Root mealybugs: white wax on the roots of a plant in slow decline

Check your plant with the questions

A potted plant that turns pale, stalls and wilts, with nothing wrong that you can find on its leaves, may be feeding a colony underground. Root mealybugs, also called ground mealybugs, belong mostly to the genus Rhizoecus. They spend their lives in the potting mix, sucking sap from roots and the base of the stem. They are harder to find than their leaf-dwelling cousins, harder to get rid of, and they travel from pot to pot in drainage water and on hands and tools. Confirming them takes one step: getting the plant out of its pot.

Clues from outside the pot

A few signs show before you unpot anything:

• a thin white powder on the surface of the mix, most often near the stem
• small cottony tufts around the drainage holes
• pale insects that crawl out through those holes after watering
• ants busy on the mix at the base of the plant, a hint that some sap feeder is at work below

None of these is certain on its own. The root ball settles it.

What the roots look like

Ease the root ball partway out and look at its outer face, where roots press against the pot wall; that is where the insects gather most, though they also work deeper in. Expect patches of white, powdery wax that can tint the mix slightly blue. The insects are tiny: NC State puts adult females at one-sixteenth to one-eighth inch, and UC IPM allows up to three-sixteenths. Each is elongated, a little flattened, rounded at both ends and plainly segmented, bluish white beneath a thin dusting of wax. They have short antennae and barely any eyes, and they crawl slowly. Unlike the mealybugs on stems and leaves, they have no fringe of wax filaments. Eggs lie in small nests of white fluff; some species give birth to live crawlers instead, and eggs that are laid tend to hatch within a day.

What else it could be

• Springtails. An NC State entomologist compares root mealybugs to large white springtails, but notes that the mealybugs move far more slowly.
• Foliage mealybugs taking shelter. A few above-ground species retreat to the roots when conditions turn poor and climb back up once the plant is growing well. If you also find cotton in the leaf joints, read the page on mealybugs.
• A different root problem. Paleness, wilting and leaf loss have many causes. Only the wax and the insects themselves identify this pest.
• Something growing on the mix. If the white on the surface looks like a growth rather than a powder, and the roots are clean, see the page on mold on soil. Small dark flies around the pot are a separate matter: fungus gnats.

The harm they cause

By feeding on fine roots and root hairs, the insects cut into what the plant can take up. Growth slows, foliage fades to pale green or yellow and wilts, and leaves may fall early. African violets and other flowering plants bloom less. Left alone, the whole plant declines. UC IPM notes that these mealybugs usually escape notice until someone exposes the roots, so the decline above ground is often the first thing seen.

Among houseplants, African violets and gardenias are named most often. NC State adds hibiscus, ginger and a long run of palms, among them kentia, lady, fishtail, majesty and MacArthur palms.

What to do

Extension services are frank about this one: root mealybugs are more stubborn than the kinds on foliage, and throwing out the plant is frequently the easiest way out.

1. Separate the plant and inspect its neighbors. Move it, with its saucer, away from other pots. Unpot any plant that stood beside it or shared a tray, because UC IPM treats nearby plants as likely infested too.
2. Decide whether to keep it. Freeing a root system of every insect is difficult. A new, clean plant in fresh mix removes the doubt.
3. Clean what it touched. Discard the old potting mix. Disinfect or scald the pot, saucer, tools and the shelf it stood on, since crawlers can linger on any of them. Use new, packaged mix for repotting, and keep water that drains from the affected pot away from other plants.
4. If you keep it, use a systemic insecticide. NC State finds these mealybugs susceptible to systemics such as imidacloprid, applied as a soil drench or in other labeled forms, provided the label covers residential use. Some are labeled for outdoor landscape use only; in that case NC State suggests treating the plant outdoors, in shade so tender leaves do not scorch. Follow the label.
5. Deal with ants. They carry mealybugs between plants as well as guarding them.

Nurseries also kill root mealybugs by immersing pots in hot water held at a precise temperature. Research on potted lady palms in Hawaii found it cleared them without serious harm to the plants, but UC IPM recommends trying any heat treatment on a few plants first, and it depends on measuring the temperature inside the root ball.

Checking whether it worked

Root mealybugs are slow: UC IPM gives two to four months from egg to adult at typical soil temperatures, and adults live one to two months. Judge results over that span, not days. Slide the plant out every few weeks and look at the outer roots, where colonies reappear first. Fresh wax at the drainage holes or on the surface means they are still there. Keep checking the plants that stood nearby, and inspect the roots of any new plant before it joins the others.

Plants whose sources name root mealybugs

Fourteen plants here are tied to root mealybugs by their source page: NC State Extension for twelve of them, the RHS for two. Most are palms, which fits NC State's own host list.

• Areca palm (NC State Extension)
• Bamboo Palm (NC State Extension)
• Canary Date Palm (NC State Extension)
• Christmas Palm (NC State Extension)
• Desert Rose (NC State Extension)
• Fishtail Palm (NC State Extension)
• Gloxinia (NC State Extension)
• Gold Dust Dracaena (NC State Extension)
• Ivy Geranium (RHS)
• Kentia palm (NC State Extension)
• Lady palm (NC State Extension)
• Madagascar Jasmine (RHS)
• Majestic Palm (NC State Extension)
• Parlor palm (NC State Extension)

When a plant is failing and the roots look clean, work through the diagnosis to narrow down the cause.

Answer a few questions about your plant