Aphid Hotspot, Natural Enemies and a Knapsack
Mrs Moyo grows 0.8 ha of cabbage, now six weeks from transplant and three weeks from harvest. She scouts 40 plants: the field average is 8 aphids per outer leaf, but the four rows on the windward edge average 45 aphids per leaf with honeydew and sooty mould. Ladybird larvae are on 30% of plants and hoverfly larvae on 15%, and some aphids are swollen and brown, a sign of parasitism. Last season she sprayed the same pyrethroid five times and by the end it gave almost no control. The action threshold is 25 aphids per leaf. Her options are: insecticidal soap at 10 g/L (contact only, no harvest interval, $8/kg); dimethoate 400 EC at 0.5 mL/L (broad-spectrum, 7-day interval, $15/L); imidacloprid 200 SL at 0.5 mL/L (systemic, 14-day interval, toxic to bees, $40/L). The knapsack holds 16 L, she sprays 250 L/ha, labour is $12 per day and one worker covers 1 ha per day.
1. Decide whether she should spray at all, and if so, where. Justify your answer with the scouting figures.
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Field average versus hot spot. The field average of 8 aphids per leaf is well below the threshold of 25, so a whole-field spray is not justified on the scouting data. The windward edge at 45 per leaf is nearly twice the threshold and is producing honeydew and sooty mould, which directly downgrades the heads she will sell.
Where the damage is. Aphids arrive on wind, so the windward edge acts as the entry point and the population there builds first. Treating the whole field because of a localised problem wastes money, exposes the whole crop and the natural enemies to pesticide, and selects for resistance across the entire population.
Biological control already working. Ladybird larvae on 30% of plants, hoverfly larvae on 15% and mummified aphids show that predators and parasites are established. In the rest of the field they are holding the population at 8 per leaf. A broad-spectrum spray would remove these free workers and trigger a rebound worse than the original problem.
Decision. Do not spray the whole field. Treat only the four windward rows, roughly 0.1 ha, and re-scout the whole block after five days. Record the counts and the beneficial insect levels so the next decision rests on data rather than fear.
2. Choose the product she should use on the hot spot and justify the choice, including what you advise about the pyrethroid history.
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Insecticidal soap is the best first choice.
- Harvest interval. She is three weeks from harvest, and soap has none, so there is no residue risk to the marketable heads.
- Safety to natural enemies. It has little residual activity, so ladybirds, hoverflies and parasites survive in the rest of the field.
- Resistance. It is a contact, non-selective detergent that physically disrupts the insect cuticle, so the resistance mechanism that defeated her pyrethroid does not apply. There is no cross-resistance.
- Limits to accept. It only kills aphids it hits, has no systemic or residual action, and can scorch leaves in hot sun, so she must spray in the early morning or evening with thorough coverage of leaf undersides and repeat after five to seven days.
Why not the others. Dimethoate is broad-spectrum and would kill the predators that are currently suppressing the field population, risking a secondary outbreak of diamondback moth or a later aphid rebound. Imidacloprid works systemically and its 14-day interval still fits her three-week window, but it is toxic to bees, selects strongly for resistance, and is unnecessary where a hot spot can be treated in contact.
Pyrethroid history. Five applications of one pyrethroid in one season is textbook selection pressure, so she should not use that group again on this block. If a chemical is ever needed again, rotate to a different mode of action and never apply the same group twice in a row.
3. Calculate the quantity and cost of her chosen treatment for the hot spot, and compare it with a whole-field dimethoate spray. What does the comparison show?
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Hot spot with soap. Area = about 0.1 ha. Spray volume = 0.1 × 250 = 25 L, which is just over one and a half knapsack loads. Soap = 25 L × 10 g/L = 250 g = 0.25 kg, costing 0.25 × $8 = $2.00. Labour = 0.1 of a day × $12 = $1.20. Total ≈ $3.20, plus a repeat application a week later, roughly $6.40 for the programme.
Whole field with dimethoate. Volume = 0.8 × 250 = 200 L, which is 12.5 knapsack loads - a full day of spraying. Product = 200 L × 0.5 mL/L = 100 mL, costing 0.1 × $15 = $1.50. Labour = 0.8 × $12 = $9.60. Total ≈ $11.10, before any repeat.
What the comparison shows. The whole-field dimethoate spray costs roughly three times the spot treatment, but the difference in product price alone is trivial. The real costs of the broad-spectrum route are hidden: loss of the predators now working for free, a likely rebound requiring yet more sprays, and continued selection for resistance in a population that has already beaten one chemical group. The cheapest can of chemical is not the cheapest decision. She should also keep the cost of the sprayer, protective clothing and water carriage in her records so that the true cost per hectare is visible next season.