9.1 IPM Principles, Economic Injury Levels & Action Thresholds

Integrated Pest Management (IPM) Principles

Integrated Pest Management (IPM) is a comprehensive, ecosystem-based strategy that focuses on long-term prevention of pests or their damage through a combination of techniques such as biological control, habitat manipulation, modification of cultural practices, and use of resistant varieties. Pesticides are used only after monitoring indicates they are needed according to established guidelines, and treatments are made with the goal of removing only the target organism. Pest control materials are selected and applied in a manner that minimizes risks to human health, beneficial and nontarget organisms, and the environment.

The core principles of IPM include:

1. Identify Pests and Monitor Progress: Accurate identification is the first and most critical step. Not all insects, weeds, and plant diseases require control. Many organisms are innocuous, and some are even beneficial. Monitoring, or scouting, involves regular field inspections to estimate the pest population density and distribution, as well as the crop stage and vigor. This helps in determining if intervention is necessary.
2. Set Action Thresholds: Before taking any pest control action, IPM first sets an action threshold, a point at which pest populations or environmental conditions indicate that pest control action must be taken. Sighting a single pest does not always mean control is needed. The level at which pests will either become an economic threat is critical to guide future pest control decisions.
3. Prevention: As a first line of pest control, IPM programs work to manage the crop, lawn, or indoor space to prevent pests from becoming a threat. In an agricultural crop, this may mean using cultural methods, such as rotating between different crops, selecting pest-resistant varieties, and planting pest-free rootstock. These control methods can be very effective and cost-efficient and present little to no risk to people or the environment.
4. Control: Once monitoring, identification, and action thresholds indicate that pest control is required, and preventive methods are no longer effective or available, IPM programs then evaluate the proper control method both for effectiveness and risk. Effective, less risky pest controls are chosen first, including highly targeted chemicals, such as pheromones to disrupt pest mating, or mechanical control, such as trapping or weeding. If further monitoring, identifications and action thresholds indicate that less risky controls are not working, then additional pest control methods would be employed, such as targeted spraying of pesticides. Broadcast spraying of non-specific pesticides is a last resort.

Understanding Economic Injury Level (EIL) and Economic Threshold (ET)

The concepts of Economic Injury Level (EIL) and Economic Threshold (ET) are fundamental to the decision-making process in IPM. They provide a quantitative basis for determining when a pest population has reached a point where the cost of control is justified by the savings from prevented yield loss.

Gain Threshold

Before calculating the EIL, it is necessary to understand the Gain Threshold. The Gain Threshold is the beginning point of economic damage. It is defined as the amount of yield loss that exactly equals the cost of a pest control measure. It is usually expressed as a quantity of harvestable yield (e.g., bushels per acre) or as a percentage of the total expected yield.

Gain Threshold Formula: Gain Threshold = Cost of Pest Control ($/acre) / Market Value of the Crop ($/bushel or $/pound)

For example, if the cost to apply an insecticide is $15 per acre and the expected market value of soybeans is $10 per bushel, the gain threshold is 1.5 bushels per acre. This means the pest population must be capable of causing at least a 1.5 bushel per acre yield loss before applying the insecticide becomes economically viable.

Economic Injury Level (EIL)

The Economic Injury Level (EIL) is defined as the lowest pest population density that will cause economic damage. It is the point at which the value of the crop destroyed exceeds the cost of controlling the pest. The EIL is a theoretical value that changes depending on the cost of control, the value of the crop, crop susceptibility to the pest, and the effectiveness of the control measure.

EIL Formula (Pedigo's Formula): EIL = C / (V * I * D * K)

Where:

• C = Cost of the management tactic per unit area (e.g., $/acre).
• V = Market value of the crop per unit of yield (e.g., $/bushel).
• I = Injury units per insect (e.g., percent defoliation per insect per acre).
• D = Damage per unit of injury (e.g., yield loss per percent defoliation).
• K = Proportionate reduction in pest population achieved by the management tactic (e.g., 0.8 for 80% control).

The EIL is dynamic. If the price of the crop goes up, the EIL goes down (it takes fewer pests to cause economic damage). If the cost of control goes up, the EIL goes up (you can tolerate more pests before spraying is justified). If a crop is heavily stressed (e.g., drought), it may be more susceptible to injury, effectively lowering the EIL.

Economic Threshold (ET)

The Economic Threshold (ET), also known as the action threshold, is the pest population density at which control measures should be initiated to prevent an increasing pest population from reaching the EIL. The ET is always a lower pest density than the EIL.

The ET is a practical, operational rule. It takes into account the time delay between scouting, decision-making, and the actual application of the control measure, as well as the time it takes for the control measure to become effective (e.g., time for the insect to consume a lethal dose of insecticide). If you wait until the pest population reaches the EIL to apply a control, the population will likely exceed the EIL and cause economic damage before the control measure fully suppresses it.

Therefore, the ET acts as a buffer. In some systems, the ET is set at a fixed percentage of the EIL (e.g., 75% or 80% of the EIL). In dynamic models, the ET is determined by predicting the pest population growth rate. If the population is growing rapidly, the ET must be set further below the EIL than if the population is growing slowly.

In summary, you scout to determine if the population is at the ET. If it is, you take action to prevent it from reaching the EIL, which is where economic losses occur. Understanding and accurately applying these concepts ensures that agricultural producers maximize their profitability while minimizing unnecessary pesticide applications, aligning with the core philosophy of Integrated Pest Management.