5.1 Principles of Integrated Pest Management (IPM), Economic Thresholds, and Injury Levels

Principles of Integrated Pest Management (IPM), Economic Thresholds, and Injury Levels

For decades, pest control relied heavily on scheduled, prophylactic calendar spraying—applying chemical pesticides on predetermined calendar dates regardless of actual pest presence or field pressure. While this approach initially produced dramatic crop yields and pest suppression, it inevitably triggered catastrophic secondary problems: rapid evolution of chemical resistance, destruction of natural predator populations, secondary pest flare-ups, elevated operational costs, and unintended environmental contamination of Wisconsin's groundwater and surface water resources.

To overcome these systemic vulnerabilities, modern agriculture, forestry, turf management, and structural pest control operate under the framework of Integrated Pest Management (IPM). Under Wisconsin law and the regulatory standards enforced by the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP), certified commercial and private applicators are expected to understand and implement IPM principles as the primary strategy for responsible pest control.

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1. Defining Integrated Pest Management (IPM) & Ecological Philosophy

Integrated Pest Management (IPM) is a comprehensive, decision-making process that coordinates knowledge of pest biology, environmental monitoring, and multiple compatible control tactics to prevent unacceptable pest damage while minimizing risks to human health, non-target organisms, and the environment.

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|                        IPM PARADIGM VS. CALENDAR SPRAYING                   |
|                                                                             |
|   TRADITIONAL CHEMICAL ERADICATION           INTEGRATED PEST MANAGEMENT     |
|   - Goal: Total elimination (100% kill)      - Goal: Suppression below EIL  |
|   - Trigger: Fixed calendar dates            - Trigger: Action Thresholds   |
|   - Tool: Routine single-chemistry sprays    - Tool: Multi-tactic hierarchy |
|   - Ecology: Disregards beneficial species   - Ecology: Conserves predators |
|   - Result: Resistance & pest flare-ups      - Result: Sustainable control  |
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The Core Ecological Philosophy of IPM

1. Pest Suppression, Not Eradication: IPM recognizes that pests are an integral component of agricultural and natural ecosystems. Complete eradication of an established pest is biologically impossible, economically wasteful, and ecologically destructive. Instead, IPM strives to keep pest populations below economically damaging levels.
2. Tolerating Non-Damaging Populations: Low pest populations serve an essential biological function: they provide a continuous food supply and host reservoir for beneficial predatory insects, parasitoids, and entomopathogenic microorganisms. Eradicating every single pest starves out beneficial natural enemies, leaving the crop defenseless against subsequent reinvasions.
3. Chemicals as a Targeted Last Resort: Pesticides remain an indispensable tool within IPM, but they are deployed judiciously and selectively only when non-chemical cultural, mechanical, and biological tactics prove insufficient to prevent economic loss.

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2. The 5 Core Steps of a Systematic IPM Program

A successful IPM program is not a static recipe; it is an active, iterative management cycle comprising five sequential phases.

+-----------------------------------------------------------------------------+
|                         THE 5 CORE STEPS OF AN IPM PROGRAM                  |
|                                                                             |
|   [1. IDENTIFICATION]    ---> Accurate diagnosis of pest & beneficials      |
|            |                                                                |
|            v                                                                |
|   [2. MONITORING]        ---> Field scouting, trapping, degree-day tracking |
|            |                                                                |
|            v                                                                |
|   [3. THRESHOLD EVAL]    ---> Compare field densities against ET and EIL    |
|            |                                                                |
|            v                                                                |
|   [4. IMPLEMENTATION]    ---> Deploy cultural, mechanical, bio, or chemical |
|            |                                                                |
|            v                                                                |
|   [5. EVALUATION]        ---> Assess efficacy, update records, refine plan  |
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Step 1: Accurate Identification of Pest & Beneficial Species

Management cannot begin until the target organism is correctly identified down to the species level. Different species within the same genus often exhibit vastly different life cycles, feeding behaviors, host preferences, and susceptibilities to control tactics.

