How should I treat spider mites in a greenhouse in spring? - briefly
Use a contact insecticide such as neem oil or a licensed miticide, applying it to leaf undersides and repeating every 7‑10 days until mites disappear. Simultaneously lower humidity, increase ventilation, and discard heavily infested foliage to prevent reinfestation.
How should I treat spider mites in a greenhouse in spring? - in detail
Spider mites proliferate rapidly when greenhouse temperatures rise in early season, especially under low humidity. Immediate detection and decisive action prevent extensive foliage damage and yield loss.
Identify infestation by inspecting the undersides of leaves for tiny moving dots, fine webbing, or stippled discoloration. Use a 10 × 10 cm white card to tap leaves; the falling mites confirm presence. Record infestation levels to gauge treatment intensity.
Cultural measures reduce population growth:
- Keep relative humidity above 60 % during daylight hours; dry air favors mite reproduction.
- Maintain daytime temperatures between 20 °C and 25 °C; temperatures above 30 °C accelerate life cycles.
- Provide adequate air circulation with fans; turbulent flow disrupts mite settlement.
- Remove heavily infested plants or severely damaged foliage promptly.
Biological options offer sustainable control:
- Release predatory phytoseiid mites (e.g., Phytoseiulus persimilis, Neoseiulus californicus) at a rate of 10–20 predators per square meter.
- Introduce predatory insects such as lady beetle larvae (Stethorus punctillum) when mite density exceeds 5 mites per leaf.
- Apply entomopathogenic fungi (e.g., Beauveria bassiana) as a foliar spray at 1 × 10⁸ spores ml⁻¹, repeating every 7 days until populations drop below economic thresholds.
Chemical interventions should be reserved for severe outbreaks and integrated with non‑chemical tactics:
- Use miticides classified as low‑risk (e.g., sulfur, neem oil) at label‑recommended concentrations, applying early in the morning to avoid photodegradation.
- Rotate products with different modes of action (IRAC groups 1, 2, 4) to delay resistance.
- Observe a pre‑harvest interval of at least 48 hours for edible crops.
Sanitation practices support all control methods:
- Clean greenhouse benches, benches, and trays with a 10 % bleach solution weekly.
- Disinfect propagation trays before reuse.
- Quarantine new plant material for 14 days, inspecting daily for mites.
Monitoring continues after treatment. Sample leaves weekly, counting mites per leaf area. When counts fall below three mites per leaf for two consecutive samplings, reduce predator releases and cease miticide applications. Maintain humidity and temperature regimes to keep the environment unfavorable for resurgence.