Pink bollworm damage in late-season BG-II cotton at Khamgaon

Bt failure. BG-II expresses two Bacillus thuringiensis proteins, but pink bollworm is the weak point of the technology in India: selection pressure from continuous Bt cotton over large areas has produced field-level resistance, and late-season larvae feeding inside bolls are exposed to lower doses in older tissue. He also grew no non-Bt refuge, which is the standard resistance-management tool intended to keep a susceptible moth population alive to mate with resistant survivors. With no refuge, resistant moths mate with resistant moths and the proportion of resistant individuals climbs each generation.

Spray-programme failure.

1. Same mode of action twice: emamectin benzoate on 5 and 12 September is a single MoA group applied back-to-back, which selects resistance fast and gives no coverage of a different target site.
2. Pyrethroid tank-mix on 20 September: pyrethroids are weak against bollworm feeding inside bolls, and they flare secondary pests by killing natural enemies, so the field loses its biological control at exactly the wrong time.
3. Timing: once larvae are inside green bolls, contact insecticides cannot reach them. Sprays applied after entry are largely wasted money; the effective window is against the adult and neonate stage.
4. Canopy: a dense, closed canopy with the last irrigation three days old keeps humidity high inside the crop, which favours adult survival and egg laying.

Diagnosis is the key judgement: a pest that no longer responds to two different modes of action, at threshold-level trap catches, in a non-refuged Bt field, should be treated as resistant, not as a spraying error.

Decision: do not plan another conventional bollworm spray as the main response.

The evidence is far above any action threshold: 34 moths/trap/night against the commonly used threshold of about 8 moths/trap/night for three consecutive nights, 14% boll damage, and 10-12 rosette flowers per 10 plants. But thresholds tell you the pest is economic, not that another spray will work - he has already had two failed MoA applications.

Immediate plan:

1. Sanitation first. Send labour through the field twice a week to pick and destroy rosette flowers and holed green bolls in a sealed bag, or bury them. This removes the next generation from the field and is the cheapest reliable action at this stage.
2. Mass trapping. Raise the gossyplure trap number to 20-40/ha and change lures on schedule; this suppresses adults and keeps monitoring honest.
3. If a spray is unavoidable, rotate to a different MoA group from the ones already used - for example a diamide or a spinosyn-type product - and apply it only once, at label dose, in the evening, with a surfactant, targeting adults and neonates. Never repeat the same group in the same window.
4. Stop the late irrigation to open the canopy and reduce humidity.
5. Plan the harvest: pick the first flush promptly and do not leave seed cotton or damaged bolls in the field; destroy crop residue after the last pick and do not ratoon.
6. Assess economics: at 2 ha, repeated sprays at Rs 2,000-3,000 per round for a pest already inside bolls will not pay back; the money is better spent on picking labour and sanitation.

1. Non-Bt refuge. Sow about 5% of the cotton area (or a border row block) with the same non-Bt parent variety, not sprayed for bollworm. Susceptible moths emerging from the refuge mate with any resistant survivors from the Bt area, and their offspring are heterozygous and much more likely to be killed by the Bt protein. Without refuge this dilution never happens.

2. Timely sowing and early termination. Sow in the normal June window, and destroy the crop after the last pick - do not ratoon and do not leave stubble standing into February-March. Pink bollworm overwinters in seed cotton and in trashed bolls; a clean termination breaks the bridge between seasons.

3. Monitor from 45 days, not from damage. Install gossyplure traps at 8/ha from square formation, record catches twice a week, and act on the 8 moths/trap/night for three nights rule rather than waiting for rosette flowers.

4. Insecticide rotation by MoA. Keep a written record of the MoA group of every spray and never use the same group twice in a row or more than twice a season. This is the only way to slow resistance once it has started.

5. Conserve natural enemies. Avoid pyrethroid tank-mixes and unnecessary early sprays; predators and parasitoids suppress the population between generations.

6. Variety and rotation. Rotate cotton with a non-host crop such as soybean or a cereal for one season in the worst blocks, and consider a non-Bt or refuge-based programme if resistance in the area is confirmed.