Flies at base of a willow tree in Missoula

Someone posted on "nextdoor" the following:

Help! Swarming Flies! Hundreds of flies are swarming around the bottom of my big Willow Tree. Part of the tree is dead but the other part is very much alive. The flies are at the base of the alive part. We sprayed them with insecticide the other day and they died but later came back. They look like small black house flies. Can anyone tell me anything about this, please?

I'd like to give them some non-toxic solutions, or even educate them if I could as to why this is happening, but I really have no idea. Does anyone here have thoughts?

Original post is here if you want to try to respond directly: https://nextdoor.com/news_feed/?post=189019637&is=tpe&mar=false&ct=Q3aVAOruEOO3cqYt18y7bjywJf_3eg9bGHiT0Qxcx-If7vDOTDkBZQ7tpaO5XYgD&ec=OWKiQRDj9vEHAYwTV6YMARldwuFdgGkeefhwfGYAE0s%3D

(link may not work.. I don't know if one has to be logged in, or what ever).

Here are some threads that might help you explain an organic or better solution:

https://permies.com/t/161563/fly-control

https://permies.com/t/162364/Fly-control-pig-pen

Thanks Anne.
I guess flies just happen. I'll try to pass along the idea of using traps vs poison.