Treat whiteflies on Plumeria by using insecticidal soap or neem oil, applied directly to the pests. Insecticidal soap suffocates whiteflies, while neem oil disrupts their life cycle. Introduce beneficial insects like ladybugs and lacewings, which are natural predators of whiteflies. Regularly inspect the plant for signs of infestation, such as tiny white insects flying around the foliage, and treat as necessary.