The flies themselves don't eat. Their only purpose in their adult life is to mate and lay more eggs. It's only the larvae that eat. So when they begin to pupate, I'd assume that their nutrition value is headed downhill fast. That's why the fly itself only lives about four days.
I've not seen anywhere that talks about the nutrition value of the fly itself, so I certainly would not make them the mainstay of your frogs/toads diet. But the larvae sound like a good thing nutrition wise.
However I can't say whether they, either the fly or the larvae are appropriate for your frogs/toads. None of the people that raise your frog/toads appear to frequent this sub-forum so the question might be better ask in the
Frog sub-forum or the
Toad sub-forum. Or they just don't know the answer that I'm just guessing at from some quick google searches.
So if no one gives you a better answer, I think you are in the experimental realm
Certainly AfricanBullFrog2's comment might be the most telling statement so far as to the larvaes appropriateness for food, which I take as a negative.
I know that many online sources are offering the larvae as a source of food, but I don't know for what they intend the larvae for. I just don't think they are appropriate for my FBT's. Which is a shame considering the nutrition the larvae have.