A sticky film on leaves, or on the floor and furniture below the plant, is usually honeydew. It is the sugary waste of sap-sucking insects such as aphids, scale insects, whiteflies and mealybugs.
Find the insect
- Aphids: small soft insects, often green, black or yellow, clustered on new growth and buds.
- Scale: small brown or tan bumps on stems and the undersides of leaves that don't move and can be scraped off.
- Whiteflies: tiny white flies that flutter up when the plant is shaken.
- Mealybugs: white cottony patches in leaf joints.
A black, sooty mould can grow on honeydew. It is a sign of the pest problem, not the cause.
What to do
- Isolate the plant.
- Wipe leaves with a damp cloth to remove honeydew and any insects you can see. Rinse the plant in a shower if it is small enough.
- Treat with a mild soapy solution or a plant-safe insecticidal soap, following the label, covering undersides and stems.
- Repeat weekly for several weeks.
- Remove badly infested stems and leaves.
When to scan
Take a close-up of the underside of an affected leaf, in daylight. Naming the pest correctly makes treatment much more effective, and a clear photo helps the Plant Doctor do that.