With spring and summer just a few months away, we thought it a good time to revisit Integrated Pest Management (IPM), economic injury levels, and economic thresholds (ET) or action thresholds (AT). Growers are constantly hearing about spraying for "plant health" or making "insurance" insecticide treatments, and we wanted to tell the other side of the story. In insect pest management, we are interested in reducing losses from insect pests in a way that is effective, economically sound, ecologically responsible, and socially acceptable.
This is the Ecological Paradigm for the College of Food, Agricultural, and Environmental Sciences of The Ohio State University.
We use two approaches in managing insect pests, the first being taking preventive action a priori, without knowledge of a pest's presence or status. This is the preferred approach, especially when using cultural or ecological tactics including host plant resistance, crop rotation, tillage, and manipulation of planting or harvesting dates.
Normally, the use of an insecticide as a preventive measure should not be considered an appropriate IPM tactic. The second approach is using therapeutic ("cure") tactics, most likely the use of insecticides. Pesticides are a completely acceptable management tactic when used appropriately. The basis for choosing this approach is by basing the decision to use them on the EIL, economic injury level, and the use of economic or action thresholds.
The EIL is the lowest number of insects or injury that will cause economic damage. This is when the cost of control equals the cost of treatment, the gain threshold; it's the break even point. The basic equation is EIL = cost of control/(value of the product X how much damage the insect will do). The most important part of the equation is cost of control/value of the product, the gain threshold.
But keep in mind, a grower wants to control the insect population before reaching this level. This point should come well before the EIL is reached, and is usually set at a much lower level known as the economic threshold, or the action threshold! The ET, or AT, is the number of insects or amount of injury that should trigger management action so that economic damage does not occur, that is, so the EIL is not reached. A good example comes from soybean when the soybean aphid is the concern. The current ET or AT, when you should treat, is 250 aphids per plant with a rising population. However, the EIL, when economic damage will occur, is not reached until you have >800 aphids per plant. A peak density of 250 aphids per plant will not cause economic losses. But last summer when soybean almost doubled in price, the action threshold was kept at 250 aphid per plant. Although the EIL probably fell from >800 aphid per plant to perhaps around 400-500 aphids per plant, we felt we could keep the action threshold at 250 aphid per plant. We still felt that this level was a good ET or AT for growers to use.
As growers make decisions on managing insects or other pests, they should always remember to consider the four points of the Ecological Paradigm that are explained above. The pesticides that are used will usually be effective in controlling the pests, there is not that much of a question about that. However, wasting money, especially in these economic times, is not a good decision. And making unnecessary pesticide applications is not only economically unsound, but is also ecologically irresponsible which will lead to those pesticides becoming socially unacceptable in the long run.
And growers should always remember that the proper usage of pesticides is based on knowledge of EILs and ETs/ATs.