• Distinguishing Pests from Beneficials: Applicators must differentiate pest damage from feeding by beneficial insects (e.g., mistaking predatory syrphid fly larvae or lady beetle larvae for pest caterpillars).
• Identifying Vulnerable Life Stages: Most control tactics are effective only against specific developmental windows (e.g., small weed seedlings prior to the 4-leaf stage, or early-instar insect larvae before they bore into plant stems or fruit).

Step 2: Monitoring and Field Scouting

Regular, systematic monitoring provides empirical data on pest density, spatial distribution, crop developmental stage, natural enemy activity, and microclimatic conditions.

• Sampling Tools: Standardized sweep nets (e.g., for alfalfa weevil or potato leafhopper), sticky cards, pheromone lure traps, soil core samplers, drop cloths, and 10x hand lenses.
• Phenological Tracking: Utilizing Degree-Day (DD) accumulation models and biological indicator plants to forecast pest emergence and peak activity windows rather than guessing based on calendar dates.

Step 3: Economic Threshold Assessment

Scouting data are compared against established research-based economic guidelines to determine whether the pest population justifies active intervention. If pest numbers remain below the action threshold, no chemical treatment is applied, and scouting continues.

Step 4: Multi-Tactic Implementation

When intervention is warranted, applicators integrate multiple compatible management tactics—selecting cultural, physical, mechanical, and biological controls first, and using selective, low-toxicity chemical applications when thresholds are breached.

Step 5: Evaluation and Record Analysis

Following treatment, applicators re-scout the site to evaluate efficacy, assess non-target impacts, verify that pest numbers dropped below the threshold, and document all findings in formal records to improve future management decisions.

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3. The Economic Injury Level (EIL): Mathematical Derivation

The theoretical foundation of IPM decision-making was formalized by entomologists V.M. Stern et al. (1959) through the concepts of the Economic Injury Level (EIL) and the Economic Threshold (ET).

Defining Economic Injury

• Injury: The physical effect of pest activities on host plant physiology (e.g., leaf area consumed, xylem fluids sucked, roots severed, stems tunneled).
• Damage: The resulting measurable loss of host utility, quality, or yield value (e.g., bushels of grain lost per acre, reduced market grade of fresh fruit).
• Economic Damage: The amount of injury that will justify the financial cost of artificial control measures.

The Mathematical Formulation of EIL

The Economic Injury Level (EIL) is defined as the lowest population density of a pest that will cause economic damage. It is expressed mathematically as:

$EIL = \frac{C}{V \times I \times D \times K}$

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|                      EIL MATHEMATICAL COMPONENT BREAKDOWN                   |
|                                                                             |
|   VARIABLE   DESCRIPTION                      UNITS                         |
|   ───────────────────────────────────────────────────────────────────────   |
|      C       Cost of management per area      $ / acre                      |
|      V       Market value of the crop         $ / bushel (or $ / lb)        |
|      I       Injury per pest density          % defoliation / insect / plant|
|      D       Damage per unit injury           Bushels lost / % defoliation  |
|      K       Proportionate pest reduction     Fraction (e.g., 0.80 for 80%) |
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The Gain Threshold ($GT$)

A simplified intermediate parameter is the Gain Threshold, which represents the minimum yield savings required to offset the cost of applying a control measure:

$\text{Gain Threshold } (GT) = \frac{\text{Cost of Control } (C, \$/\text{acre})}{\text{Market Value of Crop } (V, \$/\text{bushel})} = \text{Bushels/acre required to pay for treatment}$

Dynamic Influences on the EIL

The EIL is not a fixed, static number; it fluctuates continuously based on economic and environmental variables:

• If Crop Market Value ($V$) Rises: The EIL decreases (fewer pests are needed to cause financial loss equal to control costs, justifying earlier treatment).
• If Crop Market Value ($V$) Drops: The EIL increases (a higher pest population can be tolerated before control becomes economically viable).
• If Cost of Control ($C$) Rises: The EIL increases (control is more expensive, requiring higher pest numbers to justify the investment).
• If Control Efficacy ($K$) Drops (e.g., due to pesticide resistance): The EIL increases.

