What's eating my stenocactus multicostatus?

Stenocactus multicostatus, the brain cactus grown indoors, faces its biggest pest threat from mealybugs hiding in its densely ridged ribs. Scale insects and spider mites can also take hold under warm, dry indoor conditions. Root mealybugs are a secondary risk if watering drainage is poor.

Cottony white clusters nestle deep in the tight rib folds, weakening the plant and causing rib discoloration. Use a soft brush dipped in rubbing alcohol to work into crevices; follow with neem oil spray and improve air circulation.

Brown or tan waxy bumps adhere to ribs; infested areas yellow and the cactus grows more slowly. Scrape off with a toothpick or soft brush; treat with horticultural oil or systemic imidacloprid.

Stippled, dull patches on rib surfaces; very fine webbing may appear between spines in heavy infestations. Spray with diluted neem oil or insecticidal soap; increase humidity slightly and repeat applications weekly.