Yarb Tales - Scale Insects
January 11, 2016

The Heritage Herb Garden at the Ozark Folk Center graces the park with visual colors and textures, sweet and pungent aromas, and helps us to interpret the history of the human use of plants.


Scale is a pest that afflicts many evergreen and deciduous woody plants. We are currently treating bay, (Laurus nobilis), citrus trees, Schefflera, Gardenia, and tea shrub, (Camellia sinensis) to control this sap-sucking organism. All of these plants are indoors or in the greenhouse.

There are over 200 species of scale insects. They are all hard to see at first glance. The female adults look like hard or soft scabs on the bark, stems, leaves and fruit of plants. The bodies have no segments and usually no legs. (They often look like a small, oval-shaped, brown mound.) The adult males are small flying insects. The larvae are small and soft, are mobile, and have thread-like mouthparts.

The appearance of a black, sticky, sooty mold on plant surfaces usually accompanies scale infestation. The mold grows on the honeydew secreted by the pests as they feed. This black mold is usually the first noticeable sign of infestation.

Scale control is difficult because the eggs and first-stage larvae are microscopically small and often cluster under the females’ bodies. Adult females have hard, waxy exoskeletons that repel water. Though there are effective sprays that will kill the female adults and many crawling larvae, the eggs and larvae tucked under the wax shell of the female scale and in deep plant crevices will survive. Scales hide on all plant surfaces, as well as the container that they are in. Be sure to spray the pot, potting medium surface and all plant surfaces, especially the underside of the leaves and into bark crevices. Scrub empty pots that you wish to reuse with hot soapy water to improve the odds of preventing scale on future plants.

Before undertaking scale control it is good to get into the right frame of mind. Derive pleasure from the process of caring for your indoor plants. Turn on your favorite music. Place the specimen on a pedestal so that it can be turned and admired. Work in a well-lit space. Orient the plant so that you can work on it without straining your body. Use a magnifying glass to see what you are looking at. Practice patience, observe details and remember to breathe in the essential oils if you are working on an herb. This is quality time.  

Female adults can be scrubbed away with a soft brush or thumbnail and water. Lightweight horticultural oil and neem oil sprays are effective if used thoroughly and repeated on a weekly basis. The horticultural oil and neem coat the insects’ exoskeletons and suffocates them. Soap sprays soften the waxy coating of the insects. A combination of physical removal of scale and thorough spraying will work if practiced diligently.

Within a week of oil spray, we follow up with a soap spray (one tablespoon dish or Murphy’s Oil Soap™ combined with one gallon of water) followed by a strong water spray to dislodge the coating of sooty mold on leaves, rinse away scale adults and soap residue. Spray your plants in the shade, early in the morning or late in the afternoon so that the spray does not burn the leaves. Both soap and oil sprays can burn plant leaves.

Proper cultural conditions, including the right light, air circulation, timely transplanting, pruning and feeding, support the overall health of your plants. In addition to good, balanced fertilizers, be sure affected plants have sufficient, balanced quantities of calcium and iron in the mix. Vitamin C sprays may be beneficial at a rate of 1 to 3 teaspoons ascorbic acid per gallon. (The USDA published information online regarding research results of vitamin use on crops at http://www.ams.usda.gov/sites/default/files/media/Vitamins%20B1-C-E%20TR%202015.pdf.)

Scout for pests on a weekly basis as you enjoy the presence of plants in your life. If I don’t see you in the future, I’ll see you in the pasture.