@LouisIngenthron the people who dont want them flying around your food?
@freemo Both the meaning of the idiom and choice of the word "catch" imply a desire to attract rather than repel.
@freemo It's true that it is used in that context, but I think that's more of a softening euphemism for "trapping". Kind of like how we say "pest control" instead of "rodent genocide".
Most uses I know of imply a live catch, and then you decide what to do with it (i.e. catching fish, butterflies, fireflies, dogs).
@LouisIngenthron it may be a euphemis but it is the language you use. "I put out a fly trap to catch flys" i cant imagine anyone thinking this means anything other than an undesirable insect.