Mealybug

What you are looking at

Blobs of white fluff, two or three millimetres across, pushed into the angle where a leaf meets a stem. They look like specks of cotton wool or a patch of mould, and most people treat them as one of the two for several weeks before finding out otherwise.

Pull the fluff apart with a pin. Underneath is a soft, flat-oval insect, pale pink or grey, plainly segmented across the back, with a fringe of short waxy filaments round the edge and sometimes a pair of longer ones at the tail. Prod it and it moves, slowly. A scale insect in the same position would not.

The wax is the animal’s own secretion and it is the reason for everything that follows. It is water-repellent, so a spray runs off it, and it forms a sealed sac around the eggs.

Where they hide

They sit wherever two surfaces meet and the air is still. Leaf axils and stem forks first, then the underside of a leaf along the midrib, the crown of a cactus or a succulent, the sheaths of an orchid, the underside of the pot rim and the drainage holes. A plant that looks clean from three feet away can be carrying two hundred insects in places nobody looks.

One group has given up on the foliage entirely. Root mealybugs live in the compost, feeding on the roots, and produce white powdery deposits against the inside of the pot. The plant sulks, stops growing and slowly declines, and because nothing shows above the surface the owner usually treats it for something else for a year.

Why they keep coming back

A female produces one to two hundred eggs inside a waxy sac, and the crawlers that hatch from it are small, mobile and pale enough to be invisible against a leaf. Indoors, with no cold season to interrupt them, generations overlap continuously and there is never a point at which the population is only adults.

That is why a single treatment fails. Whatever you apply reaches the insects you can see and nothing else, and ten days later the eggs hatch. Anyone who cleans a hoya thoroughly, declares it dealt with and finds fluff again a fortnight later has met the arithmetic rather than made a mistake.

Four rounds, a week apart, is the shape of a treatment that works.

What the damage amounts to

They feed on phloem sap and a light infestation costs an established plant almost nothing. What builds up is chronic. Growth slows, new leaves come through small and puckered, older leaves yellow and drop, and after a year the plant has stopped looking like itself.

Honeydew is the visible nuisance. It falls onto the leaves below and onto the furniture, black sooty mould grows on it, and the mould blocks light without infecting anything. Wash it off with plain water once the insects above it have gone.

Small plants, seedlings and cuttings can be killed outright. So can anything carrying root mealybug, which removes the root system while the leaves are still telling you the plant is merely unhappy.

What to do about them

Get a cotton bud and a bottle of surgical spirit. Touch each insect directly. The alcohol dissolves the wax on contact and the animal dies where it sits, and you can see that it has worked because the white blob collapses to a wet smear. It takes ten minutes on a hoya and it is more reliable than anything sold in a bottle.

Work through every crevice while you are there, and go back weekly for a month. Wash a firm-leaved plant down in the shower between rounds to take off the honeydew and any crawlers.

Where a whole shoot is coated, cut it out. On a fast-growing plant this costs a few weeks and removes a source that would otherwise reinfest everything you have just cleaned.

When you repot, look at the roots. If they carry white powder, wash every trace of the old compost off under a tap, scrub or replace the pot, and start again in fresh compost. Nothing applied to the leaves gets anywhere near a root mealybug.

Be willing to give up on one plant to save the rest. A heavily infested specimen with mealybugs in every crevice sitting on a shelf of clean plants is a decision rather than a patient.

Why no spray is listed here

The wax coating repels water, so a water-based spray beads off the insect and runs down the leaf. Getting a spray to work at all means wetting it to the point of running, into crevices a droplet cannot reach, on the underside of every leaf, repeatedly. That is a poor use of a product that also kills anything else soft-bodied it lands on.

Fatty acid and plant oil products work by smothering, which means the same coverage problem with the same result, and they will kill introduced Cryptolaemus larvae as readily as the pest. A cotton bud does not.

What it gets mistaken for

Woolly aphid produces similar white fluff but sits on woody stems and branch cracks of apples, pyracantha and cotoneaster outdoors rather than in the leaf axils of a houseplant, and the insects beneath it are aphids with cornicles.

Scale insects are immobile brown or yellowish domes stuck to a stem or a vein, with no wax fringe, and they do not move at all when pushed.

Powdery mildew is a flat film across the surface of a leaf blade rather than a raised blob in a joint, and it wipes off in a clear track rather than smearing.

The white flecks in an aphid colony are shed skins, and they blow away when you breathe on them.

A white crust on the surface of the compost or the outside of a terracotta pot is mineral salts from hard water and fertiliser. It is chalky rather than fluffy, and there is nothing under it.