Pink Bollworm Control Failure in Bt Cotton
A 1.5 ha Bt cotton field in Jalna was sown on 20 June. At 95 days after sowing, the farmer finds 14 rosette flowers in 100 plants and 12% of green bolls with exit holes and pink larvae inside. Pheromone traps with pink bollworm lures catch 22 moths per trap per night. He has already sprayed emamectin benzoate 5% SG twice and lambda-cyhalothrin once, the last spray 6 days ago. The crop is under drip irrigation and the canopy is dense. Neighbouring fields are also Bt cotton sown in the same window. The farmer wants to spray a mixture of two insecticides immediately to 'finish the pest' before the next picking. Weather is dry with maximum 33°C. He has about ₹5,000 for plant protection and three pickings still expected.
1. Identify the pest and assess whether the economic threshold is crossed. Explain why the previous sprays underperformed.
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The rosette flowers, pink larvae inside bolls and trap catch identify pink bollworm (Pectinophora gossypiella). The common action thresholds are about 8 moths per trap per night, 5–10% rosette flowers and 10% boll damage; the field is above all three at 22 moths, 14% rosettes and 12% boll damage. The previous sprays failed for several reasons. The larvae feed inside rosettes and bolls, so contact insecticides reach them poorly. Bt toxin expression in cotton declines after about 90–100 days, reducing control of neonate larvae. Two sprays of the same mode of action plus a pyrethroid select resistant survivors, and the dense canopy makes coverage uneven. The last spray was also applied before the current larval flush was fully exposed, so timing was poor.
2. Recommend a practical 10-day IPM and chemical plan. Justify the choice and rotation.
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First remove and destroy rosette flowers and affected green bolls daily; this reduces the breeding population and is cheap. Keep 20 pheromone traps per hectare for mass trapping and monitoring. Do not tank-mix two insecticides. Spray one effective mode of action, such as chlorantraniliprole 18.5% SC at the label dose (group 28), thiodicarb 75% WP (group 1A) or profenophos 50% EC (group 1B), using a sticker and directing the spray to bolls in the evening. Rotate to a different group after 10–14 days only if live larvae or fresh damage are still above threshold; avoid pyrethroids. Follow the label pre-harvest interval before the next picking. This plan targets exposed neonates, rotates chemistry and preserves natural enemies better than an indiscriminate mixture.
3. The farmer proposes uprooting the crop now and sowing a short-duration pulse. Evaluate this decision.
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At 95 days after sowing, with three pickings still expected and only 12% boll damage, uprooting would destroy most of the remaining income. The cost of one or two carefully targeted sprays is small compared with the value of the standing crop, and the pest can still be managed by rosette destruction and timely picking. Uprooting is justified only if most bolls are destroyed and no marketable pickings remain, which is not the case here. A short-duration pulse after uprooting would also face residual pink bollworm moths and late sowing risk. The better decision is to protect the bolls, pick promptly, and destroy crop residue after the last picking to reduce carry-over.