Pink bollworm in Bt cotton: spray decision and crop termination
Jaswinder Singh has 6 acres of Bt cotton sown on 10 May in sandy loam. He placed four pheromone traps per acre. Catches rose from 3–5 moths per trap per night in mid-August to 30–35 moths per trap per night in the week ending 5 September. He has made five sprays so far: two of a pyrethroid, two of emamectin benzoate and one of profenophos, mostly tank-mixed with a liquid fertiliser at slightly reduced doses. On 8 September he examined 100 green bolls and found exit holes and pinkish larvae in 14, plus a few rosette flowers. The top branches still carry a good boll load and he wants a sixth spray of the same pyrethroid. He also plans to keep picking until December to recover his costs. His neighbour terminated the crop in mid-October last year and had visibly fewer moths this season.
1. Interpret the trap counts and boll damage, and decide what the sixth spray should be.
Read the worked answer
Thirty to thirty-five moths per trap per night is several times the usual action threshold for pink bollworm, and 14% of green bolls damaged confirms an active, damaging population – so a spray is justified. Neither the pest nor the thresholds are in doubt; the choice of chemical is where the crop is being lost.
Repeating a pyrethroid is the worst option. Pyrethroids have a long history of pink bollworm resistance, and his record – repeated modes of action, reduced doses, tank mixing with a fertiliser – is exactly the selection pressure that builds resistance. Emamectin is also weak against larvae already inside bolls. The sixth spray should be a full recommended dose of a molecule with a different mode of action that is labelled for pink bollworm, rotated from everything used so far, in the evening, with enough water to reach the fruiting bodies. Also remove and destroy rosette flowers and infested bolls, and keep the traps running to judge whether the spray worked. He must accept that sprays only suppress an internal borer – the trap-and-scout system and timely crop termination do the rest.
2. Should he extend picking to December? Justify your answer biologically and practically.
Read the worked answer
No. Extending the crop keeps green bolls and larvae alive in the field after the surrounding crop has gone, giving the population a bridge into the next season. His neighbour's experience is the clue: early termination reduced carry-over. Late pickings also yield less per picking and cost more in labour and sprays, at a time when late-season kapas may not fetch a premium.
The correct sequence is: complete the final picking of open bolls, then terminate the crop around mid-October as recommended in the state, shred or uproot the stalks, destroy or deep-bury the residue, and never stack stalks along field boundaries or leave the crop for ratooning – both are classic pink bollworm overwintering sites. Bag and destroy infested bolls rather than dumping them near the field. Because moths move between fields, area-wide community termination is more effective than one farmer acting alone.
3. He says the extra sprays and late pickings are needed 'to recover the cost'. How would you test that argument?
Read the worked answer
Test it as a marginal, not a total, decision. The money already spent is sunk and cannot be recovered by throwing more at the crop. Compare the extra cost of each additional spray and picking round against the extra revenue it can realistically bring. A spray costs a few hundred rupees per acre plus labour and, with resistance building, gives diminishing control; late pickings on a declining crop bring low and uncertain returns. Set that against the cost of the recommended alternative – terminate, shred, destroy residue – which is small.
It is also worth weighing the next crop. A heavy carry-over of pink bollworm raises the moth pressure he will face from August next season and pushes the first costly spray earlier. Terminating on time, using approved rather than farm-saved seed, and keeping traps from the start of the season are cheaper than another round in a losing sequence. The argument 'recover the cost' usually means 'spend more on a crop that has stopped paying'.