I have some black looking bug type things on the leaves and branches of my tree. They are difficult to remove, I had to scrape them slightly but I can not find out what it is. The tree is approximately 3 years old and I have had two small crops it looks very healthy apart from some of the leaves that have been eaten by wallabys.
Myer Lemon Tree
Certified GKH Gardening Expert
This could be a type of scale, or mealybug. It would help to have a picture for identification. There are a few things that you can do to help smother them, though. Neem oil is a good way to do so, as long as you do not apply in direct sun.
This set of articles will give you more ways to control insect issues: https://www.gardeningknowhow.com/special/organic/