Thrips

Thrips are among the harder houseplant pests to clear, because part of their life cycle happens in the soil where sprays don't reach. Monstera, philodendron, pothos and other aroids are favourites.

How to tell

• Silvery, bleached or bronze streaks and patches on leaves.
• Tiny black dots in those patches: droppings.
• Distorted, spotted new leaves.
• Slender insects about 1 mm long, dark adults and pale yellow larvae. Tap a leaf over white paper to see them.

How to get rid of them

1. Isolate the plant, and check the others closely.
2. Shower the whole plant, leaf undersides included.
3. Put up blue sticky traps (thrips prefer blue to yellow) to catch flying adults.
4. Spray with insecticidal soap or a neem product, covering both sides of every leaf.
5. Repeat every 4–5 days for at least four weeks. This is the step people skip: larvae drop into the soil to pupate and emerge later.
6. Replace the top few centimetres of soil, or treat it, to break the soil stage.

Predatory mites (*Amblyseius cucumeris*) are a good biological option for collections that keep getting reinfested.

How to prevent them

Quarantine new plants for two weeks and check them with a magnifier, keep blue traps near your plants as an early warning, and remove old flowers and fallen leaves, where thrips hide.