6.2 Economic Threshold & Economic Injury Level

Economic Threshold & Economic Injury Level

Why this matters: The exam outline lists economic threshold and economic injury level as two separate leaves, and the distinction between them is the single most tested concept in the IPM domain. One is the point where you act; the other is the point where you have already lost money.

1. Bioeconomic Thresholds: EIL vs. Economic Threshold (ET)

In commercial agriculture, silviculture, and commercial horticulture, pest control is an economic investment. Applying a pesticide is economically justified only when the financial value of the crop yield saved exceeds the total cost of purchasing, mixing, and applying the chemical.

┌────────────────────────────────────────────────────────────────────────┐
│                   BIOECONOMIC POPULATION DYNAMICS                      │
│                                                                        │
│  Pest                                                                  │
│  Density                                                               │
│     ▲                                                                  │
│     │                                                                  │
│     │                         /---\  <-- Untreated Population         │
│     │                        /     \     Reaches Economic Damage       │
│  ───┼───────────────────────/───────\───────────────────────────────   │
│ EIL │                      /         \  <-- ECONOMIC INJURY LEVEL      │
│     │                     /           \     (Cost of Control = Loss)   │
│  ───┼────────────────────/─────────────\───────────────────────────   │
│  ET │                   / <─ Action     \  <-- ECONOMIC THRESHOLD      │
│     │                  /     Initiated   \     (Action Trigger Point)  │
│     │                 /      Here         \                            │
│     │   /\       /\  /                     \--- Pest Suppressed Below  │
│ GEP ┼──/──\─────/──\/                           Threshold by Treatment │
│     │ /    \---/                                                       │
│     └───────────────────────────────────────────────────────► Time     │
│        GEP = General Equilibrium Position (Natural baseline level)     │
└────────────────────────────────────────────────────────────────────────┘

Mathematical Foundations of the Economic Injury Level (EIL)

The Economic Injury Level (EIL) is mathematically defined by the Pedigo bioeconomic equation:

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

Where the variables represent:

• $C$ = Cost of Management per Area: Total cost of purchasing chemical, labor, equipment fuel, and depreciation (e.g., $$/\text{acre}$).
• $V$ = Market Value of Crop: Current commodity price per unit of marketable harvest (e.g., $$/\text{bushel}$, $$/\text{cwt}$, or $$/\text{lb}$).
• $I$ = Injury per Pest Density: Physical loss of plant tissue or foliar area per individual pest (e.g., $% \text{ leaf area consumed per caterpillar}$).
• $D$ = Damage per Unit Injury: Measurable reduction in harvested crop yield resulting from a unit of plant injury (e.g., $\text{bushels lost per } % \text{ leaf area destroyed}$).
• $K$ = Proportionate Reduction in Pest Density: Control efficacy of the management tactic (e.g., $0.80$ to $0.95$ for $80% \text{ to } 95%$ mortality).

Worked Calculation Example

Scenario: A Willamette Valley grower faces an armyworm infestation in clover seed.

• Cost of pesticide application ($C$) = $$30.00 / \text{acre}$
• Market value of clover seed ($V$) = $$2.00 / \text{lb}$
• Yield damage per caterpillar ($I \times D$) = $0.25 \text{ lb seed lost per caterpillar / sq yd}$
• Chemical efficacy ($K$) = $0.90$ ($90%$ kill)

$\text{EIL} = \frac{\$30.00}{\$2.00 \times 0.25 \times 0.90} = \frac{30}{0.45} = 66.7 \text{ caterpillars / sq yd}$

Interpretation: If armyworm density reaches $67 \text{ caterpillars / sq yd}$, the dollar value of the crop destruction exactly equals the $$30.00 / \text{acre}$ cost to spray.

Defining Threshold Parameters for Applicators

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2. Aesthetic, Nuisance, and Public Health Thresholds

While agricultural IPM relies heavily on financial profit margins and harvest yield equations, non-agricultural settings operate under fundamentally different threshold drivers.

┌────────────────────────────────────────────────────────────────────────┐
│                      NON-AGRICULTURAL THRESHOLDS                       │
│                                                                        │
│  1. AESTHETIC THRESHOLD (Landscape, Ornamentals, Turfgrass):          │
│     • Driven by visual appearance, cosmetic beauty, and client         │
│       expectations (golf course greens, botanical gardens, HOA lawns). │
│     • Often substantially lower than agricultural thresholds because   │
│       minor foliar spotting, turf browning, or leaf chewing reduces    │
│       perceived aesthetic, commercial, or property value.              │
│                                                                        │
│  2. NUISANCE & STRUCTURAL THRESHOLD (Urban Pest Management):           │
│     • Driven by human comfort, property preservation, and structural   │
│       integrity.                                                       │
│     • Termites in structural timbers: ZERO tolerance (EIL = 1 colony). │
│     • Yellowjacket nests adjacent to school doorways: ZERO tolerance.  │
│                                                                        │
│  3. PUBLIC HEALTH & SANITATION THRESHOLD (Medical & Food Facilities):  │
│     • Driven by regulatory sanitation standards and disease transmission│
│     • Cockroaches, flies, or rodents in commercial kitchens, food      │
│       processing plants, or hospitals: ZERO TOLERANCE due to pathogen  │
│       contamination risks (Salmonella, E. coli) and legal closure.     │
└────────────────────────────────────────────────────────────────────────┘

Oregon School IPM Law (ORS 634.700 – 634.750)

Oregon state law mandates that all public and private K-12 schools and community colleges implement a comprehensive Integrated Pest Management plan.

• IPM Coordinator: Every school district must designate a trained IPM Coordinator.
• Low-Impact Pesticides: Schools must prioritise non-chemical measures first. The governing body's adopted low-impact pesticide list may include any product except one whose label carries the signal word "WARNING" or "DANGER", or one classified as a human or probable human carcinogen under EPA's 1986 guidelines or as carcinogenic or likely carcinogenic under EPA's 2003 draft final guidelines (ORS 634.705(5)). A pesticide that is not low-impact may be applied only to mitigate a declared pest emergency, or by or at the direction of a public health official.
• Plan currency: ORS 634.705 was amended by chapter 377, Oregon Laws 2025 (effective January 1, 2026); plans adopted before that date must be reviewed, updated and readopted no later than January 1, 2027, or five years from the most recent approval, whichever is later.
• Mandatory Notification & Posting: Requires 24-hour advance written notification to parents and staff and prominent physical warning signs posted at application sites 24 hours prior and 72 hours post-application.

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