What's eating my fairy castle cactus?

Fairy Castle Cactus (Acanthocereus tetragonus 'Fairy Castle') is a slow-growing columnar houseplant. Mealybugs and scale insects are the most frequent pests, hiding in the densely clustered columns. Root mealybugs can go unnoticed until the plant declines. Spider mites are less common but possible in very dry indoor air.

White cottony deposits tucked between the tightly packed columns; stunted growth and yellowing at the base Use a fine-tipped cotton swab soaked in 70% isopropyl alcohol to reach between columns; follow with neem oil spray

White waxy coating on roots and at the soil line; plant wilts and declines despite correct watering Unpot the cactus, rinse roots, treat with a systemic insecticide or neem drench, and repot in fresh dry compost

Flat brown discs on the ribs of the columns; infested areas turn yellow and soft Scrape off with a soft toothbrush and treat with horticultural oil or insecticidal soap; repeat every 2 weeks