Control scale insects on Plumeria by using horticultural oils or insecticidal soaps, applying them directly to the pests. These products suffocate and kill scale insects. For severe infestations, use systemic insecticides labeled for scale control. Regularly inspect the plant for signs of scale insects, such as small bumps on stems and leaves, and treat promptly to prevent further damage. Encourage beneficial insects that prey on scale insects as natural control measures.