I've heard of the tape trick, we have deer flies but not in great numbers, depends on the location...but here on the coast we get the big green head flies....they usually go to the knees down, they can really teach you to dance, lol, can bite through a mesh wading shoe, when they get you you bleed....
it's why I never wear shorts!..

Wish I could get my dog to wear something like that. He’d be a lot happier on daily fly season walks. Lol.

In Zimbabwe, middle of nowhere, I saw a darkish blue square in the bush.. Was told it was a tse tse fly trap and that color was most attractive to them. I did get bit once and it was like a deer fly bite combined with a bee sting.

Try spraying your dog with Adams Flea & Tick spray. It's good stuff and last quite a while.

If you go to your farm store in the equestrian - horse products isle you can get a fly spray to spray or wipe onto your dog. It is for horses but it will work on your dogs to. It will keep flies, deer flies, tick and lice off your dog.

I am trapping nuisance beaver now and the horse flies will drive you crazy.... I am going to break out the hat tape trick in the morning and get some revenge.