Mealybugs: The White Fluff That Keeps Coming Back

Mealybugs look like tiny scraps of cotton wool tucked into the crevices of a plant. They are soft scale insects, and the white fluff is a waxy coating they secrete that repels water and, importantly, most sprays.

That coating is why casual treatment fails. You have to get through it or remove them physically.

Where to look, which is the whole problem

People check the leaves. Mealybugs mostly are not on the leaves.

• Leaf axils, the tight V where a leaf stalk meets the stem. Their favourite spot.
• The underside of the crown on rosette plants and succulents.
• New growth and unfurling leaves, which are soft and easy to feed on.
• Under the pot rim and in the drainage holes.
• Among the roots. Root mealybugs are a separate problem that looks like white mould on the root ball, and they are the reason a plant keeps declining after you cleaned the visible ones.

If a plant keeps deteriorating after treatment and you cannot find anything above soil, tip it out of the pot and look at the roots. This step is skipped almost universally.

What they do

They pierce the plant and drink sap. Symptoms are yellowing, stunted or distorted new growth, and sticky honeydew on leaves and on whatever the plant sits on. Black sooty mould grows on the honeydew, which is unsightly but harmless in itself.

A light infestation is cosmetic. A heavy one will kill a plant, and they spread readily to neighbours.

Treatment that works

1. Isolate the plant. Another room, not another shelf.Day 0
2. Dab every visible mealybug with a cotton bud dipped in rubbing alcohol. It dissolves the waxy coat and kills on contact. This is the most effective single action and it is worth being thorough.Day 0
3. Rinse the whole plant in the shower, undersides and joints included, to remove crawlers you did not see.Day 0
4. Spray with insecticidal soap or neem, covering every surface until it drips, especially axils and the crown.Day 0
5. Repeat the alcohol check and the spray every five to seven days, at least three times. Nothing you can safely use kills the eggs, so you are catching each generation as it hatches.Days 5 to 21
6. If it keeps returning, unpot and check the roots. Root mealybugs need the soil replaced entirely, roots rinsed clean, and a fresh pot.If it recurs
7. Keep it isolated for two weeks after the last sighting, checking twice a week.Weeks 3 to 5

Wipe down the shelf, the pot and the saucer too. They wander, and a clean plant put back on a dirty shelf gets reinfested.

Why yours came back

Four usual reasons, in order.

You stopped after one round. Eggs hatch over the following fortnight. One treatment delays the problem, it does not end it.

You missed the crown or the axils. These are physically hard to reach and a spray that hits the leaf surface does nothing there.

They are in the roots. Invisible above soil, and no amount of leaf treatment touches them.

A neighbouring plant is a reservoir. If one plant in a group has them, treat the whole group.

Prevention

Quarantine every new plant for two weeks, checking axils and the crown twice in that time. Almost every infestation arrives on a plant somebody just bought.

Inspect when you water. Turn one leaf, check one growing tip, look into one axil. Mealybugs at week one are a cotton bud and five minutes. At week six they are a project.

Plants that are stressed, particularly by overwatering or being pot bound, get hit harder, so general good care is genuinely preventative here.

If you cannot tell whether the white specks are mealybugs, scale, or just mineral deposits from hard water, photograph the affected area close up and Identium will diagnose it. The treatments differ, and scale in particular needs a different approach because of its hard shell.