* If you're talking about the act of flying: The abstract noun is flight.
* If you're talking about the insect: The abstract noun is less clear-cut. You could use flying, but it's not a common word. You might instead use a more descriptive term like insect presence or fly infestation.
Here are some examples:
* Flight is a beautiful and graceful way to travel.
* The insect presence in the kitchen was overwhelming.
* We had a fly infestation in our house.