Four Sprays and the Larvae Keep Coming Back
A 1.5 ha maize field in Nawalparasi sown with a hybrid at 60 × 25 cm is 28 days old. The farmer reports ragged holes and window-pane feeding in the whorls, moist sawdust-like frass, and larvae with an inverted Y on the head and four dark spots arranged in a square on the second-last abdominal segment. About 30% of plants show whorl damage and larvae range from newly hatched to 3 cm long. He has sprayed emamectin benzoate 5% SG at the label dose four times at 6-day intervals and says the larvae return within three days. Neighbouring farmers are doing the same. He now wants to switch to 'something stronger', and asks whether spraying in the afternoon would work better, since morning sprays 'did not work'. His maize borders a small patch of his own okra.
1. Confirm the diagnosis and decide whether further spraying is justified at all. Explain the reasoning behind the action threshold.
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Diagnosis The description is classic fall armyworm (Spodoptera frugiperda): inverted Y on the frons, four dark spots in a square on the eighth abdominal segment, sawdust frass in the whorl, and window-pane feeding by young larvae. Distinguish it from spotted stem borer (Chilo partellus), which has no inverted Y, shows body spots and lines, and produces dead heart rather than a whorl full of frass.
Is spraying justified? Yes, at 30% whorl damage with live larvae present. The commonly used pre-tasselling guideline threshold for whorl damage is roughly 5–10% of plants with fresh damage; 30% is well above it. Below the threshold, natural enemies and plant compensation handle the population, and early spraying destroys those enemies.
What the threshold actually means
- It is a scouting trigger, not a licence for weekly calendar spraying.
- Count fresh damage and live larvae in at least five points of five plants each, twice a week.
- Rescue treatment is justified when the threshold is crossed, larvae are still small (first to third instar), and the crop is before tasselling.
- With 3 cm larvae and a closed whorl, control efficiency will be poor regardless of product: late instars are less exposed and less susceptible.
So the answer is not 'no spraying' and not 'more spraying' - it is one correctly timed application after scouting, followed by re-scouting, and a plan that stops the repeated use of one mode of action.
2. Design a resistance-management and IPM plan for the rest of the season. Name specific mode-of-action groups and non-chemical tactics, and explain why the current practice is failing.
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Why the current practice fails
- Four sprays of the same active ingredient, emamectin benzoate (IRAC Group 6, glutamate-gated chloride channel allosteric modulators), is four selection events for the same resistance mechanism.
- A 6-day interval with a largely larvicidal, non-ovicidal product leaves gaps: eggs and newly hatched larvae in the whorl are protected, and survivors are the ones that get sprayed again.
- Whorl-directed coverage with low water volume misses larvae deep in the funnel.
- Spraying does nothing about the surrounding infested maize and the okra patch, which act as a refuge and source.
Resistance-management rules
- Rotate modes of action across the season; never apply the same group twice consecutively. Alternatives: chlorantraniliprole (Group 28, diamides), spinetoram (Group 5), Bacillus thuringiensis (Group 11), chlorfenapyr (Group 13), indoxacarb (Group 22A).
- Maximum two applications of any one group per season; record every spray date, product and dose.
- Avoid pyrethroids: they flare secondary pests and kill natural enemies.
- Use the locally recommended dose, never a reduced dose 'to save money'.
Non-chemical tactics
- Scout twice weekly and spray only on threshold.
- Hand-pick larvae and crush egg masses in small plantings.
- Put sand, ash or a sand-plus-lime mix into the whorls of young plants.
- Conserve and release natural enemies (Telenomus remus, Trichogramma spp., coccinellids, earwigs); avoid broad-spectrum sprays.
- Destroy stubble and volunteer plants after harvest, avoid overlapping maize plantings, and keep the okra strip checked and treated separately under its own label and resistance rules.
3. Give application, safety and residue guidance for the single planned spray and for the okra nearby. What practical errors would you correct?
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Application
- Spray in the late afternoon or evening. Larvae feed at dusk and at night, and several products, including B. thuringiensis, degrade in sunlight. Morning spraying is not 'wrong' in itself; the failure was timing relative to larval feeding, poor whorl coverage, and repeated use of one product.
- Direct the spray into the whorl using a hollow-cone or narrow-angle nozzle at the recommended water volume; flat coverage over the canopy wastes chemical.
- Spray when the crop is dry, wind is low, and rain is not expected within the label interval.
- Correct dose only - over-dosing adds cost and resistance pressure, under-dosing selects survivors.
Safety and residues
- Wear gloves, mask, goggles, boots and a long-sleeved shirt; never spray against the wind; bathe and wash clothes afterwards.
- Observe the label pre-harvest interval. Emamectin benzoate and diamides generally carry a PHI of roughly one to two weeks on maize; check the specific label and do not harvest green cobs or fodder inside that interval.
- Respect the re-entry interval, keep children and animals out, and triple-rinse and puncture containers before disposal away from water sources.
The okra patch
- Do not use the same active ingredient on okra: it shares the resistance and residue problem, and okra fruit is harvested continuously, so PHI is much tighter.
- Spray okra separately with a different mode of action, only if its own scouting threshold is crossed, and avoid drift onto the maize and onto flowering weeds where bees forage.
Record-keeping: date, pest stage, product, group, dose, water volume and result - this is what makes the next season's decisions possible.