GARDENING : Weighing Options to Rid Plants of Scale Insects
Keep on the lookout for small, white, cottony tufts on houseplants. These tufts, which might appear on plants as different as gardenia and jade, are not some mutant flower common to plants all over the world. These white creatures are “scale insects.”
Scale insects are of two types. One type, the armored scales, never have that cottony look and stay in place beneath a protective, waxy shield. The other type, which is cottony, are the naked scales, and include mealybugs and cottony cushion scale. The “cotton” is composed of wax filaments.
A scale insect eats by sticking its hollow, pointed stylet into a plant, then sucking up plant juice. Plants infested with scale are weakened, often to the point of death.
While feeding, the insects excrete a sweet, sticky liquid called honeydew. On the plant, the honeydew is food for a fungus that grows as a sooty coating on the plant. The fungus does no direct harm to the plant, but it is unsightly and can become severe enough to actually shade a plant from light.
Scale insects are easily kept in check, but hard to permanently eradicate. Using an insecticide is one recourse. Another--often justifiable to prevent spread from one plant to another--is to throw out an infested plant. At the very least, isolate infested plants from other plants.
There are less aggressive controls than chemical poisons or plant sacrifice. A sure but tedious method for killing scale insects is to dab them with a cotton swab dipped in rubbing alcohol. This method is 100% effective, as long as you are diligent.
Hot water is another way to kill these pests. Immerse the leaves and stems of a plant in water at 140 to 150 degrees Fahrenheit for five to 10 minutes. This treatment should leave the plant unharmed, but just to make sure, first test a single leaf.
You can also evict scale insects with a stream of water. The insects are just barely mobile so it is unlikely that they will clamber back up the stems once dislodged. A dousing with soapy water is even more effective in penetrating the insects’ waxy covering.
“Safer’s Insecticidal Soap” is a relatively benign insecticide that is similar to plain soap and water. Spray it on a plant, then, after about 15 minutes, wash the plant off in the kitchen sink with a spray of plain water. Again, first test soapy water or insecticidal soap on one leaf, because either material can damage some plants.
Biological control is another possibility. The Mealybug Destroyer ladybug has been imported from Australia to control scale in California citrus groves.
If the thought of deliberately bringing insects into your home is repulsive, how about a fungal disease? The fungus Beauvaria globulifera is not available commercially, but isn’t it reassuring to know that even scales have diseases to keep their numbers in check?
Diligence is needed to control scale. Although any of the above methods are effective against adults, eggs may be unaffected. Repeated control is needed to knock down each new group of hatchlings and any adults that managed to escape control.
