging to several diverse families and are called polyphagous. Some of the pests are strictly specific as regards their site of feeding and oviposition, as for example leafhoppers, leaf miners, fruit borers or root borers. They cause damage to only one part of plant. There are others, like the locust and some species of beetles that can attack several parts of the same plant simultaneously.
Types of Losses:
The losses due to insect pests can be categorised in many ways, depending upon the significance of pests and their management.
Some of them are explained below:
(1) Direct Losses:
These relate to decrease in productivity (quantitative) or intrinsic value/acceptability of the produce (qualitative). Direct quantitative losses include killing of flowers, buds, twigs of whole plant because of infestation by a pest having either chewing or piercing-sucking mouth parts, e.g., locust and grasshoppers, bollworms, fruit borers, root borers, etc.
Examples of direct qualitative losses include light infestation of fruits by the scales, puncturing of normal fruits immediately before harvest owing to feeding or ovipositional activity. Damage by the pests to the fruit trees from the blooming to harvesting periods results in quantitative losses in the earlier phase and qualitative ones in the later phase.
(2) Indirect Losses:
These are primarily of economic interest as for example decreased purchasing power of the agriculturists and those depending upon agriculture owing to reduced production. This would lead to decrease in related activities, reduced productivity of agro-based industries, expenses incurred for importation of agricultural produce and also forced acceptance of less desirable substitute products.
(3) Actual Losses:
These include the total value of losses, both direct and indirect, the cost of control measures along with the amount spent on researches for developing pest control knowledge among the agriculturists.
(4) Recognized and Hidden Losses:
These are subjective terms showing as to whether the factors determining the losses are known or not.
(5) Avoidable and Unavoidable Losses:
These are also subjective terms related to the belief as to whether a certain pest can be controlled or not.
Estimation of Losses:
The amount of damage caused by insect pests of crop plants is a function of the pest population- its characteristics of feeding or oviposition behaviour and the biological characteristics of the host plants. Each of these factors is affected differently by the environmental factors, both biotic and abiotic. It is, sometimes, rather difficult to establish correlations between the levels of pest population and plant damage. The estimation of damage is, however, critical to pest management.
The evaluation of damage is helpful in pest management in the following ways:
i. Defining the economic status of a pest species.
ii. Estimating the effectiveness of control measures.
iii. Helping in assigning priorities on the basis of relative importance of different pests.
iv. Evaluating crop varieties for their resistance to pests.
v. Helping in deciding the allocations for research and extension in plant protection.
The techniques adopted for the assessment of crop losses caused by insect pests fall into the following categories:
1. Mechanical Protection:
The crop is grown under enclosures of wire gauge or cotton cloth. The enclosures keep the pest away from the crop. The yield under such enclosures is compared with that obtained from the infested crop under similar conditions. The technique has been used with various modifications for estimating the losses caused by jassid and whitefly to cotton.
In the case of non-flying insects, sometimes, the barriers are substituted for the cages. The limitation in the case of enclosures is that the plants generally become pale and weak due to changes in microenvironment. This technique cannot be adopted on an extensive scale because it is very time consuming and impracticable on field scale.
2. Chemical Protection:
The crop is protected from pest damage through the application of pesticides. The yield of treated crop is compared with the one which has been subjected to normal infestation. This technique has been very widely used and it can be adopted on a large scale in cultivators fields.
It should, however, be ensured that the treated and untreated fields/plots are as similar as possible in soil type, manuring, variety and cultural practices. The flaw in this approach is that the crop treated with chemicals can be physiologically affected and it may give either more or less yield.
3. Pest Incidence in Different Fields:
The yield is determined per unit area in different fields carrying different degrees of pest infestation. The correlation between the crop yield and degree of infestation is worked out to estimate the loss in yield. This technique can be used for estimating crop loss due to different pests and diseases over a larger area. The drawback in this technique is that the yield in different fields can be influenced by soil heterogeneity.
4. Pest Incidence on Individual Plants:
In this case, individual plants from the same field are examined for the pest incidence and their yield is determined individually. The loss in yield is estimated by comparing the average yield of healthy plants with that of plants showing different degrees of infestation. The same data can also be used for working out a correlation equation between yield and infestation on the basis of individual plants.
The advantage in this technique over the preceding one is that soil heterogeneity factor is considerably reduced in the same field. However, different plants showing varying degrees of infestation in it is a proof that plants differs from one another in some unknown factors due to which they carry different degrees of infestation. This factor may be genetic or physiological or it may be mere soil heterogeneity in the same field. Moreover, this method is very time consuming and involves lot of labour.
5. Damage by Individual Insect:
Preliminary information on the damage caused by individual insect is obtained from studies on biology of pest species. The details regarding the amount of damage caused by different stages or ages of the pest, and the exact nature and amount of loss caused are then worked out. This technique is quite easy in the case of leaf-feeding insects. However, it is very difficult to use this technique over large areas since it is very time consuming.
6. Simulated Damage:
This technique involves simulation of pest injury by removing or injuring leaves or other parts of the plant. The simulated damage may, however, not always be equivalent to the damage caused by an insect. Insects may persist over a period of time or inject long acting toxins rather than producing their injury instantly. Feeding on margins of a leaf may not be equivalent to tissue removal from the centre of the leaves.
