I think it is just those gnat type things that fly around and come over as a big cloud of bugs. I don't think they are flies are they? I've never seen a swarm of flies...they are normally solitary unless there is rotting food there?

Lots of flies at my house, too. I noticed it on Easter Sunday when it was warm and sunny--left the door to my shed open that day and the whole ceiling of the building was covered in flies. Had to use one of those "bug bombs" in there to get rid of them. But I haven't seen too many since then, so I thought it was due to the colder weather coming back.

If the flies are inside, and if your house/building was open to the outside for periods last fall, what you may be seeing is cluster flies - big, black and sort of clumsy. They come inside to hibernate for the winter in a sheltered spot. They are a parasite on earthworms, so the number may say something about a healthy worm population? You can just leave a door or window open and they will go out, if you don't want to use chemicals.

Other flies may be the gnats/gnatlike ones - if so, it's just time for them. It's my perception that the spring was screwy as far as temperatures and the blooming times of flowers goes, so their hatching from pupae may have been delayed/speeded up, and several types may be hatching at once, now that it has rained 'most everywhere and warmed up.

Unless there were a lot of flies that were pupating in the mulch, I doubt it was the mulch. Possibly, if there was a lot of organic matter, beyond the wood, in the mulch for the larva to eat, and the flies had laid eggs, the eggs hatched, the larva went through their life cycle, pupated and hatched, which takes weeks if not months, depending on the species, so....., somehow, I don't THINK it was the mulch. Coincidences happen, things fluctuate naturally, and also get forgotten from year to year.

You can wait the "plague" out, or try to catch some and send photos/take samples to your extension agent, who if s/he can't ID them can send them to Raleigh or Clemson, depending on your state, for ID. If you want to limit them in the future, a good clean up of any food sources might help, but first you probably should know what the larvae of this particular fly eats, so it comes back to ID.... Fly swatters and tape, or the disposable traps should catch many adults. Even a jar with holes in the lid (or a plastic soda bottle, the bottom cut off and taped back on upside down, holes in the sides, and the bait in the bottom is supposed to work, I've not tried that), and either sugar water, fruit or a bit of meat inside - depending on what attracts THESE flies - might also work until you can buy a trap.

We had a fly explosion also, the cluster flies description fits exactly. I'm thinking I may have brought them indoors when I brought deck plants in last fall. My deck plants were potted with soil I know had a lot of earthworms. I guess I'll put them in sterilized soil next fall before I bring them in.

Sticky tapes worked just fine. They seemed to prefer the kitchen, so I didn't want to spray anything.

I have the same problem. About 3 weeks ago we began landscaping around our patio...using pine mulch.
Now we have a HUGE number of flies SWARMING. These aren't gnats. These are the big house fly. It's awful. We can't enjoy the patio at all for the hundreds of flies.
Perhaps there is a chemical agent in the mulch that attracts them. I can only hope that the rain will eventually wash it away. It's like we live in the Amityville Horror house.

I just planted outside in my planter boxes. All of a sudden it is the horror story of flies. They are outside my door. Gross. I think some thing in potting soil? Darn I used miracle grow. Maybe I should give them a heads up. Hope my condo neighbor does not complain. She is not a gardener more college type.