Colorado Potato Beetle That Will Not Die
A 40-acre potato field is in its third potato crop in four years. At planting, the grower used an insecticide seed treatment; later, two foliar applications of a Group 4A insecticide were made eight days apart. The label allows no more than two applications per season and requires a seven-day interval. Scout counts average 25 large larvae per plant, well above threshold, and survivors include adults, eggs, and all larval stages across the field, not just the edges. A nearby field with the same variety and product has dead beetles. The sprayer was calibrated last week, and the applicator used the labeled rate. The grower wants to add a third application at a higher rate and blame the product.
1. Is resistance, application failure, or something else most likely? What evidence would you gather before deciding?
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Resistance is the leading hypothesis: multiple life stages survive across the field, the rate and calibration were correct, and beetles in a nearby field died after the same product. Application failure would more likely leave patchy survival, skips, or edge effects. Still, verify coverage with water-sensitive cards, check nozzle output and speed, confirm the species, and review the seed treatment, because the seed treatment and foliar sprays may all be Group 4A. A simple bioassay-collect adults or larvae from the field and a susceptible population and expose both to the labeled rate-would strengthen the diagnosis. Do not assume resistance until application and species questions are ruled out, but do not repeat the same treatment while investigating.
2. The grower demands a third application at a higher rate. What should the applicator say, and what alternatives are appropriate?
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The grower cannot apply a third Group 4A application or a higher rate; both would violate the label and increase selection pressure. The applicator should say no, explain the label limit, and pivot to a different mode of action if the label allows, such as a Group 5 or Group 28 product, while checking preharvest interval, rotational restrictions, and effects on beneficials. Nonchemical options include cultivation, flaming where suitable, crop rotation, and destroying volunteer potatoes. If the population is confirmed resistant, save samples and contact extension. The applicator should also avoid treating the whole field if only hot spots remain; spot treatments and edge management can reduce selection and cost.
3. Design a multi-season plan to reduce resistance and keep costs down.
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Next season, rotate away from potatoes for at least two years if possible, plant a resistant variety, and use clean seed. Treat seed only when scouting justifies it, and do not follow a seed treatment with the same foliar mode of action. Rotate among at least two or three modes of action, never apply the same group consecutively, and keep an untreated check strip to measure control. Scout weekly, use thresholds, and preserve predators. Manage volunteer potatoes and cull piles. Keep records of every application, including mode of action, so the farm can see how many times each group was used. The goal is to reduce selection pressure, not just kill the current generation.