Built a horsefly trap 3 days ago. Day 1 killed 146 horseflies. Day 2 killed 375! We either have a terrible horsefly problem or it's a great trap.
UPDATE: The 7 day total was 1,697 Horseflies killed! They are big (1" long) and easy to count. They need to be cleaned out every day anyway so they won't rot. How it works is simple. Horseflies are visual hunters. From a distance the black bag looks like something to bite, they do a fly over and are deflected into the soapy water and drown within a few seconds.
Note: Make sure and set this in an open area where the horse flies can see it from a distance. You may need to move it around until you find the "sweet spot". Clean it out with a cat litter scoop or large slotted spoon. Also I would love it if you would pm me pics if you do built it.
NOTE: Be sure to paint the top board a dark color so the light color of the wood won't reflect in the plexiglass and make it visible to the flies.
Source: Dan Fowler from Tennessee