Seven sprays and still pink bollworm: a resistance-management decision
Jasbir Singh grows 3 ha of BG-II Bt cotton near Bathinda, sown on 20 April. From the last week of July he has followed a fixed calendar: cypermethrin, then cypermethrin again, then quinalphos, then emamectin benzoate, each at 8–10 day intervals, reaching seven sprays by 20 September at about ₹1,100 per hectare per spray. He sprays between 11 am and 3 pm at 36 °C with a flood-jet nozzle. He has never used pheromone traps. On 22 September he finds rosette flowers, 6 pink larvae per 20 plants in green bolls, stained lint and a few exit holes; many larvae are inside bolls and are not being hit by spray. A neighbour who sprayed only three times, using two different modes of action and a trap-based threshold, has visibly cleaner bolls. Harvest is due in early November and the market price is uncertain. Jasbir wants to add two more pyrethroid sprays immediately.
1. Why did the seven-spray programme fail? Give at least four reasons drawn from the case.
Read the worked answer
- Calendar spraying instead of thresholds. Seven sprays were applied whether or not the pest justified them. Each unnecessary spray selects resistant survivors - which is why the neighbour with three threshold-based sprays is cleaner.
- Repeated pyrethroids. Cypermethrin twice, then quinalphos, then a fixed sequence is exactly the pattern that builds pyrethroid resistance in pink bollworm, a pest with a long history of resistance in this belt.
- Wrong target. Once larvae are inside green bolls, contact sprays cannot reach them. Control has to hit adults and neonate larvae before entry, so a spray placed 8–10 days late misses the window entirely.
- Application losses. Spraying at 36 °C at midday, with a flood-jet giving coarse droplets and runoff, leaves a poor deposit and degrades sensitive actives; spraying in the heat of the day is the worst timing for both the product and the operator.
- No monitoring. Without pheromone traps or rosette-flower counts he had no idea when moth flights were building, so sprays were neither timed nor evaluated.
2. Design a resistance-management plan for the rest of this season and for next year.
Read the worked answer
Rest of this season:
- Stop pyrethroids completely.
- Scout twice weekly; the working threshold is about 8 live larvae per 20 plants, with rosette flowers as the early warning. If live larvae still exceed the threshold in green bolls, apply one effective product from a different mode-of-action group (for example chlorantraniliprole or spinetoram) - not a tank mixture of several groups.
- Collect and destroy rosette flowers and attacked green bolls in bags every week; this removes larvae the spray cannot reach.
- Terminate the crop as soon as the last pickable bolls open, then destroy stalks and deep-plough to bury pupae. Do not keep ratoon cotton, and do not stack sticks beside fields.
Next year:
- Pheromone traps at about 4 per hectare, checked twice weekly from square formation.
- All sprays threshold-based, with at most one spray per mode-of-action group per season and full details recorded.
- No pyrethroid at all in the programme; keep a non-Bt refuge as required and sow on time in April, avoiding a stretched late-season crop that bridges the pest to the next year.
3. He wants two more pyrethroid sprays now. Argue the decision with numbers.
Read the worked answer
Cost: 2 sprays × ₹1,100/ha = ₹2,200/ha, or ₹6,600 across his 3 ha.
Expected benefit: almost zero. The larvae counted are inside green bolls, where a contact pyrethroid does not reach; the lint that is already stained stays stained, and quality (and therefore price) is set by the boll damage already done. Adding two more pyrethroid rounds also deepens the resistance problem he must live with next season, and it costs a further two sprayer passes over a crop that will be picked in about six weeks.
Better use of the same money:
- one properly timed spray of a different mode of action, only if live larvae in green bolls still exceed about 8 per 20 plants;
- weekly collection and destruction of rosette flowers and infested bolls;
- clean picking with stained and clean kapas kept separate, which protects the price of the good lint;
- prompt crop termination and stalk destruction.
With the market price uncertain, ₹6,600 spent on sprays that cannot reach the target is the worst of the available choices; the same money spent on separation at picking will show up directly in the price he receives.