Suspected whitefly resistance in a glasshouse tomato crop
A 0.8 ha glasshouse tomato crop (cv. 'Endeavour') is on a 12-month cycle. Whitefly (Trialeurodes vaporariorum) has been controlled with imidacloprid and then spiromesifen, but resistance is suspected: 60% of yellow sticky cards catch >200 whitefly/week and honeydew is visible on fruit. The crop is at 5 trusses set, harvest in 6 weeks. Temperature is 22–26°C, RH 70–80%. The grower wants to introduce Encarsia formosa and Eretmocerus eremicus, but the current chemical program includes a persistent neonicotinoid. A biocontrol supplier can deliver 12,000 parasitoids/week at $0.20 each. The packhouse rejects fruit with sooty mould. Labour for monitoring costs $220/week. Chemical sprays cost $650/week.
1. Assess whether resistance is plausible and explain the management failures that led to it.
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Resistance is highly plausible. The grower has used two systemic insecticides in sequence-imidacloprid, a neonicotinoid, then spiromesifen-without alternating with a different mode of action or integrating non-chemical control. Whitefly has many overlapping generations in a 22–26°C glasshouse, so selection for resistant individuals is rapid. The persistent neonicotinoid also remains active against the same population and can kill parasitoids introduced later. The sticky-card data show a rising population, and honeydew on fruit indicates the population is well above a tolerable threshold. This is not just a spray-timing problem; it is a resistance-management failure caused by repeated selection pressure and lack of biological control.
2. Recommend a transition plan to integrated pest management, including chemical withdrawal, release rates and monitoring thresholds.
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First, stop imidacloprid immediately and allow its residual activity to decline before releasing parasitoids; otherwise the beneficials will be killed. Use a selective emergency treatment only if the population threatens fruit, and choose a product with a different mode of action from both previous chemicals. Introduce Encarsia formosa and Eretmocerus eremicus at a combined rate of about 3–5 parasitoids/m² per week for four weeks: for 8,000 m², that is 24,000–40,000 per week, so the supplier's 12,000/week is a starting rate that must be increased if sticky cards exceed 50 adults/card/week. Monitor cards weekly and inspect leaves for parasitised scales. Threshold: treat or release more when 10 adults per plant or 50 per card are recorded. Maintain RH and avoid broad-spectrum sprays.
3. Compare the weekly cost of biocontrol with the current chemical program and justify the decision.
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Biocontrol costs 12,000 × $0.20 = $2,400/week plus $220 monitoring = $2,620/week. Current chemical spraying costs $650/week. Biocontrol is therefore about $1,970/week more expensive in direct cost, or roughly $11,800 over six weeks to harvest. However, the chemical program is already failing: honeydew and sooty mould risk packhouse rejection of the whole crop, and continued spraying will select for stronger resistance. The relevant comparison is not just spray cost but the value of marketable fruit. If the crop is worth many thousands of dollars per week, the extra $1,970 is justified as insurance. A phased approach-chemical withdrawal plus biocontrol-is the lower-risk commercial decision.