I had a thought about harvesting live adult flies, but I don't know if it is safe from a disease/germ standpoint. If it is, I think it would be a relatively simple method.
I noticed a lot of flies buzzing around some litter I had mucked out and hauled to the compost.
My thought -- I could bait a couple of 2 liter pop bottles with a little bit of something stinky, like maybe some cat food. Let them sit out where the flies congregate.
Flies are good at going in, but not so good at getting out of 2 liter bottles. Most of them get trapped.
When there is a large number of flies in there, I could pop a lid on it, wipe the outside down with bleach, pop the whole thing into a ziploc, and put it in the freezer for an hour to kill them. Then, just shake out for the birds and start all over.
Just wanted to make sure it was safe first, no icky diseases or parasites?
Also, later in the summer with the right bait, probably could catch a lot of wasps and yellow jackets this way.