Brown Planthopper Resurgence after Repeated Neonicotinoid Sprays in Rice
A 0.4 ha block of a long-duration rice variety is at maximum tillering. The farmer has sprayed imidacloprid 17.8 SL four times in 30 days and lambda-cyhalothrin once, after finding a 'hopper burn' patch in a corner. Brown planthopper (BPH) now reaches 40-60 nymphs per hill at the base of the plants in about 30% of the field, with honeydew and sooty mould, while mirid bugs and spiders are almost absent. The field has 8 cm of standing water, and the crop has received 150 kg N/ha in three splits, the last one 10 days ago. A nursery of the same variety 20 m away was sown 60 days ago. The farmer plans a fifth imidacloprid round and another 25 kg N/ha. Neighbouring fields carry 3-5 hoppers per hill with visible mirid bug populations.
1. Diagnose what has happened in this field and explain why the fifth imidacloprid spray and the extra nitrogen would both make things worse.
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This is classical BPH resurgence, not a control failure in the ordinary sense. Several mechanisms act together:
- Predator removal. Imidacloprid is broad-spectrum and highly toxic to the mirid bugs and spiders that are the main natural regulators of BPH in rice. Four rounds in 30 days effectively cleared the field of natural enemies, while the neighbouring fields that keep 3-5 hoppers per hill still hold those enemies.
- Pyrethroid flare. Lambda-cyhalothrin is well known to aggravate BPH; it kills predators and spiders and can stimulate BPH reproduction. One pyrethroid spray in a field already stripped of predators is enough to release the population.
- Nitrogen. The 150 kg N/ha in three splits has produced lush, high-tillering and succulent plants with a dense humid canopy at the base; BPH prefers high-N plants and the abundance of honeydew and sooty mould confirms it is feeding heavily on them. The crops that suffer most damage from BPH are those on high N.
- Standing water and dense canopy keep the microenvironment at the base humid and shaded, which favours nymphs.
- There may also be a resistance effect after four rounds of the same mode of action in 30 days. A fifth imidacloprid spray would kill more of the remaining natural enemies, select for resistance, and add to cost, while the extra nitrogen would extend the lush vegetative growth that the population is already exploiting. Both would make the outbreak worse.
2. Give an immediate action plan, in order of priority, for the next 10 days, including the insecticide to use, the application method and precautions for the nearby nursery and the neighbours' fields.
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- Stop the nitrogen. Do not apply the 25 kg N/ha; the crop already has ample N and the surplus is now a liability.
- Drain the field for 2-3 days. Removing the standing water lowers the humidity at the plant base and increases nymph mortality; it also lets you see the hoppers and prevents the insecticide from going into the drainage water.
- Choose a selective insecticide. Pymetrozine 50 WG at 300 g/ha in 500 L water, or buprofezin 25 SC at 1 L/ha. Both act on BPH and are far less damaging to mirid bugs and spiders than imidacloprid. Etofenprox 10 EC is an alternative where those products are unavailable. Avoid another imidacloprid spray; avoid pyrethroids altogether.
- Direct the spray at the base of the hills. Use a flood-jet or a hollow cone nozzle with the spray lance pushed into the canopy, aiming at the stems; most of the population sits there, and a spray delivered to the leaves as a general cover will miss it. Spray in the evening.
- Do not treat the neighbours' fields or the 20 m nursery automatically. Fields at 3-5 hoppers per hill are below the action threshold (5-10 per hill) and are the reservoir of natural enemies that will recolonise this field. Check the nursery separately and treat only if counts are above the threshold.
- Check the hopper-burn patch: note its extent and take counts before and 5 and 10 days after treatment, using the tap method at the base of the plant.
- Do not drain into a fish pond or a watercourse.
3. The farmer asks whether the spray is economically justified and how he should manage chemistry over the rest of the season to avoid this happening again. Support your answer with a simple cost-benefit calculation.
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Cost-benefit: the protectable yield in an affected block is about 5 t/ha. Farm price for paddy of the order of Rs 22,000/t, so a 10% avoidable loss = 0.5 t x Rs 22,000 = Rs 11,000/ha. One targeted spray (product + labour) costs about Rs 1,500-2,000/ha, a benefit-cost ratio of about 6:1. Even a 3% avoidable loss (about Rs 3,300) justifies a single well-directed treatment. So yes, one selective spray now is justified. But four imidacloprid sprays plus a pyrethroid at Rs 1,500 each - about Rs 7,500/ha - bought no control and created the outbreak, which is the real lesson of the case.
Mode-of-action management for the rest of the season:
- Rotate by mode of action, not by brand name: imidacloprid belongs to group 4A, pymetrozine to 9B, buprofezin to 16. Do not use the same group twice in succession, and do not use more than two applications of any one group per crop.
- Apply only when the action threshold is reached, which for BPH is about 5-10 hoppers per hill; sample by tapping 10-20 hills and counting the insects on the water surface.
- Keep a spray diary with date, product, group, dose and target so rotation can be checked.
- Protect the natural enemies: avoid pyrethroids, avoid spraying the whole block when only a patch is above the threshold, and leave untreated strips.
- In the next season reduce N to the recommended level and use a variety with BPH resistance where one is available, so the field is not run at a high N level all the time.