Pink Bollworm Threshold and Insecticide Choice in Bt Cotton
Jasbir has 4 acres of Bt cotton sown on 5 May in Bathinda. On 12 August his pheromone traps for pink bollworm average 22 moths per trap per night over three consecutive nights. Scouting the same day finds 9 rosette flowers per 100 plants, 6% green boll damage and 3 larvae in 50 bolls. Whitefly counts are 4 adults per leaf. His neighbour sprayed a pyrethroid three days ago and is urging him to do the same. Jasbir's last spray was chlorantraniliprole 18.5% SC on 20 July. At home he has thiodicarb 75% WP, profenophos 50% EC and acetamiprid 20% SP. Picking is expected from 1 October. Last year pink bollworm damaged 18% of his crop despite six sprays. The forecast is 34-36 degrees Celsius with no rain.
1. Using the threshold, decide whether a spray is justified on 12 August.
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The pheromone catch of 22 moths per trap per night for three consecutive nights is well above the accepted action threshold of about 8 moths per trap per night for 2-3 nights. Rosette flowers at 9% and green boll damage at 6% confirm that larvae are already inside squares and bolls, where contact insecticides reach them poorly. A spray is therefore justified now. Waiting for more rosettes means more bolls will already be damaged, because one larva can damage several seeds and bolls. The decision is for one well-timed spray, not a return to the six-spray routine that failed last year. Whitefly at 4 adults per leaf is below its threshold of about 6 per leaf, so no whitefly spray should be added to the tank.
2. Select a product from his stock and justify it; explain why the pyrethroid and acetamiprid are poor choices.
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Choose thiodicarb 75% WP or profenophos 50% EC, both of which act on pink bollworm larvae, and rotate away from the chlorantraniliprole (group 28) used on 20 July. Use the label dose in about 200 litres of water per acre, sprayed in the evening and directed at the bolls. The pyrethroid is a poor choice: resistance to pyrethroids is widespread, it flares whitefly by killing natural enemies, and it is not a recommended rotation partner after a diamide. Acetamiprid is a neonicotinoid and is weak against larvae inside bolls; it would also add selection pressure on whitefly. Do not tank-mix all three products. One product, correct dose, correct nozzle and evening application is the principle. Record the date and molecule so that any later spray uses a different mode-of-action group.
3. List the non-chemical actions he should take between now and picking to keep pink bollworm from repeating last year's loss.
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Keep 8-10 pheromone traps per acre and inspect them twice weekly. Remove and destroy rosette flowers and damaged bolls during picking. Do not extend picking into November; terminate the crop on time because late-season larvae carry over. After the final picking, uproot and shred stalks, plough them in, and destroy ratoon plants and cotton heaps around the farm. Follow the same schedule as neighbouring fields, because moths move between farms. Conserve natural enemies by avoiding pyrethroid sprays and unnecessary whitefly sprays. Monitor with a hand lens instead of spraying by calendar. Keep a simple spray diary. If trap catches rise again, use a product from a different mode-of-action group, not the one used in July.