Sticky leaves: the honeydew tells you it is one of four insects

listLikely causes

• Scale insects: they excrete honeydew, a sugary liquid that coats the leaf beneath them and feels tacky to the touch. Look for brown or black bumps on stems and along veins.
• Aphids: soft-bodied insects clustered on new growth and flower buds, producing large amounts of honeydew.
• Mealybugs: white, cottony insects in leaf axils and root crowns, also honeydew producers, often found on the underside of leaves.
• Whitefly: small white insects that fly up when the plant is disturbed, with honeydew and sticky leaves underneath.
• Extrafloral nectaries: some species, including Philodendron and Prunus, have glands on the leaf or petiole that produce nectar deliberately. This is not a pest.
• Spider mites do not produce honeydew. Sticky leaves plus fine webbing means you have two problems, not one.

listWhat to do

• Confirm the pest rather than guessing. Turn the leaf over and look along the midrib and in the leaf axils with a hand lens or a phone macro lens.
• Scale: remove individual insects with a cotton bud dipped in rubbing alcohol, then treat the whole plant with horticultural soap or neem every seven days for four rounds.
• Aphids: wash them off with a firm jet of room-temperature water, then treat with insecticidal soap. Repeat every five days for three rounds.
• Mealybugs: the alcohol-on-a-cotton-bud method works well for small numbers. Check the root crown and the undersides of leaves, where they hide.
• Whitefly: yellow sticky traps catch the adults, but you must also treat the leaf undersides where the nymphs live. Repeat treatments every five days.
• Wash the honeydew off with lukewarm water once the pests are controlled. The sticky layer grows sooty mould, which blocks light and is harder to remove once established.
• Isolate the affected plant. All four of these pests spread to neighbouring plants, and honeydew-producing insects are the reason infestations are usually found on several plants at once.

differentialConfusable conditions

presentationWhat you are seeing

• A tacky, shiny film on the upper surface of leaves, often with a scattering of small dark specks on top of it, and sometimes a black powdery layer if it has been there for weeks.
• The stickiness is usually worst on the leaves below the infestation, because honeydew drips downward, and it is often noticed first on a windowsill or a floor rather than on the plant.

aetiologyWhy it happens

• Sap-feeding insects take in far more sugar than they need and excrete the surplus as honeydew. Scale, aphids, mealybugs and whitefly all do this. Spider mites and thrips do not.
• Because honeydew is sugar and water, it is an ideal medium for sooty mould, a black fungus that grows on it and blocks light from the leaf. Sooty mould is a symptom of the honeydew, not the original problem.
• Honeydew also attracts ants, which will farm and protect the insects producing it. Ants on a houseplant are a strong indication of a scale or aphid colony above.

managementWhat to do

• Find the source. Turn every leaf over, check the leaf axils, the growing tips, the stems and the root crown. Use a torch and a hand lens.
• Treat the specific pest. Scale and mealybugs: alcohol on a cotton bud for individuals, then horticultural soap every seven days for four rounds. Aphids and whitefly: wash off, then insecticidal soap every five days for three rounds.
• Wash off the honeydew and sooty mould with lukewarm water and a soft cloth once the pests are gone. Do not use leaf-shine products, which seal the mould in.
• Check every plant within a metre. Honeydew-producing pests spread, and treating only the visibly affected plant leaves a reservoir next door.
• Repeat the treatment beyond the point where you can see insects. Scale nymphs are 0.5 mm and invisible; the second and third rounds are what kill the generation you cannot see.