Go ahead and feed from the mite cultures. The frogs don't mind if the flies have mites or not. Generally cultures mold if they dry out too much before the larvae start to mature. I used to spray cultures when they appeared to be drying out on top. Just a couple of squirts from a misting bottle is enough. This will keep the mold at bay long enough for the larvae to mature and start to moisten the media with digestive enzymes. Adding a little vinegar to the water you mist the culture with can help sometimes too. Culturing fruit flies is an art that it will take you some time to master, be patient and realize that you may have to get some pinhead crickets or some other food source if you are having problems with the flies right off the bat.
What kind of media are you using? I have a couple good recipes that I have been using for a couple of years now if you are interested. Also, you should probably start culturing flour beetles, springtales, or crickets as an alternate food source. This will help you get through the tough times and will give your frogs some welcomed variety in their diet.
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1.1.3 D. azureus
0.0.8 P. bicolor
0.0.7 D. auratus (polka-dot)
0.0.3 D. auratus (blue)
0.0.3 D. auratus (kahlua and cream)
Shannon