Scale is an insect which is common on houseplants. Scale rarely kills a plant, but the honey dew excrement makes a sticky mess which attracts ants and even mold. If you notice sticky spots or ants gathering on surfaces below a plant, chances are the plant has scale. Look on stems and the bottoms of leaves for small, waxy brown bumps which are easily scraped off with a fingernail.
Since scale are part of the natural outdoor environment, houseplants moved outdoors for the summer often become infested. Many gardeners just assume that any plant which was outdoors for the summer is infested and treat for scale before bringing it indoors. A safe, organic way to kill scale and discourage them from returning is to spray the plant with a horticultural oil, neem oil, or insecticidal soap. Spray two applications ten days apart, making sure to follow the directions on the label.
If you know a houseplant has scale, isolate it from your other houseplants until you know it is no longer infected. Since scale insects live in soil, it might help to replace the soil in the pot. Be sure to thoroughly clean the pot before repotting the plant. Scale can be difficult to get rid of, so, unless the plant is very special to you, it is often better to just replace it than to risk infesting your other plants. Ferns, when left outdoors, almost always become infested and are nearly impossible to rid of scale.
For more information about scale on houseplants, go to http://www.ces.ncsu.edu/depts/ent/notes/O&T/flowers/note32/note32.html
Article written by Diane Puckett, Extension Master Gardener Volunteer.