Escalating pyrethroid sprays, larvae inside tomato fruit and an illegal harvest interval
On a 0.4 ha tomato plot near Salima, Mr Chirwa has sprayed cypermethrin every third day for six weeks against fruit borer. The dose has been raised from the label rate of 25 ml per 15 litre knapsack to 60 ml, and he still finds 30–40% of ripe fruit with entry holes and frass, with larvae feeding inside the fruit. He uses the same knapsack mix on his 0.5 ha maize, where fall armyworm larvae are feeding in the whorls of about half the plants. His last tomato spray was on Tuesday and he picks and sells fruit to a lodge on Thursday. He has no protective clothing beyond a cloth over his mouth, and the knapsack leaks at the lance. A neighbour keeps bees and has complained of dead bees. He asks whether he should switch to a 'stronger' product, and which one.
1. Explain why his spraying is failing, and why a 'stronger' product at a higher dose is not the answer.
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Several failures are acting together.
- The target is protected. Once a larva has entered the fruit, a contact insecticide never reaches it. Cypermethrin has little or no systemic or translaminar movement, so killing larvae inside fruit is not a dose problem.
- Selection for resistance. Six weeks of a single pyrethroid, at every third day, at more than double the label rate, is close to a resistance-breeding programme. Surviving moths pass on tolerance, so each increase in dose works less well than the last.
- Timing. Control must be aimed at eggs and young larvae before they enter the fruit; a fixed three-day calendar spray ignores whether larvae are at a vulnerable stage.
- Natural enemies destroyed. A broad-spectrum pyrethroid kills predators and parasitoids that would otherwise suppress the borer, so each spray can be followed by more pest pressure.
- Residues and safety. Spraying every third day and picking two days later breaks the pre-harvest interval on the label, exposes him through a leaking knapsack, and kills his neighbour's bees.
'Stronger' is the wrong question. The dose is already off-label, the problem is target access, resistance and timing, and a larger dose increases residue, cost and resistance without reaching the larvae inside the fruit.
2. Recommend a practical control programme for the tomato plot, including product choice and harvest timing.
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Monitor before spraying. Twice a week inspect 20 plants across the plot, looking at flowers, young fruit and shoot tips; spray only when live larvae or fresh damage are found, not on a fixed calendar. Hang a pheromone trap if available to detect moth flight.
Remove the refuge. Pick and destroy infested fruit in a closed container or bury it deeply; larvae inside dropped fruit are the source of the next generation.
Rotate modes of action and use translaminar or ingestion-active products. Move away from pyrethroids. Use a Bacillus thuringiensis product, or a diamide such as chlorantraniliprole, or emamectin benzoate, and rotate between these groups rather than repeating one. These act on larvae that feed on or in treated tissue and are far safer for bees and natural enemies when used as directed. Apply at the label rate - never above it - with correct pH water and good coverage of flowers and young fruit.
Respect the pre-harvest interval. The label interval for most of these products is several days. If the interval is seven days, fruit picked on Thursday after a Tuesday spray cannot be sold; either delay harvest or plan the spray so it falls outside the harvest window. Tell the lodge buyer what was used and when.
Cultural measures: stake and prune for airflow, avoid overhead irrigation in the evening, and remove crop debris after harvest.
3. Address his safety and bee concerns, and advise on the fall armyworm in the maize.
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Safety and bees
- Stop using the leaking knapsack until the lance and seals are replaced; a leaking lance delivers an uncontrolled dose onto the operator.
- He needs at minimum a long-sleeved shirt, trousers, boots, gloves, goggles and a proper respirator - a cloth over the mouth does not stop fine spray.
- Never spray into wind, never spray in the heat of the day, and never spray a crop in flower while bees are foraging. Late evening is safest for bees and for the operator.
- Warn the neighbour before any spraying so hives can be closed, and keep a bucket of water and soap at the site.
- Do not re-use pesticide containers, and wash immediately after spraying.
Fall armyworm in maize
- Scout: check 10 plants in five places. The ragged whorl damage is distinctive, but small larvae are the target - large larvae in the whorl are hard to kill.
- Direct the spray into the whorl rather than over the canopy, early in the morning or evening when larvae are active.
- Keep the products used on maize in a different mode-of-action group from those used on tomato, and do not carry over the same mix to both crops.
- Hand-picking and a pinch of dry sand or ash into the whorl of small plantings are useful where the infestation is light.
Do not treat the maize with the tomato mix as a matter of convenience; that is how resistance and residues spread between crops.