Moths are insects that lay their eggs in clothing. Once the larvae hatch, they eat the cotton/wool and thereby make holes in garments. Rust
can be translated in other ways. The idea is that of perishing. Food that is stored perishes over time.1 Taking both together, Jesus is referring to all those agencies and processes that cause earthly treasure to diminish in value over time and finally to disappear completely.2
19 “Do not lay up for yourselves treasures on earth, where moth and rust destroy and where thieves break in and steal,