1. Any plant heavily infested with scale is the Typhoid Mary in your collection. It will be a source of infection and needs to be isolated and treated if you are not prepared to simply bin it.
2. A single treatment is almost never enough. Everyone wants a one shot solution – it isn’t going to happen. Treat fortnightly until you are convinced the problem is eradicated.
3. Scale will stick to both the plant and the pot – even after you have killed them. Pick, scrape and brush them off. They like to hide in crevices and hard to get spots – be vigilant, turn the plant around and inspect from different angles and in good light. Check the pot also, scale will hide in the rim, the base and in the top layer of soil. If re-using a pot, it is essential you disinfect the pot (I put them in the dishwasher).
Scale are covered in a protective coat, either a hard protective cover (Hard Scale) or a soft waxy covering (Soft Scale). Both types of scale tend to stay put once they find a site to feed from. They hold tightly to the plant, and may need to be brushed or picked off, even after treatment.
Treat fortnightly with cotton buds dipped in rubbing alcohol and brushed over the scale, neem oil (diluted as per bottle instructions) or insecticidal soap sprayed onto the scale. These are all safe to use indoors. Be thorough and persistent. Check all the crevices, nooks and crannies. They are devious little buggers and have made hiding into an art form. If possible, trim off and bin any heavily infested leaves.
Pure rubbing alcohol (isopropyl alcohol) applied with a spray bottle is very effective against scale (and mealy bug) on succulents which are easily damaged by other methods of removal/ treatment.
Scale life cycle is 2 to 3 months, and you can have all stages on the one plant. The eggs are all but invisible and the emerging nymph is also tiny and hard to see, so you may need to treat for 3 months to get all the scale. Stop treatment once you are sure all scale has been eliminated. Check the rim and the base of the pot, plus any decorative cache pots and saucers. If the scale keep coming, then remove top 2cm of soil, disinfect exposed pot, and add in fresh potting mix.