 Hi, welcome back to the class. In the last class, we understood about what is a pest, so what are the different reasons for the outbreak of the pest and how the insects can be classified based on the economic importance. Now, in today's class, let us look at when an insect is called as a pest. So, in order to understand that, so we need to understand a little bit about the ecology of the insect and how the population is going to fluctuate in a natural ecosystem and probably that I can explain that with this graph. If you look at this graph, the insect population, there will be an increase in the population for some time and later it decreases, comes down, again it increases and then comes down. So like this, the population will be fluctuating over a period of time. And why this fluctuation happens, so the fluctuation is mainly dependent on the abiotic factors and as well as the biotic factors. What are the abiotic factors? Abiotic factors like temperature, rainfall and relative humidity, wind, etcetera will sometimes favor the population in order to increase their population number or sometimes they will be detrimental, so that the population crashes down and at the same time the biotic factors such as the fecundity rate of the insect population or the availability of the food in a habitat and as well as the natural enemies will also contribute for this kind of a fluctuation. And this fluctuation will be hovering around a one position which we call it as a general equilibrium position and any organism for that matter not only the insect, so they all will be fluctuating around this general equilibrium position. What do you mean by general equilibrium position? It is nothing but the average population over a longer period of time around which an insect population tend to oscillate due to the biotic and the abiotic factor. One interesting thing that we always have to note it is that any increase in a population do not reach beyond a certain point and which we call it as the carrying capacity. Once the population reaches the carrying capacity then one of these factors will actually interfere and then bring down the population. So when an insect population is in a general equilibrium position which will be normally happening in the natural ecosystem, so whether with that population is going to cause a significant loss in a particular crop or not is a matter of concern. Now so we have to find or we have to put an add stick that so what population can be considered as the one which causes an injury or what we call it as an economic injury. For that let us understand two important terminologies telling you about one is the damage boundary and another one is the economic injury level. This also can be best explained with this graph and in this graph you are looking at the level of injury along the x axis and the yield of a crop along the y axis. Suppose the crop x has got a this much yield potential here and an insect population has started feeding on this crop and the crop can sustain the damage caused by the insect population to up to a certain level and any yield loss which happened due to this type of injury is a just a measurable loss. To say that like any interventions that we can take sometimes the cost of this intervention will be much more than the benefit that we are going to get. So this stage we call it as a damage boundary, so this damage boundary can be defined as the lowest level of a damage which can be measured but however if the damage continues to occur and then leads to a certain reduction in the yield in the crop to a certain level beyond which so this will cause an economic loss. So here at this point the population which causes the a significant yield loss or an economic loss we call this as an economic damage or the injury level. This is an economic injury level and any further losses which caused by the population will referred as an economic damage. So economic injury level can be defined as the lowest population density of an insect which will cause an economic damage. Now let us take a simple example how this economic injury level can be calculated. So there is a formula for calculating the economic injury level as EIL is equal to C divided by VIDK where C is the cost of management activity per unit production and V is the market value per unit yield and I is the crop injury per insect likes the percent defoliation or the percent damage it is going to cause D is the damage or the yield less per unit injury that is how much yield reduction happens with this much of cost and K is the proportional reduction in the injury from the pesticide usage and understanding this concept would be quite easier if you go with a one simple worked example. Let us look at this let us take one worked example that the problem like calculate the economic injury level in terms of the best population per hectare with the following figures where the C is around 3000 rupees per hectare V is around 1000 rupees per ton and I is 1 percent defoliation per 100 insects and D is a loss caused by unit injury that is 0.05 ton loss which is going to be caused by this 1 percent defoliation and K is a proportional reduction in the injury by the pesticide application that means by applying the pesticide. So, how much control we can gain say 0.8 that is 80 percent control now with the formula what when we calculate we arrive to an answer called 7500 insects per hectare it means to say that when an insect population reaches 7500 numbers per hectare then that population can be called as an economic injury level. So, suppose like there are 7500 plant numbers or the plant population in one hectare then one insect per plant can be considered as an economic injury level. Now the more in question arises is that when are we going to take up the control measure are we going to take up the control measure at economic injury level no. If we actually take up the control measure at economic injury level then already the economic injury has been started. So, that much amount of loss will also be incurred by the farmer. Hence we have to find out a point beyond or below this economic injury level. So, that so we have to prevent the pest population for reaching the economic injury level let us look at it. So, this is what actually the another important question is that is that economic injury level is the same at all the time for all the crops no it is not the same economic injury level is an a fluctuating thing or is a relative thing which mainly depends upon the market value of the crop which is a primary factor as we all know that when an economic value of a crop increases then the EL is always going to decrease and vice versa and the management injury by the insect when a management cost increases then the EL will also increases. Then the degree of injury per insect which is a secondary factor where it tells that on which part of the plant the pest or the insect is feeding. Suppose if the insect is a defoliator then definitely the economic injury level will be higher and for those which are actually feeding on the reproductive parts like fruits then economic injury level will be lower and in some cases where the insects acting as a vector for the plant diseases then economic injury level will be much much lower. Then the fourth point is the crop susceptibility to the injury like if the crop is at an anger stage then it is quite susceptible to even a lower population of insect in such cases the EL will be lower and if the crop is much older or if it has already crossed the reproductive period then it can sustain a higher population in such case EL will be higher. Now the important question which comes is that when exactly we are going to fix or at what point we are going to fix the what is called as an a level at which a management should be taken. Now this has been actually decided at a point what we call it as an economic threshold level or the action threshold level. So probably this economic threshold level can be better explained through this graph once again. If you look at here so in this graph we have an EL which you already understood and there is a time period and the population of the insect. So when a insect population keeps increasing and when it is at close to the economic injury level we have to intervene at such point or some point where not allowing the insect population to reach the economic level or the injury level. So this point which it is where the intervention is taken is referred as the economic threshold level thus economic threshold level can be defined as the pest density at which the control measures should be applied to prevent an increasing pest population from reaching the economic injury level where if you start intervening much before this then what happens the cost will be more than the benefit. So we have to intervene at such a point where the benefit should be more than the cost keeping this in view so we have to actually fix an economic threshold level. So basically all the interventions are made at an economic threshold level keeping the pest population below the economic injury level. Thank you.