Pink bollworm build-up in Bt cotton
Harjit Kaur grows 2 hectares of Bt cotton (BG-II hybrid) sown on 10 May on sandy loam soil. She installed 8 pheromone traps per hectare and counts them every morning. On 20 August the catch rose from 5 to 30 moths per trap per night. Scouting 20 random plants that morning she found 6 rosette flowers and, on dissecting 20 green bolls, 14 pink bollworm larvae. Her neighbour's field, sprayed seven times since July, now shows few rosette flowers but is covered in whitefly and the leaves are sticky with honeydew. Harjit plans to tank-mix a synthetic pyrethroid with an organophosphate, use 200 litres of water per hectare, and repeat every five days for four rounds. Money is short because cotton is already the year's main borrowing. She also wants to keep the cotton sticks standing until January to save labour, because her buffaloes will need fodder.
1. Is Harjit's decision to take action justified? Examine her spray plan and identify what is wrong with it.
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The decision to act is justified; the plan is not.
Justification: a trap catch jumping from 5 to 30 moths per trap per night, rosette flowers and 14 larvae in 20 bolls all show a population well above the level at which control is advised. Rosette flowers appear only after larvae are already inside the plant.
Fault 1 - the molecule: synthetic pyrethroids are the group pink bollworm has developed the strongest resistance to in Punjab, and they also flare whitefly and kill the natural enemies that were keeping it down. Her neighbour's whitefly outbreak after seven sprays is exactly that pattern.
Fault 2 - tank-mixing: mixing two products without an approved compatibility check adds cost, risks phytotoxicity, and is done to save a spray rather than to manage resistance.
Fault 3 - water volume: 200 L/ha is far too little to penetrate a full cotton canopy; bolls are where the larvae sit and they will not be reached.
Fault 4 - repeat interval: the same treatment every five days for four rounds selects for resistance and destroys beneficial insects.
2. Prepare a 30-day action plan for the field, including steps that cost no money.
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Weekly, free: hand-pick and destroy rosette flowers and infested green bolls twice a week and carry them out of the field. Larvae inside these structures are protected from sprays; removing them cuts the next generation directly.
Monitoring: continue the traps and record moths and rosettes. Spray only when counts cross the recommended threshold, not on a calendar.
Chemical choice: use only an approved insecticide from the current package of the state agricultural university, and rotate modes of action - one product once, then a product from a different group. No pyrethroid at all in August-September.
Application: 375-500 L water per hectare, nozzle directed into the canopy, evening spray, and stop spraying during peak bee hours.
Sequence: one round, wait the label interval (usually 10-14 days, not five), scout again, and decide the next round on counts. Alternating effective molecules, together with rosette picking, slows resistance far better than any tank-mix.
3. Harjit wants to keep the cotton sticks standing until January for fodder. What is your advice, and why does it matter beyond her own field?
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Advice: do not keep the crop standing into January. Terminate the crop after the last picking and destroy the stalks before the notified cut-off date.
Why: pink bollworm survives the off-season as larvae and pupae in the stalks, in seed cotton left in the field, and in gin trash. Stalks kept standing or stacked near the field become a carry-over nursery from which moths emerge at the start of the next season. Ratooning or extended cropping does the same.
How: shred or chip the stalks and incorporate or compost them; where burning is permitted, burn only after confirming with the agriculture department. Clean picking bags, transport carts and store rooms, and do not dump gin waste near fields.
Beyond her field: the pest is mobile. A community-wide stubble destruction drive works; one farmer doing it alone does not, because moths fly in from neighbouring fields. This is why the village should coordinate the date.
Fodder gap: plan fodder separately - berseem, oats or maize fodder can cover the buffaloes without keeping a pest reservoir standing.