Mealybugs tend to seem like white cottony masses, but if you look carefully, you can make out the individual pests. Source: http://www.biocontrol.co.za
Mealybugs are small gray or pink insects, but covered with fuzzy white wax and therefore look like balls of cotton wool. They pierce plant leaves and stems and feed on plant sap, weakening the plant, often causing deformed or yellowed leaves, reduced bloom, stunted growth and, in extreme cases, the death of the affected plant.
The two most common species found on houseplants are the citrus mealybug (Planococcus citri) and the long-tailed mealybug (Pseudococcus longispinus). (There is also a root mealybug [Rhizoecus spp.], but it’s very different in its habits and treatment. Read Root Mealybugs: Death From Below for more information on its control.)
Both aerial species are generalists and will attack almost any plant. Fortunately, they’re of tropical origin and won’t survive winter outdoors in temperate climates, but when it comes to houseplants coddled by warm air year round, there isn’t much to stop them.
Where Do Mealies Come From?
Discovering mealybugs on a plant is always a shock. How could they possibly have made it to your house? After all, female mealybugs can’t fly, and even the nymphs, the most mobile phase, can only crawl from one plant to another over short distances. You can almost end up believing it’s a case of spontaneous generation!
But it isn’t, of course, the culprit is …you! A mealybug infestation almost always follows the purchase of an infested plant. Of course, once you have one plant with mealybugs, it’s still usually you who transports them from one plant to another on infested hands, clothes or tools (pruning shears, watering cans, etc.).
That’s why the first thing to do to try and prevent a mealybug infestation is to always carefully inspect all plants before you bring them home. Especially if they’re on sale!
And the second thing is …to always put any newly purchased houseplant into quarantine to make sure it doesn’t harbor any enemies. You can set the plant in a room where there are no other plants or even in the same room, but far from others (again, mealybugs don’t fly). Personally, I seal most new arrivals inside a transparent plastic bag and put them with the other plants, since I don’t really have a place where there are no plants. After 40 days, check the plant carefully, especially the leaf axils and underneath the leaves. If no mealybugs are present, you can move it closer to other plants.
The third thing to do is always wash your hands with soapy water after handling any plant and also to sterilize your houseplant tools with isopropyl alcohol (rubbing alcohol) before moving to another plant. And when you water, never let the watering can touch the plant! (Been there, done that!)
Toss Infested Plants
Mealybugs are very difficult to control. In fact, nearly impossible. So, if you see one on a plant, the best advice is to …toss it into the trash (tropical climates) or compost (temperate climes) without delay! Then clean the spot where it sat with a soapy cloth, because often the female lays their eggs elsewhere, like in pot saucers and on windowsills.
Another possibility would be to cut the plant back severely, almost to the ground (presuming it’s a plant capable of growing back from the base), and to repot the stump into in a clean pot, replacing most of the old soil (either the potting soil or the old pot could be hiding eggs!). Now, wash the stub well with soapy water … and let it grow back.
In spite of this radical treatment, you still have to put what’s left of the plant into isolation and this time, not just 40 days, but at least 6 months!
For the Brave, the Persistent…And the Naive
You just can’t convince yourself to toss the plant or cut it back so severely? Well, there is another possibility.
You can try to control mealybugs with rubbing alcohol. It works, because because alcohol melts the insect’s waxy white covering, a protection against dry air, causing the creature to die of dehydration.
But touching each individual insect with a cotton swab dipped in alcohol as recommended on so many websites is just an exercise in frustration, because you’ll only be treating the adults and the fluffy egg clusters, both of which are white and fairly visible. The nymphs, tiny and yellowish, are rarely seen, not only because of their diminutive size, but because they tend to hide in leaf axils and clefts in stems. Thus, they survive the cotton swab treatments and then the infestation starts up all over again.
Instead, try spraying the entire plant with 70% rubbing alcohol. Or a solution of 1 cup (250 ml) of rubbing alcohol and 1 cup (250 ml) of water, adding a teaspoon (10 ml) of concentrated insecticidal soap so the solution sticks better. Apply in a well-aerated room and repeat every 3 days as needed.
If, after a month or two of regular sprays, mealybugs keep coming back, however, resign yourself to throwing the plant away. Sometimes that’s the only logical solution!