Black scale (Saissetia oleae) is a small parasitic insect, only .2 inch long, that attacks mainly citrus and olive trees in Florida, California and throughout the Gulf of Mexico. The insect is dark colored, and many people often miss identifying it because it can look like a natural deformity of plant leaves and stems. Scale can be present during all seasons, and they excrete a sticky substance called honeydew, which causes sooty mold on the plant. This fungal disease makes it hard for the plant to photosynthesize, resulting in poor health and general ugliness. Treating black scale when you first notice them is key to getting rid of this destructive insect.
Remove any ants you notice on your citrus or olive tree and any other plants. Ants do not directly harm your plant, but they transport insects such as black scale. When you notice ants, blast them with a sharp stream of water to knock them off the tree, then smear a thick layer of an organic product called Tree Tanglefoot around the lower trunk---the ants will be unable to crawl over it. Alternatively, you can insert ant stakes in the soil around your tree.
Spray your tree with insecticidal soap, or make your own by combining 1 tablespoon of mild dish-washing soap with 1 quart of water. This will help kill young black scale insects that have soft, permeable shells and are in the "crawler" stage.
Mix 1 tablespoon of canola oil or a horticultural oil with your insecticidal soap and spray your tree again. The oil will coat older, armored insects and make it impossible for them to breathe.
Introduce beneficial predator insects such as the purple scale predator to your affected tree. This insect does not harm plants, but eats black scale, mealybugs and the eggs of other insects.
Treat severely infested trees in mid-July after eggs hatch with a pesticide such as Sevin mixed with a narrow range oil. Spray pesticide products in the early morning or at night, especially when daytime temperatures rise above 90 degrees Fahrenheit.