Intro to Barnacles / Scales / Wax Scale
These gross bugs hang out on your stems and under your leaves. They suck out the life of stems and leaves, which can start to weaken your plant if it gets too bad. They release a sweet sticky substance known as "honeydew" (very similar to an aphid) which can cause dark patches of mold known as sooty mold to start appearing on your plant. The honeydew can also attract ants.
Solution to Scale Insects
The way to treat barncales or scale insects is almost exactly the same as what you would do for aphids.
1.) Remove all barnacles / scales with a power sprayer
The first step to treating this insect infestation is to remove all the bugs from your plant. The easiest way to do that, in my experience, is to use a "One-Hand Pressure Sprayer" to physically spray off all the bugs. You can also remove heavily infected parts of the plant, and you can rub them off, but I've found you can remove 99% of them with a power sprayer. To double the effectiveness, use one of the insecticides below while spraying, so you're getting rid of them and treating the plant against future attacks at the same time.
2.) Insecticidal soaps
Fatty acid salts or insecticidal soaps can be a good choice against barncales and wax scale insects. They weaken the outer shell of scales but are safe to use on your plants and they don't leave much of a residue.
With soaps, coverage is very important as it does not stay on your plant for long, so follow-up applications may be necessary. Although this is considered safe, avoid getting any on your buds!
3.) Neem Oil
Neem Oil will leave an unpleasant taste/smell on buds when used to treat flowering plants, so again, don't let this stuff get near your buds! There's also some evidence Neem oil may be harmful to humans so use with care! That being said, Neem oil is an all-natural remedy that is very effective against many different types of bugs and mold including scales. You will need a mister (also called a "One-Hand Pressure Sprayer") to spray all the leaves evenly, since neem oil and water can separate easily.
Spinosad Products (safe & organic) - Spinosad products are organic and completely harmless to pets, children, and plants. Spinosad products can be used directly to kill scales on contact and should be sprayed liberally anywhere you see scales and especially under the leaves. Although maybe not as strong against pests as some of the more harsh insecticides, it does work and it's very safe for plants, animals and humans!
Recommended: Monterey Garden Insect Spray with Spinosad
Spinosad is an organic insecticide made from the fermentation of a specific soil bacteria (actinomycete Saccharopolyspora spinosa) and kills scales via ingestion or contact by effecting the insect's nervous system. Spinosad can be a good choice for organic and outdoor growers, because it is very toxic to barnacles and scales, but is less toxic to many beneficial insects and spiders.
Note: Most spinosad products are effective for only about 24 hours after being mixed with water, so only mix as much as you will need per application. Anything left over will be waste.
You will need a mister (also called a "One-Hand Pressure Sprayer") to cover all the leaves evenly when spraying them with spinosad products.
5.) Essentria IC3
Essentria IC3 Insecticide is a mix of various horticultural oils that is organic and safe for humans. It is often marketed as a "bed bug killer" but it can be effective against broad mites or russet mites when the plants are treated regularly. Unfortunately it only stays effective on the plant for about 8 hours so you will want to either apply this daily or combine with other options. You will need a mister (also called a "One-Hand Pressure Sprayer") to spray all the leaves evenly.
6.) Beneficial Insects
Beneficial insects, such as lady beetles, lady bugs, and lacewings may eat large numbers of scales and are welcome guests in the garden. Although you can order ladybugs to release around your plants, they tend to fly away in just a day or two. Additionally, the reproductive capability of scales is so great that the impact of the natural enemies may not be enough keep scales at or below acceptable levels after an infestation has already gotten started. But they definitely don't hurt!