Thrips bite Haworthia leaves and transmit pathogens that cause leaf stunting and can make the leaves rot. If a flower stem is bitten, it often will wilt. Applying systemic insecticides periodically is an excellent preventative measure against thrips.
Acephate is an effective contact insecticide against thrips. Use a 0.11% solution and spray the infested plant once a week. Yellow sticky traps can be used for capturing adults too.
Systemic insecticides are absorbed by a plant, transported to all the tissues and remain active for a period of time (often in months). Acephate also has residual systemic activities. 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 0.11% solution until the growing medium is drenched. Acephate 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.