Sticky residue on the leaves or the floor
Honeydew. Something above is drinking sap and excreting the sugar, and the sticky patch is the clue that finds it.
This one is worth acting on today rather than at the weekend. A week of waiting changes what is left to save.
What it usually is
1 Scale insects
| How to tell | Small brown or fawn bumps, two to five millimetres, along stems and leaf veins and on the underside of leaves. They do not look like insects and do not move, which is why they are missed. A fingernail lifts them off and there is a soft body underneath. |
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| The fix | Scrape them off with a fingernail or an old toothbrush, then wash the plant with soapy water. Repeat weekly for a month to catch the crawlers as they hatch. On a badly infested woody stem, surgical spirit on a cotton bud works as it does for mealybug. |
2 Mealybugs
| How to tell | White waxy fluff in leaf joints and under leaves, with the stickiness below it. See the entry on white fluff in the joints for the full routine. |
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| The fix | Dab each insect with surgical spirit on a cotton bud, wash the plant, and repeat weekly for four weeks. |
3 Aphids on soft new growth
| How to tell | Clusters of small green, black or pink insects on the newest shoots and flower buds, with the stickiness worst directly below them and often ants in attendance. |
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| The fix | Wash them off under a tap, which removes most of a colony, and repeat every few days until none return. Check the growing tips, which is where they always start. |
Through the year. Scale and mealybug are all year indoors and worst in warm rooms. Aphids arrive with soft spring growth and with plants brought in from outside.
Stickiness is never the problem itself. It is honeydew, the sugar water excreted by an insect that has been drinking sap, and it lands on whatever is below. So the first useful move is to look upwards: at the leaf above the sticky one, at the stem above that, and at the plant on the shelf above if there is one. The pest will be there.
Three insects account for nearly all of it indoors, and the shape of the culprit tells you which.
Scale is the one most often missed, because it does not look alive. Adult scales are immobile brown or fawn discs that sit along stems and leaf veins looking like part of the plant, and people sometimes spend months wiping honeydew off a windowsill without ever recognising them. Lift one with a fingernail: underneath is a soft body and a damp patch.
Mealybugs announce themselves with white wax in the joints, and have their own entry here.
Aphids gather on the newest, softest growth in visible clusters, and are the easiest of the three to deal with because they wash off. Ants marching up a stem are a strong hint: ants farm aphids for the honeydew and will defend them.
Left alone, the honeydew grows a black sooty mould. The mould does not attack the plant, but it coats the leaf and blocks the light, so a badly affected plant slows down. It washes off with soapy water once the insects are gone, and comes straight back if they are not.
The routine for all three is the same in shape: remove what you can physically, wash the plant thoroughly including the undersides, and repeat weekly for at least a month, because every one of them has a protected egg or crawler stage that a single treatment misses. Isolate the plant while you work, and check the two plants that were standing next to it, because by the time you noticed the stickiness they had been neighbours for weeks.
Entries that name it
What the journal says about this symptom on each plant.
| Cape leadwort | Cottony cushion scale, which excretes honeydew that then grows a black sooty mould. Spray thoroughly with horticultural oil to smother the scale. The sooty mould clears on its own once the scale is under control. |
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| Escallonia | Scale insects feeding on the sap and excreting honeydew, which then grows a sooty black mould on the leaf surface. Spray thoroughly with horticultural oil in spring or summer to smother the scale. The sooty mould is cosmetic and wears off on its own once the scale is controlled. |
| Ginseng ficus | Scale insects on the stems and undersides, which figs attract. Wipe the stems over with a damp cloth and check weekly. The sticky film is what the insects excrete rather than sap from the plant. |
| Golden dewdrop | Whitefly, which feed on the underside of the leaves and excrete honeydew. Check the leaf undersides regularly and treat early with insecticidal soap or horticultural oil. Improving airflow around a crowded planting makes reinfestation slower. |
| Laurustinus | Aphids, which rarely do serious harm but can leave a sticky honeydew that grows sooty mould. Tolerate a light infestation and let ladybirds and hoverfly larvae clear it. Squash heavy colonies by hand on a small plant rather than spraying. |
| Moth orchid | Scale or mealybugs on the leaf undersides and flower stems. Wipe every surface with a damp cloth, check the leaf joints, and treat weekly with an insecticidal soap until nothing new appears. |
| Nasturtium | Blackfly, which colonise the stems in early summer. Squash the first colonies and wash the rest off with water. Many gardeners plant nasturtiums near beans on purpose, because the blackfly go here first. |
| Pot marigold | Blackfly on the flower stems, drawn to soft growth. Rub them off with finger and thumb, or knock them off with a jet of water. Stop feeding, because soft growth is what brought them. |
| Rubber plant | Scale insects on the stems and leaf undersides. Scrape them off with a fingernail or a cloth, wipe every surface clean, and treat with an insecticidal soap weekly until nothing new appears. |
| Wax plant | Nectar dripping from the flowers, which is normal, or mealybugs, which are not. A flowering hoya drips sweet nectar and it is harmless. If there are no flowers, look in the leaf joints for mealybugs. |
| Weeping fig | Scale insects on the stems and leaf undersides. Wipe every surface clean, scrape the scales off, and treat weekly with an insecticidal soap until nothing new appears. |
Questions from the margin
- Why is my plant sticky?
- The stickiness is honeydew, the sugar water excreted by an insect drinking sap, and it lands on whatever is below. Look upwards from the sticky patch: scale, mealybug or aphids will be on the leaf or the stem above it.
- What is the black stuff growing on the sticky leaves?
- Sooty mould, growing on the honeydew rather than on the plant. It does no direct damage but it coats the leaf and blocks the light. It washes off with soapy water once the insects are gone, and returns if they are not.
Checked against RHS, Scale insects; RHS, Aphids.