Fall armyworm in whorl-stage maize with a market date and a beehive nearby
Amar Singh has 0.8 hectare of hybrid maize sown on 5 July in Lalitpur district. Today is 12 August; the crop is about 38 days old and at the 8–10 leaf stage. Scouting 20 plants at five spots in the field, he finds 7 plants with fresh ragged holes in the whorl and moist, sawdust-like frass, and he collects three larvae about 2–3 cm long with an inverted Y mark on the head and four dark dots on the last abdominal segment. His neighbour has already sprayed chlorpyriphos 20% EC twice on a more heavily damaged field. Amar owns one knapsack sprayer, has ₹2,000 to spend and no gloves or mask; his children often run through the field. He plans to start picking green cobs for the festival market from 5 September. The product offered at the shop is emamectin benzoate 5% SG, whose label states a waiting period of 7 days. Rain is forecast within 48 hours, and a honeybee box is kept 200 m away.
1. Using the scouting data, decide whether a spray is justified today, and explain what should not be copied from the neighbour's field.
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The threshold is crossed, so action is justified. Damage is 7 out of 20 plants, i.e. 35 per cent of plants with fresh whorl feeding. The economic threshold for fall armyworm at the whorl stage is roughly 10 per cent of plants damaged with live larvae present. He is more than three times above it, the frass is moist (so the damage is fresh, not old), and the three larvae found are still small - 2–3 cm - which is the stage at which they are easiest to kill and before they do the heaviest feeding.
Identification fits fall armyworm, not stem borer: the inverted Y on the head, four dots in a square on the last segment, ragged whorl feeding and moist frass are characteristic. A stem borer would show dead-heart and bore holes in the stem.
What not to copy from the neighbour: two rounds of chlorpyriphos 20% EC. Chlorpyriphos is an organophosphate with poor reach into a tightly folded whorl where the larva feeds, it is broad-spectrum and kills the natural enemies that would otherwise hold the pest down, and two consecutive sprays of the same mode of action select for resistance. The neighbour's field is more damaged, not less - that is the evidence, not an argument for the same product.
Before spraying, one check: look for earwigs, ladybird beetles and parasitised larvae. If natural enemies are abundant and the larvae are mostly large, a selective product still wins, because a broad-spectrum spray would remove the free control.
2. Recommend the product, dose and timing, and resolve the conflict between the rain forecast, the market date and the honeybee box.
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Product and dose. Emamectin benzoate 5% SG at the label dose (of the order of 0.4 g per litre of water, about 200 litres of spray per hectare). Read the label on the pack he actually buys and follow its figures rather than any verbal advice. Direct the spray into the whorl using a hollow-cone or flooding nozzle, because the larva lives inside the funnel and a flat fan over the top of the canopy wastes most of the chemical.
Timing versus the rain. Spray in the evening, ideally tonight. Rain forecast within 48 hours is not a reason to wait two days: the larvae are small now, and every extra day of feeding inside the whorl costs leaf area that will never regrow. Evening application avoids the heat of the day, reduces evaporation, and - importantly - is when bees are not foraging. If heavy rain falls within a few hours, do not blindly repeat the spray; re-scout three to four days later and spray again only if live larvae are still present, using a different mode of action (for example chlorantraniliprole or spinetoram) so the same chemistry is not used twice.
Market date versus waiting period. He plans to pick cobs from 5 September, which is 24 days away - well beyond the 7-day waiting period, so there is no conflict this time. Write the spray date on the wall, and never harvest green cobs inside 7 days of the last spray, because the residue then goes to the market.
Bee box. Do not spray during the day near the hive, do not spray flowering weeds at the field edge, and warn the beekeeper so the hive can be closed for the evening.
3. He has ₹2,000, no protective equipment and children in the field. Give him a safe, integrated plan and say what he should do differently next season.
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Safety comes out of the ₹2,000 first. Buy a mask, gloves and goggles (a few hundred rupees), and keep the children and the family's cattle out of the field for at least 48 hours after spraying. Spray standing upwind, never eat or smoke while spraying, wash hands, face and clothes afterwards, and bury the empty containers in a pit rather than throwing them into the field or the canal. One spray of about ₹600–800 plus the equipment leaves him within his budget.
Non-chemical measures that can run alongside:
- Hand-pick larvae and crush egg masses in the early morning, especially in plants at the field border.
- Place a pinch of dry sand mixed with ash or lime in the whorl of damaged plants; the larvae cannot move easily and are exposed to the next spray - this also helps where the sprayer does not reach.
- Install pheromone traps at about 5 per hectare to track moth activity, and put up bird perches.
- Keep broad-spectrum sprays off the field so earwigs, ladybirds and parasitic wasps survive to work on the pest.
Next season: sow on time and uniformly with the neighbours so the crop does not bridge the pest's generations; choose a hybrid with good whorl tightness; scout twice a week from 15 days after emergence instead of waiting for visible damage; rotate crops and destroy stubble after harvest so pupae are not carried over; and rotate insecticide groups from the first spray, not after resistance has already been selected.