The ball, usually a blue color attracts dear/horse flies. Once they land on it they can't leave as they take off vertically and the net catches them. You see them in areas with livestock to protect them from the flies.
https://www.amazon.com/Bite-Lite-Professional-Control-Chemicals-Electricity/dp/B00T6MS6LI
These black ball things are supposed to attract and trap them: https://www.amazon.com/Bite-Lite-Professional-Control-Chemicals-Electricity/dp/B00T6MS6LI
there are various instructions on DIY sites for making your own. Haven't done it so I can't comment on effectiveness.
Those thing are ridiculously expensive on Amazon $300 USD. We have pretty few horse flies so it's not critical. What about common house fly traps? I haven't found anything yet that gets all of them...
I want to try one of these and see how many it catches around here.
https://www.amazon.com/Bite-Lite-Professional-Control-Chemicals-Electricity/dp/B00T6MS6LI
Horse fly catcher? https://www.amazon.com/Professional-Control-without-Chemicals-Electricity/dp/B00T6MS6LI
Homemade version that is.