Once established, scale insects can be difficult to control. Smother them with a solution of hortocultural oil.
Indoor plants stay healthier if they're kept clean.
So once a month, give them a shower with tepid water in your kitchen sink or bathtub. You'll remove dust, which can interfere with photosynthesis, and knock off certain insects. To remove fertilizer buildup in the form of excess salts, fill each container with water, and let it drain through. Repeat twice more. Repot every few years with fresh potting soil to provide nutrients.