the black barrel composter, is it a trash can??
There should be NO flies.
What kind of drainage do you have in it??
I am thinking it is TOO wet.
Now, the grubs....
You MUST keep your kitchen compost covered with things like grass clippings, yard waste type stuff. I highly reccomend bagging leaves in the fall and leaving the bags around the compost pile, after you dump your scraps into it, cover them with leaves.
our lawn mower is broken, so I have been dumping my pail then hacking down large leafy weeds to cover it. This will keep the bugs from laying eggs on it.
If you have grubs, i would leave it for another year. They are probably some sort of beetle. SO you may want to find some bag a bugs if htey are japanese beetles.