Pink bollworm surviving single-gene Bt cotton after eight seasons
A 4.0 ha block of single-gene Bt cotton (Cry1Ac) was sown on 25 May and is in its eighth consecutive cotton season. No refuge was planted. Four pheromone traps per hectare catch 38–52 moths per trap per night for three consecutive nights. Scouting finds 14% rosette flowers and 22 larvae per 50 green bolls; several larvae are actively feeding inside Bt bolls. The farmer has sprayed emamectin benzoate 5% SG twice and thiodicarb 75% WP once. Spray volume was 250 L/ha with worn hollow-cone nozzles, and spraying was done around 10 a.m. in hot weather. A neighbouring field has late-sown okra and cotton. The farmer now proposes chlorantraniliprole plus a pyrethroid at double the label dose and wants to know whether the block can be saved.
1. Interpret the trap catch and field data. What is the most likely diagnosis, and what evidence makes a simple spray-timing failure less likely?
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The trap catch is roughly five to six times the commonly used action threshold of about 8 moths per trap per night, and 14% rosette flowers exceeds the usual 10% scouting threshold. More importantly, live larvae are feeding inside Bt bolls. A single late spray or a missed window can produce a flush of larvae, but it does not explain sustained high moth catches across the block, repeated rosettes, and larvae surviving on Cry1Ac tissue after three insecticide applications.
The history fits selection for pink bollworm resistance: eight seasons of the same single-gene trait, no refuge, and continuous cotton allow resistant individuals to survive and mate. The worn nozzles and 250 L/ha volume at full canopy also reduce coverage, but poor coverage alone would leave untreated surfaces, not larvae thriving inside Bt bolls. The nearby okra and late cotton can act as alternative hosts and refuges for the pest. Confirm with a laboratory dose–response test if available, but the field evidence is strong enough to stop treating the block as a routine spray-timing problem.
2. Evaluate the proposed chlorantraniliprole plus pyrethroid double-dose spray and give a resistance-management spray plan.
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The proposal is poor on three counts. First, doubling the label dose is off-label and increases selection pressure without improving control of larvae already inside bolls. Second, pyrethroids have a long history of resistance in pink bollworm and are not a reliable partner. Third, adding two modes of action at once removes the option of rotating them later.
A defensible spray plan targets early instars and exposed stages, not mature larvae in bolls. Use only label-recommended doses, calibrate nozzles, and increase coverage to 500–700 L/ha with a directed spray in the evening. Rotate modes of action-for example, chlorantraniliprole followed by thiodicarb or another effective group-and do not repeat the same group in consecutive sprays. Spray only when scouting thresholds are crossed, and stop spraying after the crop reaches the termination window. Remove and destroy rosette flowers and infested bolls where practical. The block may still be salvaged partially, but the farmer should not expect the same yield as a clean crop; the priority is to prevent the resistant population from carrying over.
3. Design a season-long IPM and resistance-management programme for the next cotton season.
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Before sowing: Destroy the previous stalks and bolls, shred and deep-plough, and do not allow ratooning. Rotate away from cotton for at least one season where possible, and avoid planting next to a late cotton or okra crop. Use a double-gene hybrid with two distinct Bt proteins and plant a structured non-Bt refuge-about 20% of the area or a border block-so susceptible moths can mate with resistant survivors.
Crop stage: Sow in a community window, not late. Install four pheromone traps per hectare and record catches twice weekly. Scout for rosette flowers and green-boll larvae. Use hand removal of rosettes and infested bolls in small blocks. Conserve natural enemies and avoid early prophylactic sprays.
Spray decisions: Treat only at threshold, rotate modes of action, and keep a written record. Do not use pyrethroids as a default. Ensure correct dose, water volume and nozzle condition.
After harvest: Terminate the crop by the agreed date, destroy stubble, and coordinate with neighbours. A community-wide termination and refuge programme is more useful than any single insecticide because it reduces the bridge that lets resistant pink bollworm survive between seasons.