A knapsack, a six-fold overdose and a collapsing sprayer operator
Sunil grows cotton on 1.6 ha and owns a 16-litre knapsack sprayer. The label of his insecticide, a 50% WP formulation of a contact organophosphate, recommends 500 g a.i. per hectare in 600 litres of water. He fills the tank to the 15-litre mark, adds 150 g of the powder, and sprays until the leaves drip, covering his 0.4 ha block in three tank loads. He wears a lungi, chappals and no gloves; when the nozzle blocks he clears it by mouth. Two hours later he has blurred vision, profuse sweating, pinpoint pupils, vomiting and muscle twitching. His neighbour advises milk and forced vomiting, and says the dose should be doubled next time because ‘the pest has become resistant’. The sprayer's pressure gauge is broken and the nozzle is a worn hollow-cone type. Picking in the block is due in ten days.
1. Calculate the correct quantity of product per 15-litre tank and for his 0.4 ha block at the label water volume, and list his mixing, calibration and protective-equipment errors with the consequence of each.
Read the worked answer
What the label implies 500 g a.i./ha of a 50% WP means 1,000 g of product per hectare, in 600 L of water. So the spray liquid should contain 1,000 ÷ 600 = 1.67 g of product per litre.
Correct quantities
- Per 15 L tank: 1.67 × 15 = 25 g.
- His 0.4 ha block: 0.4 × 1,000 = 400 g of product in 0.4 × 600 = 240 L of water (16 tanks of 15 L, or 400 g in whatever water volume is finally used).
What he actually did
- 150 g in 15 L = 10 g/L, i.e. six times the label concentration. Three tanks delivered 450 g of product over 0.4 ha, about 560 g a.i./ha, so both the concentration and the per-hectare dose were wrong.
- Water volume was only 45 L/ha instead of 600 L/ha, so coverage was patchy and most of that concentrated dose ran off the leaf onto soil and skin.
Errors and consequences
- Six-fold concentration - phytotoxicity, residues far above tolerance, and the picker exposed ten days later.
- Blocked nozzle cleared by mouth - direct ingestion of concentrate, the most likely route of his systemic poisoning.
- Lungi, chappals, no gloves - large dermal exposure of legs and hands.
- Broken pressure gauge - he has no idea of output and cannot calibrate; a worn hollow-cone gives a coarse, drifting pattern with variable dose.
- Doubling the dose next time - the opposite of resistance management; it selects harder and kills natural enemies.
What he should do Calibrate properly: measure nozzle output for one minute, walk a measured 50 m, compute litres per hectare, then mix 1.67 g per litre. Use a separate measuring jug, keep a nozzle-cleaning brush, and wear gloves, boots and a mask.
2. Diagnose his clinical state, state what the first aid should and should not include, and explain why the neighbour's advice is harmful.
Read the worked answer
Diagnosis The combination of blurred vision, profuse sweating, pinpoint pupils, vomiting and muscle twitching, two hours after dermal and oral exposure to an organophosphate, is a cholinergic crisis from acetylcholinesterase inhibition. Pinpoint pupils and sweating separate it from pyrethroid poisoning (burning, salivation, but dilated pupils) and from heat exhaustion.
First aid - what to do
- Stop exposure: remove contaminated clothes and wash skin and hair with soap and plenty of water; flush the eyes.
- Get him to a hospital immediately and keep him lying and calm; convulsions and respiratory failure are the killers.
- Carry the label or the bottle so the doctor knows the active ingredient.
What not to do
- Do not give milk or oil: fats increase absorption of the solvent and the pesticide, and milk is not an antidote.
- Do not induce vomiting: he is already vomiting and drowsy, and the petroleum-distillate solvent can be aspirated into the lungs, causing chemical pneumonia.
- Do not give anything by mouth if he is drowsy.
Why the neighbour's advice is dangerous It delays the only effective treatment. The specific therapy is atropine sulphate given to the end-point of dry secretions and repeated, with pralidoxime (2-PAM) added early; neither works if the family wastes an hour on milk and forced vomiting.
Prevention For a knapsack user handling a class I/II organophosphate: gloves, boots, apron, face mask, a spare nozzle and brush, no eating or smoking during work, and a bath with soap immediately after spraying.
3. The pest population survived. Advise him on resistance management, nozzle and calibration checks, and the pre-harvest interval, given that picking starts in ten days.
Read the worked answer
Resistance management
- Rotate modes of action, not dose. Organophosphates (IRAC group 1B) should not be followed by another OP or by a pyrethroid (3A). Rotate with a different group such as a diamide, a spinosyn or a juvenile hormone mimic, and never apply the same group twice in a row.
- Spray only above the economic threshold for the actual pest, not on a calendar and not ‘just in case’.
- Conserving natural enemies is part of resistance management: six-fold overdoses wipe out predators and parasitoids and leave a resistant remnant to breed.
- A border or untreated refuge slows selection.
Calibration and equipment
- Mark a 50 m run, spray water, measure the volume used, and compute L/ha; compare nozzle output with the manufacturer's figure.
- Replace the worn hollow-cone nozzle: a 1.0 mm and a 1.5 mm nozzle deliver very different volumes and droplet sizes, and a broken gauge means he cannot control pressure or drift.
- Check coverage with water-sensitive paper on upper and lower leaves; drips mean run-off, not coverage.
- Keep a separate jug and a brush; never clear a nozzle by mouth.
Pre-harvest interval The label PHI governs when he can pick, and for most OP and pyrethroid products on cotton it is measured in weeks, not days. With picking due in ten days he must read that label before spraying again: if the PHI is longer than ten days, the block must be held back and picked later, or the produce diverted, because residue in lint and on pickers' hands is a real hazard. Cotton is also picked by hand, so workers re-entering before the re-entry interval need gloves and should not work in wet foliage.
Decision rule If he cannot respect the PHI, he should not spray that product at this stage at all; a different group with a shorter PHI is the correct choice.