My ducks have been very effective at controlling deerflies and horseflies on my property. Not by eating the adults, but rather by their feeding activities along the margins of my 2 acrepond and in various boggy areas where their feeding activities target deerfly and horsefly larvae that live in mud and shallow water. When I first moved here, during deerfly season (May) I would typically have 2 to 3 flies buzzing around my head whenever I was outside. Within 2 years after getting 14 ducks, deerfly populations have dropped to where I see less than 5 of them during the entire season. Horsefly populations in late summer have also dropped to where I only see 1 or 2 during their season.
The duck's feeding activities have changed the profile of the pond's shoreline. Where there used to be a constant slope entering the water where it becomes a mud floor sloping away into deeper water, there is now a mini 1 foot high cliff with the several inches of water at its base. The duck's mud filtering dabbling feeding activity has removed the loose mud in the shallows and into the bank along with any fly larvae living in it.