Scale are flat, tiny, reddish-brown insects that suck plant juices, leaving a sticky coating on leaves. Adult scale insects develop a hard, round shell which can make them difficult to kill.
Most scale are found attached to the stems and the backs of leaves, usually along central leaf veins. You may notice the affected areas to be weak and turn yellow. Scale are especially a nuisance to ficus and ferns.
Honeydew is excreted as these insects suck on plant juices. This sticky substance can be found on leaves, and also the floor or table beneath the plant.
Young scale insects can usually be controlled with a spray of soapy water. Use mild dishwashing liquid that doesn't contain fragrance or other additives. Mix 2 teaspoonfuls with 1 gallon of room-temperature water. Use a spray bottle to spray the soapy water and coat the leaves and stems thoroughly. Spray once a week for at least a month or until you no longer find insects. Always rinse off the soapy solution thoroughly with room-temperature water.
Adult scale can be more difficult to remove. If spraying doesn't work, use an old toothbrush dipped in soapy water to scrub them off. Heavily infested areas should be pruned.
Rubbing alcohol kills many house plant insects, including scale. Dab them with a cotton swab soaked in rubbing alcohol. Repeat applications every 2 to 3 days. This works well, but use it with caution. Covering the whole leaf with alcohol damages plant tissue.
If the infestation is bad, use an Insecticidal Soap made for indoor plants. Make sure your plant is listed on the product label. Read the label carefully and follow the manufacturer's directions for use.