Soil mealybugs on African violets — Laidback Gardener
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Soil mealybugs on African violets
Root Mealybugs: Death From Below
Root mealybugs. Some species, like the one above, lack eyes.
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 1/16 th 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!
Here you see the results of a root mealybug infestation: a few adults, lots of dead white skin, egg sacs and “mealybug powder”.
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.
White markings with a bluish tinge to them show this pot is hosting soil mealybugs.
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.
You can save your plant by taking a cutting. Here a cactus cutting is being allowed to callus over before being potted up.
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.
Make sure you inspect the root ball before you buy a plant.
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!
Solutions for controlling mealybugs on African violets: Ask OSU Extension
By Steven J. Hudkins, OSU Extension
As we move from the outdoor planting and maintenance season to the indoor care of plants and the change in environmental conditions, there are always a number of questions about house- plants. This week, we will be addressing some of the problems that occur during the winter growing season.
Please tell me what disease looks like small bits of cotton on my African violets and what to do for it.
These are mealybugs, sucking insect pests, not a disease. Mealybugs are soft insects that have whitish, cottony or mealy substance around them. They often leave bits of this material on plant or leaf stems when they move around. Mealybugs on an African violet can be controlled fairly easily by one of three methods:
1. Use one of the ready-made houseplant insect controls labeled for mealybug control for African violets. Use the spray, which may be aerosol or finger pump, as recommended. Read all label directions and precautions. Repeat applications are usually suggested.
2. Use a cotton-tipped swab dipped in rubbing alcohol to gently dab away the pests. This may be tedious, but it is effective. Repeat treatments like this are usually necessary.
3. Use a mixture of 1 cup rubbing alcohol, 1 teaspoon Volck oil and 1 quart water in a plant mister to gently mist the African violet portions where the mealybugs are present. The alcohol will dry out the bugs.
This should be done when the violets are not in bright light. Alcohol misting may have to be repeated every seven to 10 days for complete control. Do not use straight alcohol as that may damage the plant.
Actually, a combination of the second and third methods should work well together to eliminate the mealybug infestation on an African violet. Mealybugs are among the insect pests on indoor plants that may reappear in several weeks after an infestation has been eliminated. Thus, it will be important to watch the affected plants carefully in the coming weeks.
How can I tell if my houseplants are getting the right amount of light?
One of the most common signs of inadequate light is yellow and dropping leaves. That is also one of the most common signs of too much light. To figure out if either of these situations is your problem, you will have to look more carefully at your plants.
Flowering plants that don’t flower, any plants with leaves angled toward the light and long, spindly stems, plants such as coleus whose red pigments have faded, or plants whose new leaves are smaller are all showing signs of inadequate light.
Either move these plants to a brighter spot or supplement natural light with artificial light. Foliage plants require nothing more than an inexpensive, cool, white fluorescent lamp; combine a cool white lamp with a warm red lamp to ensure the best bloom in flowering plants. When your plant’s leaves show signs of overall yellowing or have large spots of dry, dead tissue known as leaf scorch or leaf burn, the problem may be too much light. Use a diffuser curtain to reduce the amount of direct sunlight hitting the plant, or move it farther away from the window.
Even if the light level is correct for a given plant, the same symptoms may develop if it was moved straight from a low light intensity to a much brighter spot. Make such moves gradually to allow the plant to adjust to the new light levels.
Call a master gardener for gardening advice: 10 a.m. to 1 p.m. Mondays and Thursdays at 216-429-8235. Gardening information is also available at cuyahoga.osu.edu and webgarden.osu.edu. Write Master Gardeners at [email protected] for answers to questions any time.
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