Pink bollworm at 75 days: trap counts, a spent spray history and a 45-day picking window
A 12 ha block of Bt cotton in a rainfed tract is at 75 days after sowing. Pheromone traps have caught 11, 13 and 12 pink bollworm moths per trap per night on three consecutive nights. Scouting of 20 plants per acre shows 6 rosette flowers per 20 plants and 3 live larvae in 20 green bolls. The farmer has already applied emamectin benzoate twice and a pyrethroid once in the past month, and sweep counts show very few ladybird beetles or green lacewings. First picking is expected in about 45 days. He has monocrotophos and a pyrethroid in stock and wants an immediate tank-mix, followed by four more weekly sprays. Neighbouring farmers copy his practice, and last year this block lost 22% of bolls to pink bollworm despite nine sprays.
1. Is a spray justified at this point? Work through the trap counts, rosette count and larval count separately, and state what you would do.
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The three signals do not agree, and that disagreement is the decision.
- Traps: 11-13 moths per trap per night for three consecutive nights is above the commonly used action threshold of about 8 moths per trap per night for three nights. Trap catch is a monitoring signal, not a spray trigger on its own - it says adults are emerging and egg-laying is starting.
- Rosette flowers: 6 per 20 plants is at or above the usual threshold of about 5 per 20 plants. This is the most useful signal, because a rosette marks where a neonate larva has entered the flower bud. It indicates the infestation is active now.
- Larvae in bolls: 3 per 20 bolls is below the usual 8 per 20 bolls action level. Once larvae are inside bolls, most contact insecticides cannot reach them anyway.
Conclusion: one well-timed, correctly chosen spray is justified now, targeting neonate larvae and eggs while rosettes are forming. His four weekly sprays plus the tank-mix are not justified: they would destroy the remaining natural enemies, select harder for resistance, and run into pre-harvest intervals before picking.
Do not spray as a reaction to trap counts alone; use the rosette evidence to time it.
2. Build a resistance-managed, pre-harvest-interval-compliant plan for the remaining 45 days, including the non-chemical measures you would insist on.
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Constraints to respect: only about three weeks of safe spraying remain before the label pre-harvest intervals (mostly 15-30 days, longer than a week) force a stop, so the realistic budget is 1-2 sprays, not five. Pyrethroids and emamectin have already been used, so resistance selection is well advanced, and natural enemies are depleted - a pyrethroid now will flare whitefly and mealybug.
Plan:
- Spray 1 (now): one translaminar or diamide-group larvicide with a different mode of action from the previous sprays, at the label dose, in about 200 L water, directed at flower buds and young bolls. No tank-mix with a pyrethroid, no monocrotophos (high mammalian toxicity, long pre-harvest interval, widespread resistance).
- Spray 2 (only if rosettes continue): a different mode of action again, scheduled so the last application falls at least the full pre-harvest interval before first picking. After that, stop - the interval is not negotiable.
- Non-chemical, and non-optional: hand-pick and destroy rosette flowers and infested green bolls twice a week; mass traps at 40-50 per hectare for monitoring and mass trapping; maintain the non-Bt refuge prescribed for the zone (confirm the current refuge requirement on the seed packet and in the local package of practices); fix a community crop-termination date and do not extend the crop for a late flush; after harvest, uproot and destroy stubble and deep plough to bury pupae, because the 22% loss last year came from carry-over in the block.
- Community level: synchronous sowing and synchronous termination across the 12 ha, since pink bollworm moves between fields.
3. The farmer is the opinion leader whom neighbours copy. How do you handle him so that the message spreads correctly rather than being rejected?
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Do not fight him on spray number - fight on the calendar and the economics.
- Give him control of data, not just advice. Hand him the trap-count sheet and let him record catches for the block. A farmer who owns the monitoring numbers stops being a spectator and starts defending the numbers to his neighbours.
- Show the arithmetic, not the instruction. Nine sprays at roughly Rs 1,000-1,200 per hectare per spray is Rs 9,000-11,000 per hectare, against a 22% boll loss. Ask which of his sprays can be shown to have paid for itself. Resistance and enemy depletion mean later sprays have diminishing returns.
- Reframe the goal: protect the 45-day picking window rather than 'kill every larva'. State plainly that the last spray must fall a full pre-harvest interval before picking, so the choice is one or two right sprays, not five.
- Use him as co-monitor and host: hold the field day at his field, with the trap and rosette counts on a board. A field day on his land, with his numbers, carries further than a government plot.
- Manage the risk of failure: if boll damage still rises, be explicit that the cause is the region-wide infestation and the fixed termination date, not the reduced spray count, and agree in advance what 'failure' would look like. Otherwise the first bad week will be read as proof that he was right.