Fall Armyworm in Whorl-Stage Maize After Repeated Sprays
A farmer in Perambalur district has 2 ha of maize at 32 days after sowing. About 18% of plants show ragged, window-pane damage in the whorl, with moist sawdust-like frass and a few dead central shoots. Scouting finds about three larvae per 10 plants. The farmer has already sprayed emamectin benzoate 5 SG at 0.5 g/L twice at 10-day intervals and lambda-cyhalothrin once. Damage is still increasing. A neighbour advises doubling the emamectin dose and spraying every five days. The field is bordered by an older maize field that is also infested, and light rain is forecast for the next week.
1. Identify the pest, distinguish it from stem borer, and diagnose why the sprays underperformed.
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The pest is fall armyworm (Spodoptera frugiperda). The key signs are ragged whorl feeding, moist sawdust-like frass, and larvae with an inverted Y mark on the head and four dark spots in a square on the last abdominal segment. Stem borer typically shows a clean bore hole and dead heart, with frass pushed outside the hole; it does not produce the same mass of moist frass in the whorl.
The sprays underperformed for several reasons:
- The larvae feed inside the whorl, so a contact spray that wets only the leaf surface does not reach them.
- Spraying began after damage exceeded the action threshold, and older larvae are much harder to kill.
- Emamectin benzoate was repeated with the same mode of action, which selects for resistance and gives no rotation.
- Lambda-cyhalothrin is a pyrethroid; it is not a good fall armyworm material and it kills natural enemies, which can make the outbreak worse.
- The 10-day interval and likely water volume/nozzle coverage were inadequate for a whorl target.
- The neighbouring older maize field is a continuous source of moths, so re-infestation is expected.
2. Build an integrated pest management plan for the next four weeks, including threshold, biological options, chemical rotation and resistance management.
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Scouting and threshold: inspect 10 plants at five points twice a week. Treat when about 10% of plants show fresh whorl damage, not on a calendar schedule. Record larval size and natural enemies.
Cultural and mechanical: hand-pick egg masses and young larvae where practical; destroy them. Keep the field free of alternate hosts. If the neighbour cooperates, synchronise planting and destroy crop residues after harvest, because pupae survive in soil.
Biological: release Trichogramma pretiosum at about 50,000/ha at the egg stage. Apply Bacillus thuringiensis var. kurstaki or azadirachtin 1500 ppm at early instar, directed into the whorl. Conserve lady beetles, earwigs and parasitoids by avoiding pyrethroids.
Chemical, only at threshold: use a whorl-directed spray with a label dose of chlorantraniliprole 18.5 SC at 0.4 mL/L or spinetoram 11.7 SC at 0.5 mL/L. Use a surfactant if the label permits, and enough water to reach the whorl. Do not repeat the same mode of action in consecutive sprays; rotate with a different group such as Bt or azadirachtin. Observe the pre-harvest interval.
Community level: place 5 pheromone traps/ha to monitor moth flight and warn the neighbouring field. Resistance management is not just rotating chemicals; it is also keeping non-chemical mortality high.
3. Evaluate the neighbour's advice to double the emamectin dose and spray every five days, including economic, residue and legal issues.
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The advice should be rejected. Doubling the dose is an off-label use; the label dose is part of the insecticide registration, and applying above it is not a legal recommendation. It also increases cost without a proportional gain, because the main problem is placement inside the whorl, not dose.
Repeated five-day spraying with the same product:
- speeds up resistance in a pest that already has a history of resistance to several insecticide groups;
- raises residue risk in maize used for feed and food;
- increases worker exposure and the risk of phytotoxicity, especially with a surfactant in hot weather;
- kills natural enemies and can trigger secondary pests;
- leaves a shorter interval before harvest and may breach the pre-harvest interval.
A better economic comparison is the cost of three properly timed, rotated sprays plus biologicals against the value of avoided yield loss. If the field is already at 18% damage, the aim is to protect the younger leaves and the cob, not to chase every larva. The farmer should also talk to the neighbour about the shared infestation, because a five-day spray schedule on one field will not solve a two-field problem.