What's eating my scarlet cleistocactus?

Cleistocactus winteri (golden rat tail cactus) has trailing stems that drape over pot edges, making the undersides hard to inspect — a prime spot for mealybugs and scale. Root mealybugs can spread easily between the many pendant stems sharing one root zone. Spider mites occasionally appear in dry indoor air near heating vents.

White cottony clusters on the undersides of trailing stems and at areoles; stems may shrivel locally Lift and inspect all stems; apply isopropyl alcohol with cotton swab to each colony; spray with neem oil

Multiple stems wilt simultaneously despite correct watering; white powder on root mass when unpotted Remove from pot, rinse all soil from roots, treat with neem drench or systemic insecticide, repot fresh

Tiny bronze stippling along stem length; webbing at stem tips in heavy infestations Rinse stems with a gentle water spray, apply insecticidal soap or neem oil, improve air circulation