Five Years of the Same Pyrethroid in Potatoes
A grower farms 320 acres of Russet potatoes under center pivots. For five seasons he has sprayed the same pyrethroid whenever he sees adult Colorado potato beetles, typically five to six times a year. Two years ago three June applications gave about 90% control; last July the same rate gave 20-30%, and he responded by going to the highest labeled rate and shortening the interval. He now reports more aphids and beet leafhoppers than he has ever had, plus purple-top symptoms. He has one new mode of action in the shed and wants to spray it now, at the maximum rate, back-to-back, to 'clean it up.' Forty hives of honeybees sit on a neighbor's alfalfa that blooms along his east pivot.
1. Build the diagnosis. What evidence separates resistance from application failure, and what does the aphid and leafhopper flare suggest?
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Two things are happening at once. The dose-response pattern is the classic resistance fingerprint: good control at the original rate, then a stepwise collapse, with control recovering only partially even at the maximum labeled rate. Application failure would look different - uniform poor results, coverage problems, wrong carrier volume, hard water, or a nozzle/calibration fault, and it would not have degraded so smoothly over seasons.
Confirm before acting: check the spray records to see whether the product, rate, and timing were consistent; confirm the pest is actually the target (CPB adults versus look-alikes, and whether the damage is CPB or another defoliator); run a quick dose-response in a bucket or a small replicated strip against an untreated check; rule out pH or mixing problems. Five consecutive seasons of a single IRAC group is powerful supporting history.
The secondary pests are a clue, not a coincidence. A pyrethroid used five to six times a year is broad-spectrum; it suppresses the predators and parasitoids that keep aphids and leafhoppers in check, so those populations rebound while the beetle does not. That means part of the answer is doing less, not more. The worst move available is back-to-back maximum-rate applications of the one new mode of action - that is how the second group gets lost in a single season.
2. Write a season plan for the beetle that respects resistance management, thresholds, and the neighboring bees.
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1. Rotate modes of action, and only one at a time. Move off the pyrethroid class entirely this season and place the new product in a different IRAC group, at its labeled rate - not above it - and not repeated back-to-back. Rotate to another group next generation. Never mix two products from the same group; a premix counts as one of them.
2. Spray on numbers, not on sight. Scout and count (beetle populations and defoliation), then treat at the local action threshold for that growth stage rather than every time an adult appears. Early-season beetle pressure on small plants is where the threshold is tight; later, larger canopies tolerate more. Keep the untreated check strips to evaluate every pass.
3. Protect the beneficials. Reducing broad-spectrum passes lets predators do part of the work and takes pressure off the aphid and leafhopper problem. Consider at-plant or seed-applied options where the label provides them, and treat the secondary pests only if they cross their own thresholds.
4. Bees. Read the pollinator statements on each label before application, notify the beekeeper ahead of time, avoid spraying the pivot edges during bloom and during foraging hours (evening and night applications reduce exposure), and use drift-reducing nozzles and buffers on the east side. Document notification.
5. Record and review. Track IRAC group, rate, date, and outcome by field so the next rotation decision is based on data.