Pink Bollworm Threshold and Pond Water Allocation at Kalamb
Shri Deshmukh cultivates 2.5 hectares at Kalamb in Yavatmal district: 1.6 hectares under cotton and 0.9 hectare under soybean. His soil test report shows organic carbon 0.38 per cent, pH 8.2, low available nitrogen, medium phosphorus and high potassium. A farm pond of about 1,800 cubic metres capacity was filled by the last week of July. On 3 August the extension worker's eight pheromone traps in the cotton block recorded 3, then 7, then 11 moths per trap per night on three consecutive nights, and 42 mm rain fell. The district advisory treats 8 moths per trap per night for three consecutive nights as the action threshold for pink bollworm. Neighbouring farmers are spraying a broad-spectrum pyrethroid, and Shri Deshmukh wants to do the same the next morning. He also has to decide how to use the pond water, since soybean is at flowering and cotton is at square formation.
1. Do the trap catches justify an immediate spray? Work through the evidence before answering.
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Read the series, not the last number. The three-night series is 3, 7 and 11 moths per trap per night. The advisory threshold is 8 or more for three consecutive nights. Only the third night exceeds 8, so the strict threshold is not yet met; the run to watch is whether the next two nights also stay above 8. That makes the correct first action scouting and re-checking, not a panic spray.
What to do before any decision: inspect 20 plants at four spots in the cotton block for rosette flowers, and look for larvae inside squares and green bolls; check that the lures are fresh and that traps are placed at the right height above the canopy, since a stale lure or a trap hung too low can distort counts. Confirm that the 42 mm rain did not wash out the lures.
If the next two readings stay above 8, or larvae are found in squares, the threshold is crossed and a recommended selective insecticide should be used, with the correct dose, water volume and waiting period, and with the mode of action rotated from whatever was used last season. Record the trap counts, the scouting result and the spray date in the field diary.
Conclusion: the data justify intensified scouting today and a decision tomorrow, not an automatic spray tomorrow morning on the strength of one night's count.
2. Allocate the pond water between the two crops, showing your assumptions and the arithmetic.
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Step 1 - usable water. Of 1,800 cubic metres, assume 60 per cent is effectively usable after seepage, evaporation and conveyance losses: 1,800 x 0.60 = 1,080 cubic metres.
Step 2 - water needed for one protective irrigation of 5 cm depth. One hectare at 0.05 m depth needs 10,000 x 0.05 = 500 cubic metres.
Step 3 - soybean first. Soybean at flowering is at its most moisture-sensitive stage and is the smaller block: 0.9 ha x 500 = 450 cubic metres. After this, 1,080 - 450 = 630 cubic metres remain.
Step 4 - what remains will do for cotton. 630 / 500 = 1.26 ha. Cotton at square formation can survive on one protective irrigation, so irrigate the 1.26 ha that is lightest in soil, or the patch where wilting is already visible, and leave the remaining 0.34 ha unirrigated this round.
Step 5 - protect the resource. Mulch or intercultivate to conserve the moisture applied, repair the pond inlet so the next rain is captured, and plan the next allocation only after measuring the actual pond level rather than assuming it is still full. State these assumptions in the note so the farmer can see why 1.6 ha of cotton cannot be fully irrigated from this pond.
3. The farmer says a broad-spectrum pyrethroid is cheaper and his neighbours are using it. Give the technical and economic reasoning you would place before him.
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Start with the cost he is not counting. A pyrethroid spray costs less per bottle but is non-selective: it kills the predators and parasitoids that currently keep pink bollworm and other pests in check, which is why fields sprayed early often need more sprays later. Repeated pyrethroid use on pink bollworm is also the classic route to resistance, so the same money buys diminishing control.
Then the agronomic risk. Broad-spectrum sprays at square formation can flare up sucking pests such as whitefly and mites by removing their natural enemies, and they disturb pollinators in the adjoining soybean. The farmer then spends on two or three extra sprays, each with labour, fuel and water, which erases the price advantage.
Use the neighbours' fields as information, not as a template. Their spray timing reflects their own trap counts and crop stage, which may differ. Copying a calendar spray regardless of threshold is exactly what the pheromone traps are meant to prevent.
Recommendation to place before him: follow the trap series for two more nights, scout for larvae, and if the threshold is confirmed use a recommended selective insecticide at the label dose with mode-of-action rotation, correct spray volume and adherence to the waiting period. Keep a spray diary. This is both cheaper over the season and safer for the crop.