If you spray plants with a forceful spray of water you can knock many mealybugs from the leaves. Best done in early morning. Use a nozzle that will give you a fine spray of water, and thoroughly wash off the undersides of the affected leaves. It is important that you do this at least three times - 3 days in a row or every other day -- because you need to get the mealybugs that will hatch from eggs that already were laid when you began this spritzing process.
Isopropyl alcohol can be successfully used against mealybugs. The mixture includes 1 cup alcohol and 1/2 teaspoon Volck oil in 1 quart water. The Volck oil can be replaced by insecticidal soap for equally effective results. The alcohol alone may burn a few of the ornamental plants, so the mixture is safer. The Volck oil stays on the plant to smother unseen young and eggs. The alcohol mixture is sprayed thoroughly onto any plant with signs of mealybugs infestation at intervals of one to two days until the infestation was over.
Minor infestations of mealybugs on plants can be controlled by touching them with a cotton swab dipped in rubbing alcohol, which dissolves their protective coat of wax. This is much too much work if the infestation is heavy.
If you problem is in a greenhouse, try beneficial insects available from commercial sources - Green Lace-wings, Ladybugs/ladybird beetles, Trichogramma and Chalcid wasps are good at controlling mealybug populations.