3.2 Economic Thresholds, Injury Levels & Field Scouting

Understanding Economic Thresholds in IPM

One of the most important concepts in Integrated Pest Management (IPM) is that the mere presence of a pest does not automatically justify the cost and environmental risk of applying a pesticide. Crop plants can tolerate a certain amount of damage without suffering a significant reduction in yield or quality. The decision to take action is driven by economics.

The Economic Injury Level (EIL)

The Economic Injury Level (EIL) is defined as the lowest pest population density that will cause economic damage. More practically, it is the point at which the cost of applying a pest control measure is exactly equal to the loss in crop value that the pest would have caused. If a pest population remains below the EIL, it is more cost-effective to tolerate the damage than to pay for a pesticide application.

Calculating the EIL involves several variables:

• Control Cost: The total expense of the pesticide, labor, and equipment needed for the application.
• Market Value of the Crop: The expected price the grower will receive for the harvested crop.
• Injury per Pest: The amount of physical damage caused by a single pest insect or weed.
• Damage per Unit of Injury: How much that physical damage actually reduces the final yield.

The Economic Threshold (ET) / Action Threshold

While the EIL represents the break-even point, waiting until the pest population reaches the EIL to apply pesticides is a recipe for disaster. It takes time to mobilize equipment, apply the chemical, and for the chemical to take effect. During this lag time, the pest population will continue to grow and cause damage, surpassing the EIL and resulting in economic loss.

Therefore, we use the Economic Threshold (ET), also known as the Action Threshold. The ET is the pest population density at which control measures should be initiated to prevent an increasing pest population from reaching the Economic Injury Level. The ET is always a lower number than the EIL. For example, if the EIL for aphids on soybeans is 250 aphids per plant, the ET might be set at 200 aphids per plant. When scouting reveals the population has reached 200, action is taken.

The Gain Threshold Formula

The Gain Threshold represents the amount of yield that must be saved to exactly cover the cost of the control measure. It is a fundamental component of calculating the EIL. The formula is:

Gain Threshold = Control Cost / Market Price of Crop

For example, if a pesticide application costs $20 per acre and the crop is worth $5 per bushel, the Gain Threshold is 4 bushels per acre ($20 / $5). The pest control measure must save at least 4 bushels per acre to be economically justified.

To translate this into a pest population (the EIL), you incorporate the loss caused per pest: EIL = Gain Threshold / Loss per Pest If each pest reduces yield by 0.1 bushels, the EIL would be 40 pests (4 / 0.1).

Field Scouting and Monitoring Techniques

To know if a pest population is approaching the Economic Threshold, applicators must actively monitor the field. This process is known as scouting. Scouting must be systematic and accurately reflect the entire field, not just the edges.

Sampling Methods

• Random Sampling: Taking samples from entirely random locations throughout the field to avoid bias. This is useful for pests that are distributed uniformly.
• Transect Sampling: Walking a specific pattern (like a 'W', 'V', or 'X') across the field and taking samples at regular intervals along that path. This ensures all areas of the field (edges, middle, low spots, high spots) are represented.
• Grid Sampling: Dividing the field into a grid and taking samples from each grid square. This is highly accurate and useful for precision agriculture but is time-consuming.

Scouting Tools

Different pests require different tools for accurate monitoring:

• Sweep Nets: A sturdy net swung through the crop canopy in a 180-degree arc. Useful for collecting active, foliage-dwelling insects like leafhoppers, lygus bugs, and certain beetles. Pests are counted per a specific number of sweeps (e.g., pests per 10 sweeps).
• Sticky Traps: Yellow or blue cards coated with a sticky substance. Insects are attracted to the color and get stuck. Useful for monitoring flying insects like whiteflies, winged aphids, and thrips in greenhouses and orchards.
• Beat Sheets (Drop Cloths): A white cloth placed on the ground between crop rows. The plants on either side are vigorously shaken or beaten with a stick, causing insects to dislodge and fall onto the white cloth where they can be easily counted. This is standard for monitoring caterpillars and true bugs in row crops like cotton and soybeans.

Degree-Day Calculations

Insects are cold-blooded (ectothermic); their development rate is directly tied to environmental temperature. Rather than using a calendar to predict when a pest will appear, IPM relies on Degree-Days (DD). A Degree-Day is a measure of heat accumulation above a specific developmental baseline temperature for a given pest.

Daily Degree-Day = ((Maximum Temp + Minimum Temp) / 2) - Developmental Threshold

By tracking degree-days from a specific biofix point (like the first capture of a moth in a pheromone trap), applicators can accurately predict when eggs will hatch or when the vulnerable larval stage will be present. This allows for precisely timed pesticide applications, maximizing efficacy and minimizing the need for repeated sprays.