Fly predators, I love them!! I encourage them any way I can. I even sacrifice a couple of tomato plants to hornworms each year, so that I can be assured the predators have an ample supply of hosts.
If you have a curious mind, it's interesting to see them hunting and carrying their victims around. Seeing their eggs on a host is also a learning experience.
As far as the garlic goes, I've heard mixed reviews. The last time I used it was in an Arabian show barn during the last century (
I crack me up every time I say that!!!
) We also added vinegar to our feed, water, and after-workout rinses. We had few flies, and a very comprehensive prevention program. The Moehlman's were all natural before it was popular to do so.
You do have to be careful not to overfeed garlic, as it can cause anemia. Here is a link to a VERY brief paper, that includes maximum dosages:
https://avmajournals.avma.org/view/journals/ajvr/66/3/ajvr.2005.66.457.xml
When I use fly sprays, I use the PYRETHRIN based formulas, as they're derived from certain species of Chrysanthemums.
A multi-pronged approach, along with removal of manure piles to the most distant place on the property that you can, is the most effective.
And for those who use flysheets, here's some interesting research about the effectiveness of zebra-striped coats:
https://www.discovermagazine.com/planet-earth/zebra-stripes-protect-against-flies-now-we-know-how
As if one didn't have enough stuff in the tackbox, here's the justification for new zebra wear!