What's eating my giant barrel cactus?

Giant Barrel Cactus (Echinocactus platyacanthus) is a slow-growing houseplant well armored by formidable spines but still susceptible to mealybugs tucked into the dense spine clusters and to scale on the ribs. Root mealybugs can go unnoticed for months. Overwatering that leads to root rot is a bigger overall threat than most insects.

White cottony masses hidden deep within dense spine clusters; prolonged feeding causes yellowing ribs. Use a thin cotton swab soaked in 70% isopropyl alcohol to reach between spines. Spray neem oil solution; repeat weekly for a month.

Unexplained wilting and pale color despite correct watering; white waxy residue on roots when unpotted. Unpot, rinse and inspect roots, let dry, repot in sterile fast-draining mix. Drench with systemic insecticide if needed.

Brown or grayish lumps along ribs; localized scarring and yellowing around feeding sites. Scrape off scales and swab area with alcohol. Apply neem oil spray and inspect monthly; the large size of this cactus requires thorough coverage.