Mealybugs can destroy a Haworthia in months if left unattended. Besides constant inspections, applying systemic insecticides periodically is an excellent preventative measure.
Surface mealybugs tend to hide in between the leaves or where leaves attach to the stem. Look for their white flecks or cottony residues. Root mealybugs dwell underground and are hard to detect. Regularly lift random plants out of containers and carefully check the roots.
For an infested plant, clean as much of the growing medium out of the roots and completely discard it. Some roots may be torn off but the plant will be fine. Soak the whole plant in a warm 1% solution of potassium fatty acids (often branded as insecticidal soap) for 2+ hours and then rinse it in warm water. This kills any adults and eggs. Let the plant dry out in a shaded and cool place before planting in a fresh growing medium.
Systemic insecticides are absorbed by a plant, transported to all the tissues and remain active for a period of time (often in months). Imidacloprid is a very effective systemic insecticide against mealybugs. Apply it once at both spring and autumn, when Haworthias are in their growing seasons and are actively taking water from growing medium. Water with a 0.004% solution until the growing medium is drenched. Imidacloprid can be toxic to animals, and the run-off water (including the next 3 waterings) should be collected and disposed to a hazardous waste disposal site.