The critical factor which determines the damaging capacity or otherwise of an insect pest in its population level. The concept of injury level was propounded to enable us to identify the population level at which an insect would cause damage to a crop.
Factor # 1. Economic Injury Level (EIL):
The critical factor that determines the damaging capacity or otherwise of an insect is its population level. The concept of injury level was propounded to enable us to identify the population at which an insect could cause damage to a crop.
According to Stern et al. (1959), it is the lowest pest population density that will cause economic damage. It is the level at which damage can no longer be tolerated and, therefore, at that point or before reaching that level, it is desirable to initiate deliberate control operations.
Although expressed as numbers of insects per unit area, the EIL, in reality, is a level of injury. Because injury is difficult to measure in a field situation, however, number of insects is used as an index of that injury. It may, therefore, be more useful to express EIL in standard units of injury. The standard units of injury are the injury equivalent, i.e., the amount of injury that could be produced by one pest through its complete life cycle, and equivalency, i.e., total injury equivalents (for a population) at a point of time.
If management action (insect suppression) can be taken quickly and loss averted completely, EIL may be expressed as follows:
EIL = C/VID … (1)
Where, EIL = No. of injury equivalents per production unit (insects/ha)
C = Cost of management activity per unit of production (Rs/ha)
V = Market value per unit of product (Rs/kg)
I = Crop injury per pest density
D = Damage per unit injury (kg reduction/ha)
These primary variables are affected by a number of complex variables. In instances, where some loss from the insect is unavoidable, the relationship becomes
EIL = C/ V × I × D × K … (2)
Where, K represents proportionate reduction in injury (e.g. 0.6 for 60%)
Factor # 2. Economic Threshold Level (ETL):
It is the pest density at which control measures should be applied to prevent an increasing pest population from reaching the economic injury level. Control measures are taken at this stage so that the pest does not exceed the economic injury level.
ETL is the best known and most widely used index in making pest management decisions. Although expressed in insect numbers, ETL is, in fact, a time parameter, with pest numbers being used as index for when to implement management strategies. Just as with EILs, ETLs can also be expressed in insect equivalents.
ETL is a complex value based on EIL, population dynamics of the pest, weather forecasting, and pest’s potential for injury. The relationship between ETL and EIL is shown in Fig. 12.2. When no action is taken at ETL, population exceeds EIL, while when management steps for pest suppression are taken as the population crosses ETL; the population is forced down before it could reach EIL. ETL is a direct function of EIL and as such is subject to changes in EIL variables. In addition, ETL varies with logistical considerations associated with time delays that may vary from one situation to another.
The concept of EIL and ETL gained wide acceptability from the time it was presented. However, implementation of the concept in practice has been very slow. This is due to a number of serious limitations in the concept.
Some of these limitations are given below:
(i) The terms EIL and ETL are themselves misleading because both are defined in terms of population densities, while former represents an injury level and the latter the time for taking control measures. This limitation may be overcome by defining these levels in terms of injury equivalents. Moreover, it would then be possible to describe the same type of injury for many pest species.
(ii) There is a lack of rigorous definition of economic damage, i.e., the amount of injury that will justify the cost of control.
(iii) The EIL concept overlooks the influence of other production factors that can affect the crop/pest systems. The externalities left out include interpersonal dynamics, biological relationships with other pests and natural enemies, environmental contamination by pesticides, resistance to pesticides, effect of control in neighbouring fields and health problems relating to pesticides.
(iv) Decision levels for management of some types of pests cannot be determined with EILs. Besides medical and veterinary pests, it includes most vectors. It is very difficult to place a monetary value on the reduction in aesthetic value associated with a given type of injury. A similar problem exists with respect to forest pests.
Almost all components of EILs are difficult to estimate for forest pests; accurate market values are a problem, management costs may vary greatly and frequently include mere environmental and social costs and the injury/crop response relationships may be difficult to determine because the growth of the crop spans many years.
