Toomas

Two causes that are not your fault, for completeness. They eventually try to pupate, and an isolated superworm will go to beetle; pupae change colour naturally as they develop, but a dead pupa goes dark and shrivelled, usually because the larva went in without enough moisture reserves.

And contamination: pesticides, cleaning products, treated veg. Clean tub, clean food, kept away from any sprays. But in your case it is the fridge, and the fix is free.

Moisture is the other axis and it goes wrong in both directions. Too much, from wet pieces left in, condensation and poor airflow, grows mould and bacteria and kills them fast. Too little and they dehydrate, because the veg is where they get their water.

So small pieces of carrot or potato, replaced often rather than left to go soft, anything mushy or mouldy out immediately, and a tub with real ventilation.