Pink bollworm after six pyrethroid sprays
A 6 ha Bt cotton field in Sirsa, sown on 5 May, is in its third picking. The farmer has sprayed lambda-cyhalothrin or cypermethrin six times since 20 August, each costing ₹1,050/ha, because he saw moths and 'worms in bolls'. Now he finds rosette flowers in 8 of 10 plants sampled, 30% of green bolls with exit holes, and a pheromone trap catch of 90 moths/trap/week. Whitefly nymphs are dense on lower leaves with honeydew and sooty mould, and about 10% of plants show upward leaf curling. The crop has 3-4 pickings left, worth about 1.5 q/ha, with lint at ₹7,200/q and picking cost ₹1,200/q. Neighbouring fields are already being terminated. He wants two more pyrethroid sprays plus one 'combination' product to save the last pickings, and asks whether he can keep the crop for ratoon.
1. Explain why his spray strategy failed, naming the biological and resistance reasons, and why the pyrethroids may have made the whitefly worse.
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Three separate failures.
Biology. Pink bollworm larvae bore into bolls and feed on seed. Foliar contact insecticides never reach them; only eggs and newly hatched larvae are exposed, and by the time rosette flowers appear the larvae are already inside.
Resistance. Six applications of one mode of action is the textbook way to select resistance. Pyrethroid resistance in pink bollworm is well documented in north India, so the sprays were both mistimed and increasingly ineffective.
Natural-enemy destruction. Pyrethroids are broad-spectrum. They kill Trichogramma, predators and spiders that would otherwise suppress whitefly, so whitefly flares; honeydew and sooty mould then cut photosynthesis and lint quality. The 90 moths/trap/week confirms a heavy population, and the leaf curling suggests whitefly-transmitted virus as well.
2. Recommend a plan for the remaining crop, including whether to continue or terminate, and support it with the economics given.
Read the worked answer
Stop pyrethroids immediately. Remove and destroy rosette flowers twice a week, since they carry larvae and pupae; keep pheromone traps for mass trapping. If harvestable bolls remain, use one spray from a different mode-of-action group, such as spinosad or emamectin benzoate - not a pyrethroid and not an unlabelled combination.
Economics of termination. Remaining yield about 1.5 q/ha × ₹7,200 = ₹10,800/ha gross; less picking ₹1,200/ha = about ₹9,600/ha net. One more spray plus labour costs ₹1,500-2,000/ha, and the crop still carries a large larval population into next season.
Decision. Terminate by mid-December, shred stalks, destroy ratoon and deep plough. The carry-over cost to next year's crop is larger than the value of the last picking, which is why neighbours have already terminated. Do not keep the crop for ratoon.
3. List the preventive measures for next season, with the reason for each.
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- Sow on time in May, so the crop's fruiting window does not extend into the cool, late season when pink bollworm multiplies.
- Plant a 5% non-Bt refuge to slow resistance selection.
- Install pheromone traps from about 45 days after sowing and act on trap catch, not on sight of damage.
- No pyrethroids, especially early season, to protect natural enemies and avoid selecting resistance.
- Rotate modes of action and never use the same group twice in succession.
- Avoid prophylactic sprays; conserve Trichogramma and predators.
- Destroy stubble, avoid ratoon and deep plough to break the pest's carry-over.
- Synchronise sowing across the village so one field does not bridge the pest between seasons.
- Choose a variety with tolerance to whitefly and leaf curl virus, since the field already shows both.