5.6 Why Applications Fail: Factors Influencing Pesticide Effectiveness
Key Takeaways
- ATCP 29.30(6)(d) requires knowledge of both factors influencing pesticide effectiveness and pest resistance, and the practical skill is distinguishing them, because most control failures are application failures rather than resistance.
- Resistance is suggested by a failure that is patchy within a field, worsens over seasons, affects only one mode-of-action group while others still work, and occurs at a correct rate under good conditions.
- Carrier water quality changes efficacy: hard-water calcium, magnesium, and iron cations antagonize some weak-acid herbicides, and high pH accelerates alkaline hydrolysis of many organophosphate and carbamate insecticides in the tank.
- Spray volume and droplet size trade off against each other: contact products need higher carrier volume and finer droplets for coverage, while drift management pushes toward coarser droplets, and resolving that tension is a per-job decision.
- Weed size at application is usually the single largest controllable efficacy factor for postemergence herbicides, and applying at label rate to oversized weeds is a common cause of failure misread as resistance.
The Diagnosis Everyone Reaches For
When a treated field still has pests two weeks later, "resistance" is the first word out of most people's mouths. It is usually wrong, and getting it wrong is expensive twice over: the grower switches away from a chemistry that still works, and the actual cause — a plugged nozzle, hard water, an oversized weed, a rain event — goes unfixed and repeats next season.
ATCP 29.30(6)(d) deliberately bundles "factors influencing pesticide effectiveness and pest resistance to pesticides" into one competency, because you cannot evaluate the second without ruling out the first.
The Resistance Test
Before blaming resistance, check whether the failure has the resistance signature.
| Observation | Points toward resistance | Points toward application failure |
|---|---|---|
| Pattern in the field | Patchy survivors scattered among dead individuals of the same species | Uniform failure, or failure in strips, edges, or one part of the field |
| Other species | Other susceptible species controlled normally | Everything survived |
| History | Same mode of action used repeatedly for several seasons | New product, new field, or a changed practice |
| Other modes of action | A different MoA group still works | Nothing works |
| Rate and conditions | Correct rate, good conditions, good coverage documented | Rate, timing, water, or coverage questionable |
| Trend | Worsens year over year | One-off event |
A single uniformly failed application in the same season a new sprayer was put into service is an equipment story, not an evolution story.
The Eight Non-Resistance Causes
1. Misidentification
The most complete failure mode: a perfect application of the wrong product. A grass herbicide on a sedge, a fungicide on a virus, a rodenticide aimed at a mole. Section 5.5 covers the diagnostic work that prevents this.
2. Timing Relative to Pest Stage
- Weeds: for most postemergence herbicides, weed size at application is the largest single controllable variable. A label written for weeds up to 4 inches will underperform badly on 10-inch weeds at the same rate — and raising the rate above the label maximum is never a legal fix.
- Insects: young larvae are far more susceptible than late instars, and eggs and pupae are often not susceptible at all.
- Diseases: protectant fungicides must be on the leaf before infection. A protectant applied after symptoms appear cannot cure the existing infection.
3. Rate and Calculation Errors
A misplaced decimal, a confusion between product and active ingredient, a broadcast rate applied to a banded application, or an uncalibrated sprayer all produce underdose. Chapters 6.2 and 6.3 exist to prevent this. ATCP 29.50(5) makes using equipment that cannot be properly calibrated to the label rate a violation in its own right.
4. Coverage and Spray Volume
Contact products act only where they land. A contact fungicide or insecticide applied at low carrier volume with coarse droplets can leave a third of the target surface untreated, and the survivors are not resistant — they were never treated. Dense canopies need higher volume, and undersides of leaves need droplet energy or air assist to reach.
The tension is real: coverage pushes toward finer droplets and higher volume; drift management pushes toward coarser droplets. Resolve it per job, and prefer coarser droplets with higher volume over finer droplets when both goals are in play.
5. Carrier Water Quality
Water is 95 percent or more of most tank mixes, and its chemistry is not neutral.
- Hardness. Calcium, magnesium, iron, and sodium cations bind to some weak-acid herbicides in the tank and form poorly absorbed complexes. The standard remedy for susceptible chemistry is a water-conditioning agent added before the herbicide, and the label will say whether the product needs one.
- pH. Many organophosphate and carbamate insecticides undergo alkaline hydrolysis, degrading in the tank when the carrier water is alkaline. With sensitive products the loss can be substantial within hours, which is why buffering agents exist and why a tank mix should not be left standing overnight.
- Turbidity. Suspended clay and organic matter can adsorb and deactivate some active ingredients, and dirty water plugs screens and nozzles.
Test the water rather than assuming. Wisconsin well water varies widely in hardness and pH across the state.
6. Environmental Conditions After Application
- Rainfastness. A systemic product needs uptake time; rain before that window can remove most of the dose. Rainfast periods on labels range from under an hour to many hours.
- Drought and plant stress. Stressed plants close stomata, thicken cuticles, and slow translocation, all of which reduce herbicide uptake and movement.
- Temperature. Very hot conditions increase volatilization and can cause crop injury; cold slows both pest metabolism and herbicide activity, delaying and reducing effect.
- Soil-applied residuals need an activating rainfall or mechanical incorporation to move into the germination zone. A dry two weeks after a preemergence application looks exactly like product failure.
7. Adjuvant Omission or Mismatch
When a label requires a specific adjuvant — a nonionic surfactant, crop oil concentrate, methylated seed oil, or ammonium sulfate — that requirement is mandatory language, and omitting it can cut efficacy dramatically. Using the wrong adjuvant class is equally damaging: an oil concentrate where the label calls for a nonionic surfactant can cause crop injury without improving control.
8. Reinfestation and Pest Biology
Sometimes the application worked and the pest came back. Migratory insects re-enter treated fields, a new weed flush germinates after a residual expires, structural pests re-enter from an untreated harborage, and multi-generational pests simply produce another generation. This is a monitoring conclusion, not a product conclusion, and repeating the same product on a reinfestation is how genuine resistance pressure gets built.
The Failure Investigation Checklist
When a treatment underperforms, work the list in order:
+-----------------------------------------------------------------------------+
| 1. Was the pest correctly identified? |
| 2. Was the pest stage or weed size within the label's window? |
| 3. Was the rate correct -- product vs. a.i., broadcast vs. band? |
| 4. Was the sprayer calibrated, and were nozzles worn or plugged? |
| 5. Was coverage adequate for a contact product? |
| 6. What was the carrier water hardness and pH? |
| 7. Did rain, drought, heat, or cold intervene? |
| 8. Was the required adjuvant used, at the right rate and class? |
| 9. Is this reinfestation rather than survival? |
| 10. ONLY IF 1-9 CLEAR: consider resistance and change mode of action. |
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Record the answers. A documented failure investigation is what turns one bad season into a corrected practice, and it is also the record that protects an applicator when a customer alleges negligence. It is worth remembering that under ATCP 29.30(9)(d) unnecessary use and misuse of pesticides is itself a competency: reapplying a product that failed for a knowable, fixable reason is exactly the behavior the standard is written against.
A grower applies a postemergence herbicide across an 80-acre field. Two weeks later, control is excellent everywhere except that scattered individual plants of one weed species survived throughout the field, while all other weed species were controlled. The same mode of action has been used on this field for six consecutive seasons. What is the most likely explanation?
An applicator mixes an organophosphate insecticide in well water with a pH of 9.0 and leaves the loaded sprayer overnight before applying it the next afternoon. Control is poor. What most likely happened?