I am pretty certain I kill more honey bees on my daily commute then any mantis would eat any given day, not to mention all the other natural predators. unless you are raiding hives for your entire mantis stock, then I really wouldn't think too hard on it.
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Besides if you are really worried about bee populations , you could stop using your cell phones(
http://scientopia.org/blogs/scicurious/2011/05/25/are-cell-phones-killing-the-bees/)
I dunno my roommate was giving me guf about feeding honey bees to my idolos when others flyers became less avail. So it has become kind of a pet-peeve of mine, as a little common sense will tell you that the 2-3 bees you may feed off a day just seem so insignificant in the big picture.
In my world, I try not to use feeders that have more potential to hurt my mantis, but when push comes to shove sometimes it's better then a starved to death mantis.