Organic Dairy Farms
The practical Organic Dairy Farming Manual published by Hoard's Dairyman in 2016 recommends the use of fly predators.
Here is their statement from page 124:
"In my experience, the early and regular release of fly predators (parasitic wasps) is the key management step to ensuring and maximizing their chances of successfully reducing fly load.
Fly Predators work and should be a part of any holistic, well managed fly control program.
To reiterate, the secret to success is to begin early in the season and to continue to release wasps (parasites) at recommended intervals throughout. As I'm fond of telling clients, one dead fly in the beginning of the season can mean thousands fewer at the end of it."