What is the white fluffy stuff on my plant's stems and leaf joints?

The white wax is the whole problem. It is water-repellent, so sprays run off, and it hides the insect so effectively that people treat it as mould for months while the population grows. Once you know it is an animal, the treatment becomes obvious and mechanical.

Step by step

1. Confirm it is an insect rather than a mould.Prod one with a cocktail stick. Mealybug fluff separates to reveal a soft, oval, greyish-pink insect a few millimetres long with a segmented body. Mould has no body inside and smears rather than parting. Cuckoo spit is wet and bubbly, not dry and fibrous.
2. Check where it is sitting.Mealybug hides where it is hard to reach: leaf axils, the undersides of leaves along the veins, the growing tips, under the rim of a pot, and in the compost against the stem. Woolly aphid sits on the bark, particularly in old pruning cuts, cracks and around canker sites.
3. Look for the honeydew.Both produce sticky honeydew and both are usually accompanied by black sooty mould on leaves and surfaces below, and often by ants. A sticky houseplant with white blobs is almost certainly mealybug.
4. For mealybug indoors, isolate the plant immediately.Before treating anything. It spreads by walking between touching leaves, and one infested plant in a group becomes a group problem within weeks.
5. Dab each one with surgical spirit.A cotton bud dipped in rubbing alcohol dissolves the wax and kills the insect on contact. Go over the whole plant, into every axil, under every leaf, and around the top of the compost. Tedious, and by far the most reliable domestic treatment.
6. Check the roots for the underground form.Root mealybug lives on the roots and inside the pot as white powdery patches, and is a common reason a plant declines with nothing visible above the compost. Knock the plant out and look at the rootball and the inside wall of the pot.
7. Repot a badly affected plant.Wash the roots clean, discard the old compost, wash or replace the pot, and repot into fresh compost. This removes the root population and the eggs, which no surface treatment reaches.
8. For woolly aphid outdoors, scrub and hose.A stiff brush over the affected bark to break up the wax followed by a jet of water removes most of a colony. Do it in early summer when they become active. Prune out badly infested twiggy growth and bag it.
9. Deal with the ants and encourage predators.Grease bands on the trunk of a fruit tree stop ants protecting woolly aphid, and outdoors ladybirds, lacewings and a parasitic wasp all attack it. Once the ants are excluded, natural control is often sufficient.
10. Repeat every week for at least a month.Eggs hatch after treatment. This applies to both, and it is the step that separates a successful clearance from a recurrence.

Tips

• Mealybug is one of the hardest houseplant pests to eradicate. Assume you will be checking that plant every week for two months, and that a plant which recurs three times may not be worth keeping.
• Cacti and succulents are prone to mealybug and hide it well among spines and between the ribs. Check them thoroughly before buying and after buying.
• Woolly aphid on an apple tree causes lumpy swellings on the twigs that can crack and let canker in. That, rather than the fluff, is why it is worth reducing.
• Do not confuse woolly aphid on bark with the white bloom that appears naturally on some stems and on the leaves of plants such as brassicas.

Common mistakes

• Spraying water or an ordinary insecticide at the fluff — The wax repels it. The wax has to be broken mechanically or dissolved with alcohol before anything reaches the insect.
• Treating only the visible blobs — There are always more in the axils, under the pot rim, in the compost and on the roots. Partial treatment guarantees a recurrence.
• Leaving the plant among the others while treating it — Mealybug walks. Isolation is not optional and costs nothing.
• Assuming a declining plant with no visible pest is a watering problem — Root mealybug is invisible above the compost and produces exactly the symptoms of poor watering. Knock the plant out and look.

Questions people ask