Failed pyrethroid sprays and beneficial insects in Bt cotton
At 45 days after sowing, a Bt cotton field in Vidarbha shows 8% damaged squares, 2 medium larvae per plant, and a spray history of two pyrethroid applications at 10-day intervals with no visible reduction. The farmer has already spent ₹4,800/ha on insecticides and wants a third spray, this time a pyrethroid-organophosphate mixture. A scout reports 6 ladybird beetles and 4 syrphid fly larvae per 10 plants, plus 15% parasitised larvae. The crop is at square formation, soil moisture is adequate, and the weather forecast is dry with 34°C maximum. The economic threshold for bollworm on squares is 10% damage or 1 larva per plant. Neighbouring fields are spraying every week.
1. Should the third spray be applied? Justify your decision using the threshold, resistance evidence and natural-enemy population.
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The field is above the larval threshold: 2 larvae/plant exceeds the 1 larva/plant limit, although square damage at 8% is just below 10%. So some intervention is justified, but not the planned pyrethroid-organophosphate mixture.
Two pyrethroid sprays at 10-day intervals with no reduction is strong evidence of resistance or of poor coverage/timing; pyrethroids are notorious for selecting bollworm resistance and for killing natural enemies. The scout found 6 ladybirds and 4 syrphid larvae per 10 plants and 15% parasitism. A third broad-spectrum spray would likely fail again, add selection pressure, and cause a secondary outbreak of sucking pests or whitefly. The dry, hot weather also increases the risk of spray burn and rapid evaporation.
Decision: hold the third pyrethroid mixture. Re-scout 10 plants in five spots and check whether larvae are small or large. If larval pressure persists above 1/plant and square damage is rising, use one selective larvicide from a different mode-of-action group, such as emamectin benzoate, spinosad or chlorantraniliprole, following the label. Spot-spray hotspots rather than the whole field, spray in the evening, and preserve the beneficial population. If most larvae are parasitised or large and damage is stable, delay and re-check in 3 days.
2. Design a three-week resistance-management and IPM plan for this field.
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Week 1: Stop all pyrethroids and record the products, dates and doses used. Scout twice weekly using 10 plants in five locations; set pheromone traps at 5/ha. Threshold remains 1 larva/plant or 10% damaged squares. Hand-pick large larvae and remove heavily damaged squares where labour allows. Do not spray neighbouring fields' calendar; use the field's own counts.
Chemical choice: If the threshold is crossed, use a selective larvicide with a different IRAC group from the previous sprays: for example chlorantraniliprole (group 28), emamectin benzoate (group 6) or spinosad (group 5). Apply once, not as a tank mix with pyrethroids. Ensure good coverage of squares and undersides of leaves, and spray in the evening to protect pollinators and natural enemies.
Week 2: Re-scout 72 hours after spraying. If live larvae remain, do not repeat the same molecule; switch group or use a biological option such as Bt formulation or NPV where appropriate. Conserve the ladybirds and syrphids by avoiding broad-spectrum sprays. Keep a refuge or border crop if Bt cotton is grown.
Week 3: Continue twice-weekly scouting and monitor for sucking pests that may flare after insecticide use. Destroy crop residue after harvest and avoid continuous Bt cotton on the same field. The aim is to break the resistance cycle and restore biological control, not merely to kill the current larvae.