Answer: Indoors, where light intensity is low, cactus can become stressed and show signs of insect and disease problems. The most commonly encountered insect pests on cacti include scale, mealybugs and spider mites. What you describe sounds like a combination of all three. Scale insects have a hard outer coating so insecticides aren't very effective against them. You should be able to pry them off of your plants with your fingernail or with a toothpick. Or, you can try dabbing them with rubbing alcohol. Mealybugs usually congregate in groups but occasionally they will travel on a plant when they're young, until they find just the right place to settle. Eventually they produce a fluffy white coating to protect themselves from predators. Again, peeling them off will control them, or dabbing with rubbing alcohol will kill them. Spidermites can be discouraged by washing your plants off under a strong stream of water or by spraying with insecticidal soap. Once you rid your cacti of these pests, rinse them off every couple of weeks to discourage future attacks of spidermites. Best wishes with your cacti collection!
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