You thought that no houseplant pest could be worse than the common mealybug? That just goes to show that you haven’t yet had to deal with its underground cousin, the root mealybug (Rhizoecus spp.), also called soil mealybug. Though the average one is only about 1/16th of an inch (1.5 mm) long, that is, about one-third the size of a mature common mealybug, it packs quite a punch… and to make things worse, it lives underground, out of sight. In other words, it is even harder to spot than the common mealybug that lives on stems and leaves, because how many times a year do you take your plant out of its pot to examine its roots?
Root mealybugs can be found on almost any houseplant (pelargoniums, ferns, fuchsias, etc.), but they seem to be especially fond of African violets (Saintpaulia) and succulents, including cacti.
These insects are essentially houseplant pests in most areas and I’m treating them as such here. However, in tropical and subtropical climates, they can attack outdoor plants as well.
Root mealybugs are active all year round, but their life cycle speeds up in summer (about 1 month from egg to mature adult) and slows down in winter (up to 4 months from egg to mature adult).
Root mealybugs feed by piercing a tiny hole in the plant’s root with their buccal parts and suck up the sap that flows out. This weakens the plant and can eventually kill it.
Females deposit an egg sac covered in white powder here and there in the soil, often towards the outside of the rootball. The eggs hatch into crawlers, tiny nymphs essentially identical to the mother except for their smaller size. They are the most mobile stage and can easily spread to other plants. Crawlers seem especially active when you water plants and it is often when you are watering that they are washed out of one pot and move to neighboring plants. Crawlers also wander about on their own, mostly at night.
Unlike scale insects (another close cousin) that settle down when they reach adulthood, never to move again, root mealybugs remain mobile all their life. Root mealybugs are more likely to infest plants that share a tray than plants grown in individual saucers, as a saucer, though small, still remains somewhat of a barrier to insect movement.
Root mealybugs are often said to resemble a grain of rice and, as with all mealybugs, are covered with a waxy white cottony substance, giving them the “mealy” appearance their common name implies. Some species have no eyes (you may know those under the name blind mealybugs) and in fact, it’s hard to tell the front from the back!
At the beginning of a root mealybug infestation, there is no visible symptom unless you examine the soil. When the number of insects increases, the plant starts to show symptoms of general decline: nothing too specific, but it stops growing, becomes pale, stops blooming or loses leaves. Often plant owners figure the plant needs more light or fertilizer or is suffering from rot, but any improvements made to growing conditions have no effect. Only in the most advanced cases can you actually see root mealies above the soil line, as they sometimes congregate at the very base of the plant when the root system is completely infested.
Sometimes, when the root system is severely infested, you can see tiny bits “white fluff” in the saucer when you water, usually egg sacs or remains of egg sacs. That’s a sure sign an infestation is in progress.
Even when you unpot an infected plant to examine its root system, mealybugs are not that easy to spot, especially when the potting mix contains perlite (particles of expanded white rock), because the two are easily confused. However, if you look carefully, its cottony appearance and more symmetrical shape help the mealybug to stand out from irregular particle of perlite. Or squeeze the suspicious particle between your thumb and forefinger: it goes “crunch”, you’ll know it was perlite, if it goes “squish”… well, it used to be a mealybug!
Besides root mealies themselves, you’ll discover they are rather messy insects and leave clusters of pure white cottony wax here and there, white egg sacs and also powdery white wax on roots. Often you’ll see signs of their presence rather than the pests themselves.
The pot will have traces of them as well, in the form of small bluish-white spots on the inner wall. The marks on the pots, in fact, are often what give their presence away.
First, isolate the infested plant… and check immediately any other plant nearby. These mealybugs do get around!
Next, think seriously about tossing the plant: that remains the easiest and most effective treatment.
The second easiest treatment? Take cuttings from the aerial part of the plant (root mealybugs live strictly underground, so plant stems are therefore free of them)… then throw away the infested mother plant.
You can theoretically kill root mealies by slowly pouring an insecticide solution into the soil until saturation, but personally I’d be concerned that a few would survive and start a new infestation. After all, the soil will still be full of little white specks. How will you know the pest isn’t still alive and proliferating? Or that a few egg sacs haven’t survived?
Another possibility is to unpot the plant, thoroughly rinsing the roots to remove all soil. While you’re at it, prune off any dead roots. Now soak the roots that remain in a diluted solution of the insecticide of your choice for 15 minutes. Afterwards, repot in fresh potting soil and a clean pot. Consider adding diatomaceous earth, a biological insecticide, to the potting soil at a rate of about 1 tablespoon per liter of mix as a way to keeping them from coming back.
Under laboratory conditions, scientists have controlled root mealies by soaking the roots in hot water (120˚F/49°C) for 10 minutes, but it’s difficult to maintain a steady temperature under home conditions. If the temperature drops too much, some scale insects will survive; if it becomes too hot, that can kill the roots.
Before buying a plant (especially a succulent or cacti, as there seem to be a lot of root mealybug-infested succulents on the market these days!), ask the clerk to remove the plant from its pot so that you can examine the plant’s rootball… and the inside of its pot.
When you get home, put the new plant in quarantine for at least two months and even then, remove it from its pot and carefully examine its roots before placing it among your other houseplants.
True enough, root mealybugs can be sneaky, but at least you now know what to look for… forewarned is forearmed!