Pink Bollworm Decisions in a Late, Hot Bt Cotton Crop
A 1.6 ha Bt cotton crop in Yavatmal district was sown on 5 June and is now 95 days old, in peak boll formation. Four pheromone traps are installed, and for the last three nights they have caught 11, 13 and 12 moths per trap per night. A scouting round on 100 plants finds 8 rosette flowers per 100 plants and exit holes in 6 out of 100 green bolls. Last season the same field had heavy pink bollworm damage; the farmer sprayed emamectin benzoate five times, followed by two rounds of lambda-cyhalothrin, with little effect. He still has 500 ml of lambda-cyhalothrin at home and argues that his neighbour sprays every fifth day and gets clean bolls. Picking is expected from about 150 days after sowing. Rainfall in August was 40% below normal and day temperatures are 34-36 °C.
1. Using the monitoring data, decide whether a spray is justified now, and evaluate the farmer's plan to use lambda-cyhalothrin every five days.
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Action is justified, but not with that product or that interval.
- The usual action threshold for pink bollworm is 8 moths per trap per night for three consecutive nights, or about 10% rosette flowers. Catches of 11-13 for three nights clearly exceed the moth threshold, and 8% rosettes with 6% boll damage confirm that eggs and young larvae are present. The spray window is the next 2-3 days, aimed at young larvae before they enter the boll.
- Lambda-cyhalothrin is the wrong choice. It is a pyrethroid, and pink bollworm has widespread resistance to pyrethroids in Vidarbha; the two rounds last season with little effect are evidence of that on this very field. Pyrethroids also flare whitefly and mites and kill the natural enemies that currently help.
- A five-day repeat interval is worse. It selects resistant individuals hard within two generations, wastes money, and leaves illegal residues. Reject both the product and the calendar.
Do instead: pick a larvicide from a different mode-of-action group from last season's emamectin benzoate (for example a Group 28 diamide or a Group 1 carbamate), at the label dose with 375-500 L/ha of spray for canopy penetration. Alongside it, send labour through the field twice a week to pick and destroy rosette flowers and infested green bolls - this removes the next generation at no chemical cost.
2. The crop is at 95 days and picking is expected from about 150 days. What does that mean for product choice and for the rest of the season's spray plan?
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- Pre-harvest interval drives the last spray date. Most recommended larvicides carry a 15-21 day PHI. At 95 days, a spray now is safe for a pick starting at 150 days. But working back from day 150, the last spray must be completed at least one full PHI before picking - so no spray after roughly 130 days for a 21-day-PHI product. Check the label of whichever product is chosen rather than assuming.
- Rotate modes of action. No more than two sprays of the same group in a season; keep a written spray log of date, product, group and dose so the next choice is not a repeat.
- Use the traps to time, not the calendar. Spray only when catches cross the threshold for three nights; otherwise skip and keep scouting twice a week.
- Stop spraying once bolls open. Late sprays stain lint, leave residues and do not protect bolls that are already opening; at that stage, pick promptly and frequently.
- Watch for secondary pests. With the hot, dry August, sucking pests may build up; do not use pyrethroids, which would make them worse, and treat them only above their own thresholds.
3. The neighbour sprays every fifth day. Explain to the farmer how that practice affects him, and what he should do differently next season.
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How the neighbour's practice affects this farmer:
- Repeated calendar sprays over a wide area select for resistance, and resistant moths do not respect field boundaries - they will move into this field.
- Broad-spectrum, frequent sprays destroy predators and parasitoids across the village, so secondary outbreaks of whitefly and mites become more likely for everyone.
- If the neighbour's field is a continuous supply of late-season hosts, the pest pressure on this field lasts longer.
What to change next season:
- Sow on time with the monsoon in June, and avoid a late or extended crop that carries the pest into winter.
- Keep a refuge of non-Bt cotton around the Bt block as recommended, and use certified seed of a known event rather than saved seed.
- Destroy crop residues after harvest and avoid ratoon cotton, which is a bridge for the next generation.
- Rotate with soybean or tur to break the cycle, and coordinate sowing and trap-based decisions with neighbouring farmers so the whole village acts together instead of one field spraying every fifth day.