Knocking Off Mealybugs and Scale Insects

Mealybugs and scale insects have made their annual winter appearance. They tend to lurk about on some plants year-round, and then, when conditions are just right, their populations explode.

These two related pests move slowly, or not at all, so they could easily be mistaken for plant parts or for bits of debris, especially in the early stages of an infestation. Mealybugs look like little tufts of white cotton and commonly congregate at joints between stems or leaves.

Scale insects look like little brown or whitish bumps on stems or leaves. One way to tell if a bump is a scale insect or part of a plant is to rub it off. The insect comes off cleanly, but rubbing a natural bump off a plant leaves a fresh green scar.

Rubbing is one way to get rid of these pests. This hand-to-hand combat is satisfying revenge, but infestations sometimes increase beyond where you can keep the pests in check by mere rubbing. Fortunately, other weapons are at our disposal.

One possibility is to enlist the help of another insect, a parasite with the ominous name of “mealybug destroyer” (Cryptolaemus montrouzieri, or “crypts” for short).

This species of ladybug does need warm temperatures, from 72 to 77 degrees, to thrive. Crypts also are not cheap and may even cost more than your infested plants are worth.

Crypts are not the only beasts that mealybugs and scale insects have to fear.

Tiny, parasitic wasps such as Aphytis melinus, Metaphycus helvolas and Leptomastix dactylopii are commercially available, as are larvae of the more familiar green lacewings. Any of these insects make welcome, or at least unobtrusive, house guests.

Reaching deeper into the arsenal against scale insects and mealybugs, we find a fungus and an insect growth regulator.

The fungus, also ominous-sounding (the C-3 strain of Cephalosporium lecanii but sold as Vertalec), is used commercially in greenhouses in Europe. Alas, this fungus thrives only where the humidity is very high and there is plenty of standing water. Neither condition exists in most homes, especially in winter.

The insect growth regulator upsets the development of the insect and is sold commercially as “Enstar.”

(Crypts and other beneficial insects are available from Beneficial Insectary, 14751 Oak Run Road, Oak Run, CA 96069; Biofac Inc., P.O. Box 87, Mathis, Texas 78368; and IPM Laboratories, Main Street, Locke, NY 13092.)

How about killing either pest with a plain old insecticide, such as pyrethrin or malathion? Insecticides can be effective but usually are so only during the brief period of time when the young scale insects or mealybugs crawl out from beneath their protective coverings to find a new home.

Toxic insecticides must be used with extreme caution, especially indoors, so many gardeners opt for gentler poisons, common household items such as alcohol, soap or oil. A cotton swab dipped in alcohol and then touched to the pests dissolves their waxy covering and kills them.

This method is more efficient than rubbing the insects to death with your fingers but still is too tedious for heavy infestations.

Then it’s time for sprays of insecticidal soap, available at most stores selling gardening supplies. As infestations on plants become increasingly out of control, advance from rubbing the pests off with your fingers to the alcohol swabs and on to soap sprays.

If the soap doesn’t work, try oil--”superior” or “horticultural” oil sold expressly for this purpose.

Most important in controlling scale insects and mealybugs are vigilance and diligence: vigilance to nip infestations early and to avoid spreading to uninfested plants, and diligence because most control measures must be repeated. Once the weather warms in the spring and you move your plants outdoors, environmental conditions and native parasites keep scale insects and mealybugs in check without your help.