What's eating my stenocactus crispatus?

Stenocactus crispatus (brain cactus / wave cactus) is susceptible to the typical indoor cactus pests — mealybugs, scale insects, and spider mites — which exploit its closely packed, wavy ribs as hiding spots. Overwatering also invites fungus gnats and root rot. Inspect the deep rib furrows carefully during routine care.

White fluffy masses in the rib grooves and areoles; honeydew deposits leading to sooty mould Remove with a cotton swab soaked in isopropyl alcohol; follow up with neem oil spray; treat weekly until clear

Brown or tan shell-like bumps on ribs and spine clusters; yellowing or pitting of the cactus body Scrape off manually with a soft toothbrush; apply horticultural oil spray; repeat after 10 days to catch crawlers

Larvae in moist compost feeding on fine roots; adults are an annoyance but larvae weaken the plant Allow compost to dry thoroughly between waterings; use yellow sticky traps for adults; apply Bacillus thuringiensis israelensis (Bti) drench for larvae