The most efficacious way to eliminate a significant scale infestation would be to use a systemic insecticide mixed with water. For a few plants that can't be sprayed, a soil drench using Imidacloprid will do the trick. It comes by the quart or gallon and is packaged sometimes as a 1.47% solution spray solution and sometimes as a 1.47% soil drench. Spraying works better on woody plants, but be sure to wet the entire plant.
This product can be used as a soil drench or spray:
https://bonide.com/product/ann...
There is also a product, made to be sprayed, that has a systemic insecticide (Imidacloprid), a systemic fungicide (Tebuconazole), and a miticide (Tau-valfluvinate). It's called BioAdvanced 3-in-1 for Insects Disease and Mites. I use the product immediately below primarily in fall before I bring my plants in for the winter. It controls nearly all insects with rasping/sucking mouth parts for the entire winter (indoors).
https://www.domyown.com/bio-ad...
If I do have a major outbreak, I treat twice at a 2 week interval.
A plant's best defense is a high level of vitality. The better the metabolic machinery is operating, the more natural defense chemicals a plant synthesizes. Insects have an amazing ability to detect stressed plants, and will fly by hundreds of healthy plants until they detect one that's vulnerable. Most often, plants that are suffering insect herbivory were already struggling from stress related to one or more conditions related to the cultural hand it's been dealt, often a problem in the root zone, but on the list are too much/ too little of water, nutrients, light, heat, ..... even air pollution can take a heavy toll on many plants.
Al