Soybean at pod filling: two failed sprays and 20 days to harvest
Suresh Patil farms 2.0 ha of soybean (JS 20-34) near Sihora, Narsinghpur, sown on 22 June after a 38 mm shower. By 8 September the crop is at pod filling. In the eastern 0.6 ha, 25 to 30 per cent of the leaves are eaten and 6 to 8 dark, velvety caterpillars with a black crescent on the first abdominal segment are counted per metre of row; the rest of the field has 1 to 2. He sprayed quinalphos 25 EC at 2.0 mL per litre on 20 August and chlorpyrifos 20 EC at 2.5 mL per litre on 31 August, both with a single-cone knapsack nozzle and about 180 litres of water per hectare, in the afternoon. The last spray was seven days ago and harvest is expected in about 20 days. Two honeybee boxes stand at the field edge and a flowering pigeonpea crop lies 200 m away. Each spray round costs about Rs 3,000 and he has Rs 3,000 left in his input budget.
1. Which pest most likely caused this damage, and why did two insecticide rounds fail to stop it?
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The caterpillar with a black crescent on the first abdominal segment, velvety skin and a habit of hiding by day is Spodoptera litura, the tobacco caterpillar. The failure is explainable on four counts:
- Stage and behaviour. By the time 25-30 per cent defoliation is visible, most larvae are in the 4th-5th instar. They feed voraciously, are two to three times harder to kill than early instars, feed mostly at night, and shelter in leaf axils and under clods during the day.
- Molecule choice. Quinalphos and chlorpyrifos are largely contact poisons with short residual action. On a closed soybean canopy at pod filling, neither reaches the lower leaves where the larvae rest.
- Application. A single cone nozzle delivering about 180 L/ha wets only the top of the canopy. Soybean needs 250-300 L/ha with hollow-cone nozzles angled into the sides of the canopy. Afternoon spraying in September heat increases evaporation and photodegradation.
- Selection pressure. Two rounds of related nerve poisons 11 days apart, repeated across the village, let tolerant individuals survive and reproduce.
Before spending again, walk a W pattern, count larvae per metre row at five spots, and look for white Cotesia cocoons or diseased larvae. If 30 per cent of larvae are parasitised, the population is collapsing and a chemical spray may be unnecessary.
2. With 20 days to harvest and Rs 3,000 available, should the 0.6 ha patch be sprayed? If yes, with what, when, and with what precautions? Show the trade-off.
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Yes, but only the 0.6 ha patch. At pod filling, leaves are the factory that fills seed, so 25-30 per cent defoliation with 6-8 larvae per metre row is above the usual action threshold (about 5 larvae per metre row or 20 per cent defoliation). The remaining 1.4 ha at 1-2 larvae per metre row is below threshold and should not be sprayed - blanket spraying wastes money and kills natural enemies.
Choice of molecule. The old molecules have already failed and the harvest interval is short, so:
- Prefer a newer, ingestion-plus-contact larvicide from a different mode-of-action group (for example chlorantraniliprole 18.5 SC, flubendiamide 39.35 SC, or emamectin benzoate 5 SG) at label dose, but only if the label pre-harvest interval is 20 days or less.
- If the label interval is longer than the days left, use Bacillus thuringiensis var. kurstaki or Spodoptera NPV, which act only on caterpillars and leave no harmful residue at harvest.
Application. Spray in the evening with 250-300 L/ha, two hollow-cone nozzles, spray directed at the sides of the canopy, plus a sticker. Hand-pick and destroy large larvae in the patch before spraying.
Precautions. Do not spray the flowering pigeonpea, keep at least 100 m from the pond, close or move the bee boxes and inform the beekeeper, and spray when bees are not foraging. Cost: about Rs 1,200-1,500 for 0.6 ha, which fits the Rs 3,000 budget with a reserve.
3. What should he do after this harvest and before the next kharif so that the same escalation does not repeat?
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The problem is calendar spraying, not a shortage of chemicals. Practical changes:
- Scout, do not guess. Count larvae twice a week from 30 days after sowing at five spots per field, and spray only when the count crosses the threshold. Record the date, pest stage, product, dose, water volume and nozzle in a spray diary.
- Use non-chemical checks. Install 5 pheromone traps per hectare for Spodoptera to detect the moth flight and time scouting, keep 10 bird perches per hectare, and conserve parasitoids by avoiding broad-spectrum sprays early in the season.
- Rotate modes of action. Never use the same insecticide group twice in one season, and never mix two products as a cocktail without a reason. Repeated organophosphates in the village are how tolerance starts.
- Destroy carry-over. Deep summer ploughing exposes pupae, and removing alternate hosts (weeds and stray vegetable plants) around the field removes the bridge between seasons.
- Budget realistically. Keep Rs 3,000-4,000 per hectare per season for pest management instead of spending it all in one emergency round, because a single well-timed spray at early instar is cheaper than three late ones.
- Synchronise with neighbours. Community-level scouting and a common spray window work better than one farmer spraying alone.