Brown Planthopper Patches in Aman Paddy
Bimal Ghosh has 0.80 ha of transplanted aman paddy (variety Swarna) at maximum tillering, about 45 days after transplanting, in Arambagh, Hooghly. The field has received 60 kg N/ha in two splits, and the past week has been cloudy, drizzling and humid at 28–30 °C. On a walk today he records from ten random hills at each corner: stem borer dead hearts 7%, leaf-folder damaged leaves 6%, and brown planthopper 12–15 insects per hill in three patches near the bund, with many spiders, mirid bugs and a few coccinellids on the plants. His neighbour, who sprayed imidacloprid plus lambda-cyhalothrin a fortnight ago, now has a much heavier hopper population and his crop looks scorched at the base. Bimal has ₹1,000 and a knapsack sprayer, and paddy sells at ₹2,100 per quintal.
1. Which of the three problems have crossed the action threshold and which have not? Justify your judgement.
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Comparing his counts with the commonly used action thresholds:
- Stem borer: threshold is about 5% dead hearts in the vegetative stage (2% white ears later). He has 7% - above threshold, so action is justified, though 7% in a tillering crop is still modest.
- Leaf folder: threshold is around 10% damaged leaves or 2 larvae per hill. He has 6% - below threshold; most damaged leaves are older ones contributing little to yield. Spraying here would cost more than the loss.
- Brown planthopper: threshold is about 10 hoppers per hill (some advisories use 5–10 at tillering). He has 12–15, but only in three patches, so those patches are above threshold while the rest of the 0.80 ha is not.
So two problems need action, and one of them - BPH - needs it only in patches. The correct judgement is not to convert a patchy problem into a whole-field spray. The economics support this: at 4.5 t/ha the crop is worth about ₹75,000, a cover spray costs ₹700–900, and an unnecessary one buys nothing.
2. Explain what has happened in the neighbour's field, and state what Bimal should avoid.
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The neighbour's field shows the classic result of a broad-spectrum spray in a rice ecosystem:
- Imidacloprid and lambda-cyhalothrin kill spiders, mirid bugs, coccinellids and parasitic wasps that normally hold BPH in check. BPH breeds under the plant base where it is partly protected, so it rebounds faster than its enemies - resurgence.
- Sub-lethal doses of some insecticides also raise hopper fecundity and shorten generation time.
- The scorched base is hopper burn: heavy sap sucking at the base yellows tillers, then browns and dries them, usually in circular patches, with sooty mould on honeydew. It looks like a disease but is insect damage.
- Pyrethroids under high humidity can also scorch leaves, and they flare BPH rather than control it.
Bimal should avoid pyrethroids (lambda-cyhalothrin, cypermethrin), imidacloprid and other broad-spectrum products, avoid tank mixtures and calendar sprays, and avoid spraying when natural enemies are abundant.
3. With ₹1,000 and five days, what would you tell Bimal to do and why?
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- BPH patches first. Drain the field for 3–4 days, since BPH needs water at the plant base, then keep the field alternately wet and dry. Do not spray the whole field. If a patch still holds more than 10 hoppers per hill, spot-spray only those patches, directing the nozzle at the base, with a selective hopper insecticide such as pymetrozine or buprofezin. A few hundred rupees covers three patches.
- Stem borer. Clip and destroy affected tillers, put up 4–5 pheromone traps per 0.5 ha or release Trichogramma japonicum cards, and spray only if dead hearts again cross 5–10%. Chlorantraniliprole works well but should be held back for genuine need.
- Crop management. No more nitrogen now - a lush dark crop attracts BPH; keep potash adequate and remove weeds and bund grasses that shelter hoppers.
- Scout and record. Walk the field twice a week and count hoppers per hill along with spiders, spraying only when the count justifies it. Most of the ₹1,000 is better kept for a patch spray and traps than spent on a cover spray.