An insect is attacking our mandarin tree. It appears to be scale. A. Scale insects have piercing mouthparts that suck juices out of the plant, says UC master gardener Bill Pierce. Severe infestations can cause overall decline and even death of a plant.
Control is often accomplished by natural enemies, so avoid spraying broad-spectrum insecticides.
In situations when natural enemies are not successful, spray with specially refined oils, often called narrow-range, supreme or superior oils. Mix oil and follow application directions as stated on the label. Water the plant before you spray, and do not spray if the temperature is above 90 degrees. I'd like something that stays compact and doesn't require too much care. The spot has a western exposure with several hours of afternoon sun.