Mites, a Worn Sprayer, and an Organic Neighbor
On July 20 a PCA samples a 60-acre almond orchard and finds two-spotted spider mites on 40% of leaves in the sample, with predatory mites on about 15% of leaves. The orchard has received three pyrethroid applications in the previous 18 months, and the grower reports that the last one "did almost nothing." A neighboring 20-acre certified organic vegetable farm lies 1/4 mile downwind, and the neighbor's honey bee hives sit at the shared fence. The forecast is 98°F with 10 mph wind. The grower wants to "spray everything with the strongest thing" using an air-blast sprayer last calibrated three years ago. The nozzles are visibly eroded and the pressure gauge reads 15 psi below the target. The label for the candidate product allows 100–400 gallons per acre and 12–24 fluid ounces per acre, and requires a 12-hour restricted entry interval.
1. Is a treatment justified, and what product strategy makes sense given the history and the predators?
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Treatment is defensible but not automatic. Forty percent of leaves infested is at or above the mid-season action threshold for two-spotted spider mite in almond, so some action is justified. The 15% predator level means beneficials are still working, so the decision should be made block by block rather than across all 60 acres.
Why not a pyrethroid. Three pyrethroid sprays in 18 months, with the last one ineffective, points to resistance. Pyrethroids also flare spider mites by killing predators, which would make the next outbreak worse. "The strongest thing" is the wrong instinct.
Strategy:
- Choose a selective acaricide from a different IRAC mode-of-action group than anything used in the past 12–18 months, at the full labeled rate.
- Avoid horticultural oil at 98°F; oil plus heat can burn foliage. Use it only if the label permits and temperatures moderate.
- Treat only blocks over threshold and preserve untreated refuges for predators.
- Re-sample 7–10 days after treatment to confirm the population is falling and to detect flare-ups early.
Document the sampling counts and the resistance reasoning in the recommendation; that record supports both the agronomic decision and any later drift complaint.
2. What is wrong with the sprayer, and how do you get it to deliver the labeled rate before spraying?
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What is wrong. A sprayer last calibrated three years ago, with eroded nozzles and a gauge reading 15 psi low, cannot deliver a uniform labeled rate. Worn nozzles over-apply in the worn pattern and under-apply elsewhere, and the low gauge means actual flow is below the calibration figure. Uniform coverage is what makes a labeled rate meaningful.
Fix it in this order:
- Replace all nozzles as a set - never mix new and worn nozzles.
- Verify the pressure gauge against a known-accurate gauge and replace if needed.
- Recalibrate with clean water: collect output from several nozzles for a timed interval and check nozzle-to-nozzle variation, replacing any nozzle more than about 10% off the average.
- Compute gallons per acre: GPA = (gallons per minute per nozzle × 5940) ÷ (mph × nozzle spacing in inches). At 3 mph and 20-inch spacing, 100 gallons per acre requires about 1.0 gallon per minute per nozzle.
- Set the labeled rate (12–24 fluid ounces per acre) from the calibrated output, and record the calibration date and results.
Do not compensate for a worn sprayer by raising the rate or the pressure; that increases drift and still leaves streaks.
3. What drift, pollinator, and neighbor protections are needed, and what would you record?
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Drift is the deciding factor. A certified organic farm 1/4 mile downwind and hives at the fence mean an off-target event becomes a drift complaint, a commissioner investigation, and a possible loss of organic certification for the neighbor - expensive and avoidable.
Protections:
- Spray only when wind is steady and below any label limit; measure wind at the site, not from a forecast.
- Prefer early morning or evening, and avoid inversions.
- Use drift-reduction nozzles at the lowest effective pressure, keep the air-blast directed into the canopy, and shut off the outermost downwind row.
- Maintain a buffer between the last treated row and the fence; consider not treating the downwind edge at all.
- Check the label for bee precautions and notify both the neighbor and the beekeeper ahead of time. Almond bloom is long past in July, but foragers still visit weeds and nearby crops.
Record: date, time, temperature, wind speed and direction at the site, product and rate, gallons per acre, nozzles and pressure, buffer width, calibration result, and the restricted entry interval posted. Those records are the only defense if a complaint is filed, and they are also how the next applicator knows which mode of action was used.