What's eating my ferocactus glaucescens?

Ferocactus glaucescens, the blue-barrel cactus, is susceptible to mealybugs sheltering at its areoles and root mealybugs in wet, dense soil. Scale insects may appear on the glaucous blue ribs. Spider mites can cause surface scarring in very dry indoor conditions. The attractive blue colouration makes cosmetic pest damage particularly visible.

Cottony white clusters at the areoles along the ribs; the blue surface around colonies may look bleached or scarred Remove with alcohol swabs; spray with neem oil solution; isolate the plant and check all nearby cacti; repeat weekly

Slow growth and wilting despite correct watering; white waxy fluff on roots and in the pot drainage holes Unpot completely, remove all soil, treat roots with imidacloprid drench, allow to fully dry before repotting in gritty compost

Hard, tan or brown fixed bumps on rib surfaces; surrounding skin yellows; honeydew attracts mould Scrape off manually with a soft brush; apply neem or horticultural oil; recheck after 14 days and repeat