Anyone know if those green mosquito coils are effective against horseflies?
No. Trust me on this... thats a big NO. At least the ones in New Brunswick, Canada. They were immune to anything and attacked you at will and in swarms. Usually when a big cloud of mosquitos (they are not tiny back home!) where around you, the big horse flies would see what the fuss is.
But I would only experience the horse flies in the dense brush in northern New Brunswick, along with the monster mosquitos.