Pink bollworm at 95 days in Bt cotton: spray now, and with what?
A 4 ha Bt cotton field near Sriganganagar, sown on 10 May, is 95 days old. Eight pheromone traps, one per 0.5 ha, have gone from 4 to 36 moths per trap per night in ten days. Scouting 50 plants gives 12 rosette flowers, 8% of green bolls with round entry holes and 3 pink larvae with dark bands per 50 bolls opened. The farmer sprayed a synthetic pyrethroid at 62 and 78 days; after the second spray, whitefly nymphs rose to 18 per three leaves and ladybird beetles almost disappeared. His neighbour sprayed chlorantraniliprole at 60 and 80 days; his trap catch is 22 and he still finds 4 live larvae per 50 bolls. Both fields were sown to the same Bt hybrid and neither has a refuge. The farmer asks whether to spray again this week, and with what.
1. Identify the pest and decide whether the population has crossed the action threshold. Explain why the Bt technology has not controlled it.
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The trap catch of 36 moths/trap/night against an action threshold of roughly 8, 24% rosette flowers (12 out of 50 plants) against a 10% threshold, and 8% boll damage with pink, dark-banded larvae inside locules all identify pink bollworm, Pectinophora gossypiella. The threshold is crossed several times over, so intervention is justified - but not with the same chemistry as before.
Bt cotton does not give season-long control of this pest. Cry1Ac/Cry1Ab expression in bolls is much lower than in leaves, and the larva feeds inside the boll and locule, where it is protected from contact insecticides and from much of the Bt dose. Continuous Bt cotton on both farms, with no refuge and no rotation, selects resistant individuals. The neighbour's result makes the point: 22 moths and 4 live larvae after two diamide sprays means survivors, not poor spraying.
2. Critique the two spray histories and explain the whitefly flare-up.
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Both programmes break resistance-management rules. The pyrethroid was the worst possible choice: pyrethroids are weak against larvae already inside bolls, they are well known for inducing whitefly resurgence because they remove predators and parasitoids - hence the ladybirds vanishing and nymphs rising to 18 per three leaves - and two applications in one season select resistant bollworm. The neighbour repeated a single mode of action (chlorantraniliprole, IRAC group 28) twice, which is exactly the pattern that selects resistance in an insect with 10–12 generations a year.
Neither farmer used any non-chemical measure, and the sprays were made without checking thresholds. A third spray this week with another pyrethroid, or with the same diamide, would add cost, add residues and still leave larvae inside bolls.
3. Give a three-week action plan with non-chemical measures and a resistance-rotation spray choice.
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- Trapping and disruption: keep 8–10 pheromone traps/ha for monitoring and put out a mating-disruption dispenser (PB-Rope L type) for the block.
- Sanitation: twice a week, collect and destroy rosette flowers and bored bolls - this removes larvae that no spray can reach, and it is the cheapest single measure at this stage.
- Protect natural enemies: stop pyrethroids completely; spray only at threshold, in the evening, with 500–750 L water/ha so bolls are covered.
- Chemistry: if a spray is justified, choose one mode of action not used on this farm this season - spinosad 45 SC (IRAC 5), thiodicarb (1A) or emamectin benzoate (6). Keep chlorantraniliprole (28) out of this field for the rest of the season; do not repeat a group within 21 days or use any group twice in a season.
- After harvest: uproot and shred stalks, plough soon after the last picking and do not keep the crop past December - stacked stubble and late bolls carry the next generation into the following year.