Whitefly Resistance and a Biocontrol Release Under Pressure
An 0.8-acre greenhouse produces poinsettia cuttings for a December market. Whitefly sticky-card counts rose from 8 to 45 adults per card per week; the action threshold is 10. Nymphs cluster on leaf undersides, and honeydew with sooty mold covers upper leaves in two benches. Five weeks ago the crop received an imidacloprid drench; three weeks ago, a pyriproxyfen spray. The grower wants to tank-mix a pyrethroid with the same pyriproxyfen and spray the entire house tomorrow. Night temperature is 24–28°C and relative humidity 60%. A supplier can deliver Encarsia formosa wasps in four days. The greenhouse has biological-control experience but no recent baseline residue test. The crop is three weeks from pinching, and workers scout twice weekly. The owner fears losing the cuttings and asks for a decision.
1. Evaluate the grower’s tank-mix proposal. What is the immediate treatment decision, and why?
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Reject the tank mix as proposed. The proposal repeats pyriproxyfen, so it adds selection pressure for resistance to the same insect growth regulator, and it adds a broad-spectrum pyrethroid just before a planned Encarsia release. Pyrethroids can kill the parasitoids and flare secondary pests such as mites, and the greenhouse has no recent residue baseline to predict compatibility. The immediate move is not a whole-house spray: map the two heavily infested benches, remove and bag the worst leaves, spot-treat only hotspots with a labeled product from a different IRAC group that is compatible with the biological plan, and hold the rest of the house for the Encarsia release. Check the product label for plant safety and preharvest interval, and do not tank-mix products unless the label allows it. If adult counts remain above threshold after the release, reassess in 5–7 days rather than spraying on a calendar schedule. The reasoning is that resistance management and biocontrol both depend on preserving susceptible populations and natural enemies.
2. Design a four-week integrated plan that includes Encarsia release and resistance management.
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For four weeks: week 0, scout and map; remove heavily infested plants; release Encarsia formosa at the supplier rate in hotspots, not the whole house; keep yellow cards at the same density and count weekly. Week 1, check parasitism by looking for black pupae and monitor adult counts; do not spray broad-spectrum. Week 2, if hotspots remain above threshold, spot-treat with a different MOA that has low residual activity against Encarsia; treat only those benches. Week 3, evaluate whether the adult count is falling and whether new damage is declining; if not, rotate to another MOA, not back to pyriproxyfen. Week 4, decide whether to continue biocontrol or use a clean-up product before the next crop. Keep temperature and humidity stable, avoid excessive nitrogen, and record every product, rate, date, and area. The plan must include a stop rule: if parasitism is low and damage is spreading, a different MOA is justified, but not a return to the same chemistry.
3. What monitoring data and records would you keep to justify continuing biological control if the owner demands a broad-spectrum spray in week two?
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Keep weekly sticky-card counts by bench, percent parasitism, beneficial counts, crop damage ratings, product names, rates, dates, and areas treated, plus the label and SDS for any product used. Record the decision rule in advance: continue biocontrol if adult counts trend down and parasitism rises; treat hotspots if damage crosses the economic threshold. If the owner demands a broad-spectrum spray in week two, show the trend data and the compatibility concern, and document the refusal in writing. If the owner overrides, record the instruction, the product, the rate, and the potential loss of the biocontrol program. The record should let a third party see why the original plan was reasonable and what changed.