Insect feeding is usually extended over a period of time and it is rather difficult to incorporate the concept of the rate of injury in simulation studies. Furthermore, the kind of leaf removed may be important, as for example, quality and position of leaf on the plant. In addition, the time of simulating damage with respect to the stage of plant growth is also critical.
7. Manipulation of Natural Enemies:
The manipulation of natural enemies of a pest species offers a means of evaluating plant damage. The pest is controlled by introducing predators/parasitoids into the field and the yield of such a crop is compared with that on which no such pest control measures have been undertaken. This technique has, however, not been widely used. This method is feasible only in small plots and is not practicable on a field scale.
Any of the above methods can be suitably modified and used for estimating loss in yield of a given crop. The degree of pest infestation and the damage caused by it may differ from field to field in the same season, and from season to season in the same field.
It is, therefore, imperative to work out the average values. In case the crop losses have to be worked out on the regional/state basis, the number of places from where estimations have to be made is more important than the degree of precision of the technique employed.
Extent of Losses:
Despite the annual investment of US$ 40 billion for the application of 3 million metric tonnes of pesticides, plus the use of various biological and other non-chemical controls worldwide, global crop losses remain a matter of concern. Although crop protection aims to avoid or prevent crop losses or to reduce them to economically acceptable losses, the availability of quantitative data on the effect of different categories of pests is very limited.
The generation of experimental data is time- consuming and laborious, and losses vary from season to season due to variation in pest incidence and weather conditions, and estimates of loss data for various crops are fraught with problems. The assessment of crop losses is required for demonstrating where future action is to be needed and for decision making by farmers as well as government agencies.
The first attempt to estimate crop losses due to various pests on global scale was made by Cramer (1967), who put the global losses due to insect pests at 10.8 per cent. Subsequently, Oerke et al. (1994) made extensive study to estimate losses in principal food and cash crops and total losses due to all categories of pests animals, weeds and pathogens) were estimated at 42.1 per cent.
In spite of the wide spread use of synthetic pesticides and other control measures, the losses due to insect and mite pests increased in post-green revolution era than in pre-green revolution era. Worldwide total pre-harvest losses for post-green revolution era (1988 through 1990) period value at US$ 90 billion for eight principal food and cash crops (barley, coffee, cotton, maize, potato, rice, soybean and wheat).
Since mid-nineties, the production systems have undergone a tremendous change, particularly in crops like maize, soybean and cotton in which the advent of transgenic technology has modified the strategies for pest management in some major production regions.
Therefore, Oerke (2006) updated the loss data for major food and cash crops for the period 2001-03 according to which the total losses due to all categories of pests were calculated as 32.1 per cent. The actual losses due to various pests have been estimated as 26-29 per cent for soybean, wheat and cotton, and 31, 37 and 40 per cent for maize, rice and potatoes, respectively.
Thus, there is a decline in global crop losses due to various pests from 42.1 per cent during 1988-90 to 32.1 per cent during 2001-2003. The corresponding decline in loss due to animal pests, weeds and pathogens was from 15.6 to 10.8, 13.2 to 8.8 and 13.3 to 12.5 per cent, respectively.
Losses due to insect pests in Indian agriculture have been estimated from time to time. Extensive surveys carried out during early 1960s revealed that fruits, cotton, rice and sugarcane suffered 25, 18, 10, and 10 per cent yield losses, respectively. However, during the early 2000s, the losses increased considerably due to intensification of agriculture. The highest losses were reported in cotton (50%), followed by sorghum and millets (30%), and rice, maize and oilseeds (each 25%).
In general, the losses in post-green revolution era have shown an increasing trend than in the pre- green revolution era. Overall, the losses increased from 7.2 per cent in early 1960s to 23.3 per cent in early 2000s. The maximum increase in loss occurred in cotton (18.0 to 50.0%), followed by other crops like sorghum and millets (3.5 to 30.0), maize (5.0 to 25.0) and oilseeds (other than groundnut) (5.0 to 25.0).
There has been a paradigm shift in the crop management scenario of Indian agriculture since the beginning of this century. Bt cotton was released in the country in 2002 and the area under Bt cotton increased from 50,000 ha in 2002 to 11 million ha in 2013. Currently, Bt cotton occupies about 90 per cent of the total area under this crop.
Secondly, concerted efforts were made to implement integrated pest management programmes in principal food and cash crops. As a result of these developments, losses due to insect pests in several agricultural crops have shown a declining trend (Table 2.2). However, in terms of monetary value, the decline in losses does not appear to be prominent. This is due to both the increase in production levels as well as the increase in prices of different commodities.
Thus, the Indian agriculture suffers annual losses to the tune of Rs. 900 billion due to the ravages of arthropod pests in field and during storage. This is a colossal loss and all out efforts should be made to bring the losses to the minimum so that the access to food is increased for the expanding population.
This has to be done in such a way that environmental quality is maintained and long term sustainability of the agro-ecosystem does not suffer. Integrated pest management (IPM) appears to be the only viable option to achieve these objectives.