(v) The concept is unsuitable in case of attack of multiple pests on a single crop at the same stage.
However, inspite of these limitations, EIL concept continues to offer a practical approach to pest related decision making in a broad sense.
Factor # 3. Environmental Economic Injury Levels (EEIL):
The challenge of attempting to decrease pesticide inputs further can be met by developing environmentally based EILs and their concomitant ETLs. An environmental EIL is an EIL that evaluates a management tactic based on not only its direct costs and benefits to the user but also its effects on the environment. The EIL equation (equation 2) integrates many management elements, each of which may have a role in making pest management environmentally most sustainable.
(i) Assigning realistic management costs (C):
Component C of the EIL equation represents costs associated with taking management action against a pest population, and increased costs cause EIL to increase proportionally. Generally, C does not take into account the environmental costs associated with environmental risks; it is possible to include these costs in variable C of EIL.
One approach for estimating environmental costs of pesticides through economic techniques of contingent valuation was presented by Higley and Wintersteen (1992). They estimated the level of risk posed by 32 field crop insecticides to different environmental elements (surface water, ground water, aquatic organisms, birds, mammals, beneficial insects, etc.) and to human health (acute and chronic toxicity).
They also estimated from survey data the relative importance of avoiding risk 10 each of these elements. Additionally, survey respondents (producers) indicated how much they would be willing to pay, in either higher pesticide costs (for safer pesticides) or yield losses, to avoid different levels of risk (high, moderate, low) from a single application of a pesticide.
With these data, the individual environmental costs for each insecticide were calculated as below:
Environmental EIL = PC + EC/VDIK
(ii) Manipulating Crop Market Value (V):
This could be achieved by putting a higher market value for a pesticide-free produce. The extent of increase would depend on the consumer’s willingness to pay for a safer product.
(iii) Reducing Damage per Pest (D):
Reducing D implies that less loss of yield occurs for a given amount of injury. This is possible if plant is able to tolerate and compensate for injury. Plants that can tolerate or compensate for injury do not place selection pressures on pest populations. Therefore, the benefits of tolerance and compensation in plant are sustainable and permanent. Even partial tolerance will increase EILs (by decreasing D). The need for pesticides and the risks to environment will be reduced correspondingly.
(iv) Developing Environmentally Responsible K Value:
Modified K is the proportion of total pest injury averted by timely application of a management tactic. Increasing the EIL to improve environmental quality implies that we are willing to tolerate more pests. But this is not always the case. By reducing D or K, EIL can be increased even without causing increased losses or costs.
Factor # 4. General Equilibrium Position (GEP):
It is the average population density of a pest over a long period of time unaffected by the temporary interventions of pest control. The population fluctuates around a mean level as an outcome of the influence of density dependent factors, such as parasitoids, predators, diseases, etc.
It may be understood that EIL may be at any level from well below to well above the GEP. In certain insects GEP is well below the EIL or even ETL and never reaches the latter two parameters. Such insects are rarely noticed physically but the damage caused by them through the introduction of a virus or any other disease can be most significant.
GEP touches EIL and ETL, approximately 2 to 5 years for many insect pest species. Such insects are called ‘occasional pests’. The increase in population may be due to the injurious effect of pesticides or due to favourable weather conditions. For example, in Punjab, outbreaks of the armyworm, Mythimna separata (Walker), on wheat, are recorded every 2 to 5 years.
Sometimes, the control measures are required frequently to bring down the GEP well below the EIL and ETL. Such a situation has been observed in the tobacco caterpillar, Spodoptera litura (Fabricius), which is a ‘regular pest’ of cruciferous vegetables.
The known severe pests form another category altogether and, in their case, EIL and ETL are below the GEP level. The maize borer, Chilo partellus (Swinhoe), is a severe pest of maize and its severity increases in some Himalayan valleys where the climate is mild. Moreover, it is much more severe on the hybrid corn varieties than on the local, comparatively low yielding maize.