Failed sprays and very high pink bollworm trap catches in Bt cotton
On 15 September, in a village in central India, 42 farmers with 60 ha of Bollgard II cotton report that sprays have failed. The crop is 150 days old with a good boll load. Pheromone traps in five fields average 95 moths per trap per night, up from 12 in July. Scouts find 14% rosette flowers and 20% of green bolls with exit holes; larvae are inside the bolls. Farmers have sprayed emamectin benzoate plus thiodicarb twice and chlorantraniliprole once, spending Rs 9,200 per hectare, with no reduction in damage. No refuge was planted. The nearest ginning mill will accept seed cotton until 20 October; beyond that, picking quality falls and the mill offers 12% lower prices. The agriculture department has recommended a fourth spray of a pyrethroid mixture. Rain is forecast in four days.
1. Interpret the trap and damage data, and evaluate the department's recommendation.
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A catch of 95 moths per trap per night is far above the action threshold of about 8 moths per trap per night for pink bollworm. A rise from 12 in July to 95 in mid-September describes a late-season population explosion. Rosette flowers at 14% and exit holes in 20% of green bolls mean the larvae are already inside the fruiting bodies, where contact and ingestion insecticides reach them poorly.
Three sprays costing Rs 9,200/ha with no visible effect point to two possibilities that can coexist: resistance, and larvae protected inside bolls at the time of spraying. Bollgard II expresses Cry1Ac and Cry2Ab, and pink bollworm has developed resistance to both in central and southern India. Repeated use of the same or related modes of action adds selection pressure.
The pyrethroid recommendation is poor. Pyrethroids are weak against pink bollworm, are disruptive to natural enemies, and can flare whitefly and other sucking pests; adding a fourth spray after three failures wastes money and increases resistance selection. The department's advice should be rejected and replaced with a crop-termination and sanitation plan.
2. Recommend an action plan for the next 30 days that protects income and reduces the carry-over population.
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First, stop spraying. There is no evidence that a fourth spray will pay, and the money is better spent on harvest management. Rain in four days makes spraying even less attractive because wash-off and poor coverage are likely.
Second, bring harvest forward. Pick the opened bolls in two or three rounds and aim to finish before 20 October, because the 12% price penalty after that date is a certain loss while further sprays are an uncertain one. Do not uproot immature bolls in a way that mixes trash; grade and dry the seed cotton properly.
Third, destroy the crop and stubble immediately after the final picking. Cut the stalks, shred or burn them under supervision, and plough. Do not ratoon or leave standing stalks, because they carry diapausing larvae into the next season.
Fourth, community action: all 42 farmers should terminate within the same week. A single late field can reinfest the village. Install 40 pheromone traps per hectare for mass trapping, and continue monitoring until the crop is removed.
Fifth, document the losses and spray history for the resistance-monitoring team, and report the field to the state extension authority so that the area is flagged for the next season.
3. Design next season's resistance management plan for the village, with specific practices and one monitoring indicator.
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The plan must combine genetics, ecology, and community timing.
Refuge: plant a non-Bt refuge of at least 5% of the area, as a border or in strips, and ensure it is not sprayed with the same insecticide. This is non-negotiable; the absence of refuge this season contributed to selection.
Sowing window: synchronise sowing across the village so that fruiting does not extend into the late-season pest buildup. Avoid staggered sowing that provides a continuous food bridge.
Crop termination: set a community termination date in early October and enforce it through the village committee. This breaks the diapause cycle.
Chemistry: rotate modes of action and never apply the same group twice in a season. Reserve newer molecules for threshold-based use, and use pheromone traps to time any spray.
Traits: do not rely on Bollgard II alone; consider a stacked trait with a different protein, and pair it with the refuge.
Monitoring indicator: weekly moths per trap per night, plus rosette flower counts from 20 plants in five fields. A trigger of 8 moths per trap per night should initiate scouting, and 10% rosette flowers should trigger a targeted, rotated insecticide. Report the indicator to the village committee every week.