Q: My Meyer lemon tree just contracted these yucky little bugs. I did not have them last year. I scraped them all off by hand. What are they and how do ensure they don’t return?
A: They are scale insects. They suck sap from the underside of leaves and excrete a sticky syrup that coats nearby leaves.
Black sooty mold grows on the syrup. I see the black mold on leaves below your scale insects.
The easiest way to control them is spraying under the leaves with neem oil (click for sources), which suffocates them.
Now that you know what they look like, inspect your leaves every month to see if any survive. Treat as above.