[AmityvilleHams]

A full mesh top alone may potentially still not be enough ventilation compared to the level of ventilation a barred cage or even a cage with one or more barred/meshed side(lacking cross ventilation which can make a big difference).

That said even barred cages can have ventilation issues with hideouts, tunnels, etc if they're plastic or other non breathable materials - for example plastikoted wood isn't breathable like unfinished wood would be.

I would be extremely careful using any sort of pesticides and insecticides in the room where the hamster is. They can be highly toxic for such sensitive species, or at the very least potentially irritating to their respiratory system. Food grade diatomaceous earth is one thing I would definitely recommend trying around the room in general though since it mechanically kills anything with an exoskeleton & unlike synthetic pesticides and such they can't develop a resistance or tolerance to it over time. A thorough cage clean should help too though.

ETA more natural cage setups can have potential increased moisture issues too, for example moss being best avoided if I'm honest since that can contribute to humidity being too high(even "dry" moss can rehydrate eventually and be a problem). Coconut fiber is a great natural digging material(actually safe unlike any kind of soil) but you do have to be careful to avoid it being too dry or too wet - overly wet coconut fiber can contribute to potential mold and other humidity related issues.