The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: I am wondering what causes scale on house plants. The plants I have that get scale are not near other plants. Could it be possible that the scale can come from potting soil? Only certain plants get scale, and others are not affected. What, other than washing or using alcohol, can be done to keep scale away?
Answer: There are a number of scale insects that infest house plants. Some scales prefer certain plant species. When young scales (crawlers) hatch from eggs laid by the female, they will crawl about to find a suitable place to feed. This suitable place may be the same plant they were hatched on or on adjacent plants. Once at these sites, they will settle down and begin feeding. The primary way scales get to new plants is by walking across leaves and table surfaces to new plants. They can also be spread if they get on your clothing. Males are capable of developing wings, and thus are able to fly to other plants that have crawlers which matured into females. At which time mating occurs and the infestation grows. Soil mixture contamination is very unlikely. Keep up the good work in controlling the scales with washing and alcohol. An ultrafine oil 2% works well against settled crawlers (it is preferred in the home). Read and follow all label directions when using insecticides.
-Corey Gerber & Cliff Sadof