Twelve sprays in 45 days: okra IPM on a Kanpur holding
Ramesh Singh farms 1.2 ha of okra (cv. Nirmal-101) on the edge of Kanpur and sells pods to the local mandi twice a week. Between 25 and 70 days after sowing he has sprayed 12 times, mostly lambda-cyhalothrin 5 EC and chlorpyriphos 20 EC, often only 3–4 days apart and often at double the label rate "because the borer keeps coming back". On 12 September your scouting finds third-instar Earias vittella larvae on 22 of 100 randomly sampled plants, 9% of shoots damaged, two of five yellow sticky traps per 100 m² carrying more than 40 whiteflies each, and roughly 12% of plants showing yellow vein mosaic symptoms, most of them less than 30 days old. Spray costs so far are about ₹9,400 and he has spent 22 hours spraying. He wants to know why the sprays stopped working and asks you to write an IPM plan for the rest of the season, including what to do this week.
1. Explain the most likely agronomic, ecological and operational reasons why the 12-spray schedule failed, and rank them by importance for this field.
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What the numbers say. 22% of plants carrying third-instar Earias larvae is roughly four times the usual action threshold for okra shoot and fruit borer (about 1 larva per plant, or 5% damaged shoots/fruits). The pest pressure is real, so the sprays were not being wasted on an absent pest; they were failing for other reasons.
Ranked causes.
- Selection for resistance. Twelve sprays in 45 days, almost all lambda-cyhalothrin or chlorpyriphos, often at double dose. Earias vittella has a long record of pyrethroid resistance in the Indo-Gangetic plains. Repeating two modes of action (sodium-channel and acetylcholinesterase) at high dose is the textbook recipe for field resistance.
- Destruction of natural enemies and resurgence. Pyrethroids at double dose remove Trichogramma, braconid larval parasitoids and generalist predators, but not whiteflies. The 40+ whiteflies per trap and the YVMV build-up fit a whitefly flush after broad-spectrum sprays.
- Wrong target for yellow vein mosaic. The disease is a begomovirus transmitted by Bemisia tabaci. Once a plant is infected no insecticide helps it; spraying infected plants adds cost and selection pressure with no yield benefit.
- Application quality and timing. Double-rate spraying causes run-off and poor under-leaf coverage, and a 3–4 day interval is no better matched to the pest's life cycle than a well-timed 7–10 day programme.
Not the main cause: dose too low (he doubled it) or lack of a sticker (secondary). The ranking matters because it decides the fix: change modes of action and rebuild natural enemies, do not simply buy a stronger product.
2. Write the practical action plan for the next 10 days and for the rest of the season, specifying thresholds, product groups to rotate, and cultural/mechanical measures.
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Before any spray (this week).
- Rogue out and destroy, in bags, all YVMV-symptomatic plants, starting with the young ones. They are the virus reservoir and no spray will cure them.
- Hand-pick and crush damaged shoots and fruits twice a week. Larvae inside fruits are untouchable by sprays and this costs only labour.
Whitefly decision. 40 adults per trap is high. If adults exceed about 5 per plant on leaf undersides, make ONE targeted under-leaf application from a group not yet used here - spiromesifen 240 SC, diafenthiuron 50 WP or flonicamid 50 WG at label dose. Hang 20 yellow sticky traps per hectare. Do not tank-mix this with the borer spray unless a jar test is clean.
Borer decision. Threshold exceeded (1 larva/plant, 5% damage), so one spray is justified, but rotate the mode of action: emamectin benzoate 5 SG, chlorantraniliprole 18.5 SC or flubendiamide 39.5 SC. Then do not repeat that group for the rest of the season; re-scout before the next spray and keep at least 10 days between applications.
Below threshold. Use NSKE 5% or Bacillus thuringiensis var. kurstaki, which spare parasitoids.
Abandon the calendar. Spray only when scouting justifies it, and record date, product, dose, volume, area and the count that triggered each spray.
Next season. YVMV-tolerant (not immune) varieties such as Arka Anamika or Pusa A-4, a 40-mesh nylon-net nursery, border rows of maize to slow whitefly entry, and never okra after okra in the same plot.
3. Pods in one block must be picked in three days. Would you allow the borer spray now? Justify the decision.
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Judge on three things: the pre-harvest interval, the residue risk, and whether the spray can wait.
- PHI rules. Okra is picked every 2–3 days and most recommended insecticides carry a PHI of 3–7 days. A spray three days before picking may leave residue above the MRL, and okra pods are eaten whole and are commonly sampled in market surveillance. Read the specific label PHI; do not guess or average.
- Does it protect yield? A borer spray mainly protects shoots and flowers. Pods already formed and picked clean are at lower risk, so the spray can often be delayed until after picking.
- Decision. Harvest the clean pods first, do the roguing and hand-picking now, and postpone the insecticide to after that harvest. If pressure is severe, use only a product whose label PHI fits inside three days, or a biological (Btk, NSKE) with a short interval, and pick strictly after the stated interval.
- Communication. Never advise "spray today and pick tomorrow" - that is how a consignment gets rejected, and the farmer, not the adviser, carries the loss.