What's eating my thelocactus hexaedrophorus?

Thelocactus hexaedrophorus (table tennis ball cactus) is susceptible to mealybugs and scale insects hiding in its sparse areoles, and to spider mites during hot, dry spells indoors. Its relatively smooth, blue-green body makes inspection easier than hairy cacti. Root mealybugs are a sneaky risk if the plant sits in damp soil.

White fluffy wax at the areoles and stem base; honeydew residue; stunted or puckered growth Dab with isopropyl alcohol swabs; follow with a neem oil spray; treat every 7–10 days and isolate the plant

Hard brownish discs attached to the cactus body; yellowing or small depressions in tissue beneath Remove manually with a soft brush; apply horticultural oil or neem oil; repeat after 10 days to kill emerging crawlers

Unexplained decline, drooping, or failure to absorb water; white powdery residue on root surface Unpot, inspect and treat roots with imidacloprid drench; let dry completely before repotting in fresh sterile cactus mix