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4. The Economic Threshold (ET) / Action Threshold

While the EIL defines the exact pest density where economic loss equals treatment cost, an applicator who waits until the pest population reaches the EIL before spraying will suffer substantial financial loss. Pest populations continue to grow, feed, and reproduce during the time required to arrange equipment, purchase chemical, apply the spray, and allow the toxicant to achieve mortality.

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|                 DYNAMIC RELATIONSHIP: EIL, ET, AND POPULATION GROWTH        |
|                                                                             |
|   Pest Density                                                              |
|        ^                                                                    |
|        |                                                                    |
|        |                          /\                                        |
|   EIL -|- - - - - - - - - - - - -/--\- - - - [ECONOMIC INJURY LEVEL]        |
|        |                        /    \                                      |
|        |                       /      \  <--- Untreated curve exceeds EIL   |
|    ET -|- - - - - - - - - - - / - - - - - - - [ECONOMIC THRESHOLD]          |
|        |           * * *     /          \                                   |
|        |         *       *  /            \                                  |
|        |       *          */ <=============== ACTION TAKEN HERE             |
|        |     *             \                                                |
|        |   *                \_ _ _ _ _ _ _ _  Treated curve stays below EIL |
|        | *                                                                  |
|        +---------------------------------------------------------------->   |
|        0                            Time                                    |
+-----------------------------------------------------------------------------+

Defining the Economic Threshold

The Economic Threshold (ET)—also universally known as the Action Threshold—is the pest population density at which management action must be initiated to prevent an increasing pest population from reaching the Economic Injury Level.

[!IMPORTANT] Fundamental Threshold Rule: The Economic Threshold (ET) is ALWAYS lower than the Economic Injury Level (EIL): $ET < EIL$ The difference between ET and EIL represents the critical safety margin and operational lead time needed for the control tactic to take effect before economic injury occurs.

Practical Operational Examples in Wisconsin Crops

1. Soybean Aphid (Aphis glycines):
   • EIL: $\approx 674\text{ aphids per plant}$. At this density, measurable yield loss begins to exceed the cost of an insecticide application.
   • ET (Action Threshold): 250 aphids per plant on 80% of plants across the field, with populations actively increasing, between the R1 (beginning bloom) and R5 (beginning seed) growth stages. This 250-aphid threshold provides a 7- to 10-day lead time to apply treatment before numbers reach 674.
2. Potato Leafhopper (Empoasca fabae) in Alfalfa:
   • Threshold varies directly with plant height: on alfalfa $< 3\text{ inches}$ tall, the ET is 0.1 leafhoppers per sweep (10 per 100 sweeps); on alfalfa 8–11 inches tall, the ET rises to 0.5 leafhoppers per sweep; on alfalfa $> 12\text{ inches}$ tall, the ET is 2.0 leafhoppers per sweep (or immediate harvesting to remove the food source).

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5. General Equilibrium Position (GEP) and Pest Categorization

Pest species interact differently with host crops over multi-year cycles. Ecologists classify pests by comparing their long-term average population density—known as the General Equilibrium Position (GEP)—relative to the Economic Threshold and Economic Injury Level.

