Mites can be controlled by a strong spray of water from a hose. You just have to be consistent and keep blasting them off everyday until they are completely gone. Chemicals arent really needed unless the infestation is widesprad across many plants to the point where using a hose becomes impractical.
But those photos that you posted in the other thread are still most consistent with sunburn in my opinion. Are you sure that there are mites there? Do you see the actual mites or their webs? If the cacti have been dry for a long period of time and the temps start to rise as the sun becomes stronger it can cause damage to the tips, even if the cacti have been outdoors for awhile. I've seen it happen in the springtime to inground plants that are stressed from lack of water. Just make sure that there are actually mites there before you go applying any chemicals. Best luck.
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