you dont have any old food and fruit sitting around your place do you?
leave a banana out on the counter and you'll have a bajillion of em, I used to take a container like a gatorade bottle and put a banana cut in the bottom and put shrink wrap over the top leaving a small hole, it'd get a whole bunch of em and their too stupid to fly out... then all i'd do is place the bottle over some of my dews...
feeding frenzy...
just leave the container outside, you may even get some flies... then put it in the freezer for a moment or two and they'll all immobilize. then you can pick out the big ole flies and feed your vft's by hand
the flies will wake up and trigger the rest of the vft's signals...
yum yum.