6.4 IPM Principles and Common Pest Recognition
Key Takeaways
- IPM has five linked components: identify the pest, monitor populations, compare to a threshold, act with the right tactic, and evaluate the result.
- The Economic Injury Level (EIL) is the pest density at which the cost of crop damage equals the cost of control; the Economic Threshold (ET) is set below the EIL to allow lead time before damage occurs.
- Pest identification covers four main groups in Iowa: insects, plant pathogens (diseases), weeds, and nematodes; correct ID drives the choice of tactic and product.
- Common scouting methods include whole-plant counts, sweep nets, visual rating, transect walks, and disease incidence estimators; sample size and pattern must represent the field.
- Action thresholds may be nominal (experience-based), fixed percent of EIL, or descriptive (model-based); treating below threshold wastes money and kills natural enemies.
6.4 IPM Principles and Common Pest Recognition
Quick Answer: IPM is a five-step cycle: identify the pest, monitor its population, compare to a threshold, act with the right tactic, and evaluate the result. The Economic Injury Level (EIL) is the pest density where control cost equals crop damage; the Economic Threshold (ET) is set below the EIL so action can be taken before injury occurs.
The Five Components of IPM
Integrated Pest Management is a decision framework, not a pesticide-free program. It uses chemical control as one tool among cultural, biological, and mechanical tactics. The five components are linked: skipping any one weakens the others.
- Identify the pest. Misidentification leads to the wrong tactic. A disease can look like insect feeding; nutrient deficiency can mimic herbicide injury. Confirm the pest before acting.
- Monitor populations. Use a defined scouting pattern and sample size so the count represents the field, not just the field edge.
- Compare to a threshold. The threshold is the pest density at which action pays for itself. Below the threshold, do not treat.
- Act with the right tactic. Choose the least-risky effective option: cultural (rotation, planting date), biological (predators, pathogens), mechanical (cultivation, mowing), or chemical (the labeled pesticide).
- Evaluate. Record what worked and what did not; use the result to refine the next decision.
Economic Injury Level vs. Economic Threshold
These two terms drive every IPM decision.
| Term | Definition | Practical Meaning |
|---|---|---|
| Economic Injury Level (EIL) | Lowest pest density that causes economic damage; cost of damage equals cost of control | Break-even pest density |
| Economic Threshold (ET) | Pest density at which action should be initiated to prevent reaching the EIL | Operational trigger; always set below EIL |
The EIL is a calculated value driven by the formula EIL = C / (V x I x D x K), where C is control cost per unit, V is crop market value, I is injury per pest, D is damage per injury unit, and K is the proportional reduction in pest attack from control. When control cost rises, EIL rises (treat later); when crop value rises, EIL falls (treat earlier).
The ET is set below the EIL to give lead time for scouting intervals, pest growth rate, and the delay between decision and treatment. A common rule of thumb puts the ET at about 80% of the EIL, but fast-growing pests like soybean aphid may need an ET well below the EIL.
Pest Identification: Four Groups
Iowa field and ornamental pests fall into four broad groups. Correct group identification determines the management tactic and the product class.
| Group | Examples | Key ID Clues |
|---|---|---|
| Insects and mites | Corn rootworm, soybean aphid, European corn borer, spider mites | Six legs (eight in mites), chewing or piercing-sucking mouthparts, life stage dependent damage |
| Plant diseases | Gray leaf spot, sudden death syndrome, frogeye leaf spot, tar spot | Lesions, wilts, rots, mold; often tied to wet weather or residue |
| Weeds | Waterhemp, giant ragweed, marestail, Palmer amaranth | Leaf shape, growth habit, seedhead; many Iowa populations are herbicide-resistant |
| Nematodes | Soybean cyst nematode (SCN), corn nematode | Microscopic; above-ground symptoms resemble nutrient stress; soil test required |
Correct identification matters because the wrong tactic wastes money. A herbicide will not control an insect; an insecticide will not stop a disease. Within insects, the life stage matters: a systemic insecticide may control corn rootworm larvae but not adult beetles.
Scouting Methods
A scouting plan must represent the whole field, not just the field edge or one spot.
- Whole-plant counts: count insects on a set number of plants per location (common for soybean aphid).
- Sweep nets: standard 15-inch net; 20 sweeps per location for canopy insects like potato leafhopper.
- Transect walks: a walking pattern (X, W, or zigzag) that crosses the field, stopping at fixed intervals.
- Visual rating: estimate percent defoliation or disease incidence on a standard scale.
- Soil sampling: for nematodes and soilborne pathogens; pull cores in a zigzag, mix, and send to a lab.
- Traps: pheromone or sticky traps for moth pests; counts are compared to a trap-network threshold.
Record the date, growth stage, weather, and counts at each stop. The more stops, the better the estimate; ten locations per 80-acre field is a common minimum.
Action Thresholds in Practice
Thresholds are published by Iowa State University Extension for major Iowa pests. Examples:
- Soybean aphid: 250 aphids per plant (ET) through the R5 stage; this ET is set well below the EIL because aphid populations can double in days.
- European corn borer: treatment based on percent of plants with egg masses or larvae in the whorl, by growth stage.
- Weeds: size-based thresholds (for example, postemergence herbicide when waterhemp is less than 4 inches tall) reflect the yield loss curve and the resistance risk.
- Soybean cyst nematode: no single spray threshold; management is by resistant variety, rotation, and seed treatment.
Why Treat Below Threshold Is Costly
Treating below threshold wastes pesticide cost, can kill beneficial insects, and selects for resistance by exposing a large susceptible population to sub-lethal pressure. IPM explicitly preserves natural enemies so they hold pests below the threshold for free.
Which statement correctly describes the relationship between the Economic Injury Level (EIL) and the Economic Threshold (ET)?
A soybean aphid count of 250 aphids per plant through R5 is an example of: