Nine Seasons of a Single-Toxin Bt Hybrid and a Pink Bollworm Outbreak in Mid-August
Ravinder farms four hectares of irrigated cotton in Bathinda and has sown the same single-gene Bt hybrid, carrying only the cry1Ac event, for nine consecutive seasons on the same fields; he has never sown a non-Bt refuge. In the second week of August he finds rosette-shaped flowers, larvae inside green bolls, and 40% of sampled bolls damaged. His neighbour, who sowed a two-gene hybrid carrying cry1Ac plus cry2Ab on similar land, has about 8% boll damage. Ravinder has already applied two rounds of a pyrethroid at the label dose with no visible reduction; larvae collected from his field and fed on cry1Ac diet in a district laboratory survived to pupation. Each spray costs him about Rs 1,500 per acre and he has ten weeks of the season left. He asks whether to spray a third time with a different insecticide, uproot and replant, or do something else, and what he should change next year.
1. Interpret the field and laboratory evidence. What has happened in Ravinder's field, and why did the pyrethroid fail?
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The picture is field-evolved resistance to Cry1Ac, not a spray-timing error. Bt cotton kills larvae only when they ingest the Cry1Ac protein bound to midgut receptors; the toxin forms pores and lyses midgut cells. Ravinder has exposed an entire pink bollworm population to a single toxin for nine seasons with no refuge, so every generation was selected. Rare individuals carrying resistance alleles survived, mated with one another, and their frequency rose. The laboratory result is the decisive evidence: larvae not merely surviving on plant tissue but completing development to pupation on Cry1Ac diet shows heritable, not behavioural, resistance.
The pyramid rescues the neighbour because Cry1Ac and Cry2Ab bind different midgut receptors; a single mutation rarely confers resistance to both, so two independent resistance alleles must coincide in one insect - the product of two small frequencies. The neighbour's 8% damage shows the pyramid slowing resistance, not abolishing it, and his fields are not a clean control.
The pyrethroid failed for two reasons that operate together: neonate larvae establish inside squares and bolls, under the calyx, where contact insecticide barely reaches them, and repeated pyrethroid use on the same farm has selected for knockdown resistance in the local population. Repeating a failed mode of action wastes money.
2. Advise Ravinder on the remaining ten weeks. Compare a third spray, replanting, and the alternatives on cost and expected return.
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Replanting in mid-August is not an option. Cotton sown then will not accumulate enough heat units before the season ends; he would lose the crop already set and gain nothing. Dismiss it.
A third pyrethroid spray is unlikely to pay. His larvae are inside bolls and the laboratory has already shown survival on the Bt toxin; two failures predict a third. Spending Rs 1,500 per acre, Rs 6,000 per spray on four hectares, for no measurable kill is straightforwardly uneconomic.
Better options, in order:
- Assess what is still salvageable. Bolls set before mid-August carry most of the yield; determine whether damage is concentrated in late-forming squares. If it is, terminating the crop early and concentrating resources on the set bolls beats further insecticide.
- Switch class only if larvae are still exposed. A translaminar or ingestion-active molecule with a different target site can be justified once, at threshold, on late flush - not as routine cover.
- Mechanical and behavioural control. Hand-collect and destroy rosette flowers and infested bolls; hang pheromone traps for monitoring and mass trapping; avoid leaving gin trash and stacked stalks near the field.
- Post-harvest sanitation. Destroy stalks and stubble promptly after the last pick to kill diapausing larvae, which is the single most cost-effective action he can take for next season.
He should be told plainly that biology, not more money, is now the limiting factor.
3. Explain the design logic of the refuge and of the two-gene pyramid, and state what Ravinder must do differently next season.
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Refuge logic. A block of non-Bt plants of the recommended proportion, sown close to and around the Bt crop, produces susceptible moths. These mate with any rare resistant survivors emerging from the Bt area. If resistance is recessive or partially recessive, the heterozygous offspring die on Bt plants, so resistance is flushed out each generation instead of accumulating. The refuge only works with high-dose expression - enough toxin to kill heterozygotes - combined with a refuge of adequate size and proximity. India's regulator recommended roughly 20% non-Bt refuge around the Bt field; Ravinder used zero, which is the root cause of his problem.
Pyramid logic. Two Cry proteins with distinct midgut receptors mean resistance requires two independent mutations in the same insect. If each allele has a frequency of, say, 1 in 1,000, both together are roughly 1 in a million, so evolution is delayed by orders of magnitude.
Next season Ravinder should: sow a two-gene hybrid with documented efficacy against pink bollworm, plant the recommended non-Bt refuge, rotate to a non-cotton crop in at least part of the area to break the cycle, scout from square formation rather than waiting for rosette flowers, spray only at threshold with rotated modes of action, and destroy stalks after harvest. He should also keep purchase records, since only approved events with a valid licence are legal to sow.