man, my aunt has guinea pigs, and we get shit tons of flies. if anyone think's it's cruel to be against those little buggers than you have never had to put stuff over your drink cause otherwise there'll be a thousand little fly shits every time you go for a sip.
This is not something I would encourage or try. It seems cruel to some, but those "some" probably live in warm houses with sanitary conditions and have fresh food to eat. Flies are on about the same level as germs to me.