What are the tiny white insects on my plant's roots?

This is the diagnosis that gets missed for a whole season. The plant looks thirsty however much it is watered, it stops growing, the leaves pale and it eventually dies, and every visible check comes back clean because the pest is under the compost. Two minutes with the pot off settles it.

Step by step

1. Take the plant out of its pot.Over a sheet of newspaper, so anything that falls off is visible. Do it for any container plant that is declining with no explanation, and particularly one that wilts despite moist compost.
2. Look for white powder on the roots.Root mealybug appears as white waxy powder or small cottony patches on the roots and, very characteristically, on the inside wall of the pot. Within it are soft pale insects a few millimetres long. It is common on cacti, succulents, pelargoniums and fuchsias.
3. Look for root aphid.Small pale grey, yellow or bluish aphids clustered on the roots, often dusted with a fine white waxy bloom, and moving when disturbed. Lettuce, chicory, primula, auricula and many container plants are affected, and it is worse in dry compost.
4. Distinguish them from the things that are harmless.Springtails are tiny and jump, and they feed on decaying matter, not roots. White worms are harmless decomposers. Fine white threads through the compost may be mycelium doing useful work. Fat white C-shaped grubs are vine weevil, which is a different and more destructive problem.
5. Wash the roots clean.Remove all the compost, teasing and rinsing under a tap until the roots are bare and clean. This physically removes most of the population and is the core of the treatment.
6. Discard the compost and clean the pot.Do not reuse either. Wash the pot in hot soapy water and scrub the inside wall, because that is where root mealybug wax and eggs sit. A clean pot matters as much as clean roots.
7. Repot into fresh compost, and do not feed straight away.A plant that has just had its roots washed has lost feeding roots and needs a month to rebuild. Water it in, keep it out of direct sun, reduce the top growth a little if a lot of root came away, and hold off fertiliser.
8. Treat every pot in the group.Both spread between pots standing together, along benches and through drainage holes. Check the neighbours at the same time or you will be repeating this in a month.
9. Change the watering.Both are far worse in compost that dries out repeatedly. Consistent moisture, a compost that holds water properly, and pots that are not standing dry for long periods make a real difference.
10. Use a soil drench of nematodes if it recurs.Nematodes sold for vine weevil also have some effect on other soil-dwelling pests, and a proper drench of a plant group is worth trying where the problem returns. They need moist compost above about 5°C and a fresh, in-date product.

Tips

• Any container plant that wilts in moist compost should be knocked out of its pot before anything else is tried. That single habit catches root aphid, root mealybug, vine weevil and root rot.
• Check the roots of every new plant before it joins a collection. Root mealybug travels almost exclusively on bought plants.
• Cacti and succulent collections are the classic setting for root mealybug, because plants are repotted rarely and pots are reused.
• A dry-loving plant with root pests may show nothing but slower growth for a year. Repot succulents periodically and look at the roots while you do.

Common mistakes

• Watering more because the plant wilts — The roots are damaged and cannot take it up. More water in the same compost accelerates rot on top of the pest problem.
• Repotting into the same pot without cleaning it — Root mealybug wax and eggs are on the inside of the pot. A clean plant in a dirty pot is reinfested immediately.
• Feeding to revive it — Damaged roots cannot use it and concentrated feed scorches what is left.
• Treating one plant and leaving the tray of neighbours — Both spread along a bench and through drainage. Treat the group.

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