This is the worst trouble I have. Once they come, I have no way to avoid. It may be the best to use exterminators, but they recovers its number if an insect survives and acquires medicine tolerance. There are two insects in this photograph: a big one at the center, and a little one at the right of the big one. Touching it, it moves and we recognize that it lives! So check carefully plants when we buy plants with flower spikes, scale insects are hiding in the flower spikes. Most cases they prosper inside of base of outer most leaves. In case many insects live, outer leaves tend to die faster. This can be a sign of scale insects and, by removing outer leaves, we face a fact that so many fat insects at the base! They live also on roots, flower spikes, and flowers. If you find a insect, definitely there are more somewhere in the foliage. Scale insects leave white encrustation "scale" on the surface of leaves. These encrustation might be a crawling traces or eggs, so I try to wipe out by tissue-papers by finding them. Scale insects love no trichomes leaves and hate thin leaves which are rich in trichomes. This is probably because it is hard for them to suck sap. Some plants with wide leaves with trichomes are good target for scale insects. Please refer to this site to distinguish which plants they love. Once I used exterminator for scale insects, but it worked also to plant itself and the plant is almost dying. I have heard that diluted vinegar is good exterminator, but I am afraid of further troubles. After all, I smash them one by one by hands.