I dug up some old pix on my computer files. I posted them to a thread a few years ago.
These
one way traps for top of bottle. Left one is from a set I purchased that included the bait concoction. It slips on perfectly, The one on right is one I saved from one of those Disposable traps. I would save them when I disposed of the dead flies in the throwaway bags. Have quite a few of them on hand.
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Bottle with first one installed.
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Bottle with second one installed. NOTE; I did have to enlarge the neck opening slightly. Used a hair dryer on high, and when plastic got soft, I slid neck over a funnel to enlarge. I think I could have used the plastic yellow top to do the enlarging also
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Clear plastic works best, because the buzzing flies inside are easily seen by the ones flying around outside, and attract them in.