+-----------------------------------------------------------------------------+
|                        PEST POPULATION CATEGORIES                           |
|                                                                             |
|   1. NON-ECONOMIC PESTS       2. OCCASIONAL PESTS      3. KEY / PERENNIAL   |
|   Density                     Density                  Density              |
|      ^                           ^                        ^                 |
|      |                      EIL -|- - - - /\ - - -   EIL -|- - - - - - - -  |
|  EIL-|- - - - - - - - -      ET -|- - - -/--\- - -    ET -|- - - - - - - -  |
|   ET-|- - - - - - - - -          |      /    \            |   /\  /\  /\  /\|
|      |                           |     /      \       GEP |==/==\/==\/==\/==|
|  GEP |~~~~~~~~~~~~~~~~~      GEP |~~~~/~~~~~~~~\~~        | /               |
|      +---------------->          +---------------->       +---------------->|
|               Time                        Time                     Time     |
+-----------------------------------------------------------------------------+

1. Non-Economic Pests (Sub-Economic Pests)

• Biological Dynamic: The General Equilibrium Position (GEP) lies far below both the ET and EIL at all times.
• Management: Although these organisms feed on the crop and cause minor cosmetic injury, natural predators, parasitoids, and environmental factors keep their numbers suppressed. No chemical treatment is ever justified. Applying broad-spectrum pesticides against non-economic organisms kills natural enemies and risks transforming them into secondary pest outbreaks.

2. Occasional Pests

• Biological Dynamic: The GEP remains below the ET and EIL under normal environmental conditions. However, during specific weather anomalies (e.g., severe summer drought, unseasonable heat, mild winters) or following the disruption of natural enemies, the population experiences temporary exponential spikes that exceed the ET and EIL.
• Examples: European corn borer, armyworms, two-spotted spider mites in drought-stressed soybeans.
• Management: Intensive field scouting and degree-day tracking. Treatments are applied only during outbreak years when scouting confirms that numbers have breached the action threshold.

3. Key Pests (Perennial / Severe Pests)

• Biological Dynamic: The GEP is permanently situated near, at, or above the Economic Injury Level during critical crop growth stages. Left unmanaged, key pests cause severe economic damage every single growing season.
• Examples: Western and Northern corn rootworm in continuous corn, codling moth and apple maggot in commercial orchards, Colorado potato beetle in commercial potatoes.
• Management: Requires a persistent, integrated multi-year strategy utilizing crop rotation, resistant transgenic or conventional hybrids, cultural sanitation, and targeted biological or chemical controls.

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6. Real-World Scenario: Calculating EIL and Threshold Decisions

Field Decision Case Study: Grain Corn Defoliator

A certified agricultural crop scout is evaluating a 160-acre field of grain corn in Dane County, Wisconsin, infested with an armyworm outbreak during the whorl stage.

Agronomic & Economic Data:

• Cost of Insecticide Application ($C$): $18.00 per acre (chemical + custom ground rig application).
• Anticipated Corn Market Value ($V$): $4.50 per bushel.
• Control Efficacy ($K$): 90% mortality ($0.90$).
• Biological Loss Factor ($I \times D$): Research indicates each armyworm larva per plant causes a yield loss of $0.05$ bushels per acre ($0.05\text{ bu/acre/larva}$).

Step 1: Calculate the Gain Threshold ($GT$): $GT = \frac{C}{V} = \frac{\$18.00/\text{acre}}{\$4.50/\text{bushel}} = 4.0\text{ bushels per acre}$ The management tactic must save at least 4.0 bushels per acre to break even financially.

Step 2: Calculate the Economic Injury Level ($EIL$): $EIL = \frac{C}{V \times (I \times D) \times K} = \frac{18.00}{4.50 \times 0.05 \times 0.90} = \frac{18.00}{0.2025} \approx 88.9\text{ larvae per 100 plants (0.89 larvae/plant)}$

Step 3: Threshold Interpretation: The Economic Threshold (ET) is established at 60 larvae per 100 plants (0.60 larvae/plant) with 25% whorl feeding damage. During field scouting across a W-pattern, the applicator samples 100 plants and finds an average of 72 active larvae per 100 plants with 35% whorl feeding. Because the pest density exceeds the ET ($72 > 60$) and is actively approaching the EIL ($88.9$), an immediate insecticide treatment is economically justified.