Thrips, Virus and Weeds in a Chrysanthemum Polyhouse
In a 1,000 m² polyhouse, 6,000 chrysanthemum plants were planted four weeks ago for cut flowers. Yellow sticky traps catch 40–60 thrips per card per week. Young leaves are silvery, buds are distorted, and about 5% of plants show yellow mosaic and stunting. The nursery has sprayed imidacloprid, dimethoate and acephate three times in 18 days, each at the recommended dose, with no reduction in trap catch. The floor is infested with Oxalis and Cyperus rotundus; pots also carry liverwort. Workers hand-weed only when labour is available. A new consignment of 2,000 potted gerbera is due in the same polyhouse next week. The owner asks whether to spray a fourth insecticide or increase the dose.
1. Diagnose the thrips problem and explain why the sprays failed. What do the 5% mosaic plants indicate?
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Diagnosis
The silvering, distorted buds and high trap counts fit a thrips infestation, likely a flower thrips species. The sprays failed for several reasons:
- Mode-of-action repetition. Imidacloprid is IRAC group 4A; dimethoate and acephate are both group 1B. The nursery effectively used only two modes of action, with two organophosphates in succession. This selects resistant thrips quickly.
- Poor target coverage. Thrips feed in buds, leaf axils and flower heads. A general foliar spray may not reach them, especially if application volume and nozzle direction are inadequate.
- No resistance or efficacy check. The same trap count after three sprays is evidence of failure, but the owner proposes another spray rather than a diagnosis.
- Possible wrong life stage. Eggs and pupae are protected in plant tissue or media, so adult knockdown alone does not stop reinfestation.
The 5% mosaic plants are likely infected with a thrips-transmitted virus such as TSWV or INSV. These plants cannot be cured and are a virus reservoir for the rest of the crop. They should be rogued and destroyed, not sprayed. Confirm with a laboratory test if possible, but treat the symptoms as infectious until proven otherwise.
2. Set out an integrated management plan for the next eight weeks, including pesticide rotation and resistance management.
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Immediate actions
- Rogue and destroy the 5% mosaic plants, bagging them before removal so thrips do not disperse.
- Do not increase the dose. That accelerates resistance and may cause phytotoxicity.
- Increase scouting to twice weekly. Use blue and yellow sticky cards and tap buds over a white tray. Set an action threshold with the buyer's cosmetic standards in mind, not a zero-thrips target.
Cultural and biological control
- Screen vents and keep the polyhouse door closed. Remove weeds that shelter thrips.
- Release Amblyseius swirskii or Orius laevigatus where compatible with pesticide residues. Check product compatibility before release.
- Improve ventilation and reduce leaf wetness; liverwort indicates excessive moisture.
Chemical rotation
Use no more than two consecutive applications of the same IRAC group, and rotate among groups such as spinosad (5), abamectin or emamectin (6), diafenthiuron (12A), flonicamid (29) and cyantraniliprole (28). Because abamectin and emamectin share group 6, treat them as one rotation partner. Add a suitable adjuvant and direct sprays into buds and leaf undersides. Observe pre-harvest intervals for cut flowers and record every spray: date, product, group, dose, area and result.
3. How should the nursery manage weeds and the incoming gerbera consignment to avoid worsening the problem?
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Weeds
- Hand-remove Cyperus rotundus tubers and Oxalis before they seed. Repeat every 10–14 days; a single weeding will not control tubers.
- Mulch walkways and cover bare floor with weed mat or gravel, but keep the mat free of silt so drainage is not blocked.
- Reduce liverwort by improving ventilation and avoiding standing water. Liverwort is a symptom of wet, shaded surfaces, not just a weed.
- Avoid herbicide use inside the polyhouse around chrysanthemum and gerbera; drift and volatilisation can cause flower distortion. If a herbicide is absolutely necessary, use a registered product, close vents, and test on a small area first.
Incoming gerbera
- Quarantine the 2,000 plants in a separate bay for 7–10 days. Inspect leaves, buds and media; place sticky traps nearby; check roots for pests and disease.
- Reject or treat any consignment with thrips, mites or virus symptoms before it enters the main house.
- Do not mix the new gerbera with the infested chrysanthemum crop. Keep separate tools, workers and irrigation equipment where possible.
- Record supplier, batch, inspection result and any treatment. If the gerbera carries the same thrips, the nursery will have bought a second problem rather than solving the first.