Mealybugs can destroy a Haworthia in months if left unattended. Besides constant inspections, regularly applying systemic insecticides is an excellent preventative measure.
InspectionSurface mealybugs tend to hide in between Haworthia 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. Periodically lift random plants out of containers and carefully check the root system.
For an infected plant, clean as much of the soil out of the roots. The soil should be discarded completely because it often contains mealybug eggs. Soak the plant in ~1% warm solution of potassium fatty acids (often branded as insecticidal soap) for several hours and then rinse it in warm water. Let it dry out before replanting.
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 an very effective systemic insecticide against mealybugs. Apply it once half an year at spring and fall, when Haworthias are in growing seasons and are actively taking water from soil. Drenching the soil with ~0.004% solution. Imidacloprid can be toxic to animals. Collect the runoff and dispose it to a hazardous waste disposal site. Continue to collect in the subsequent 3 waterings so that the Imidacloprid residues are mostly flushed out.