OK.
I wasn’t sure if thy were like grasshoppers that ate everything they saw.
Understandable.
Grasshoppers actually have a different lifecycle than cicadas. Grasshoppers are hemimetabolous insects, with 3 life cyles: egg, nymph, and adult. The nymphs and adults, the vast majority of their lives, both eat plants.
Cicadas have these same life cycles too, but the nymphs remain underground for years, harmlessly feeding on tree roots. By the time they emerge, they’re about done.