Just like any mites, grain mites will compete with your flies and maggots for resources. The vendor in question does NOT state a mite free guarantee, but still, it's too bad they have chosen to turn their back to the problem.
Here is a cut and paste of some PM's I have exchanged with somebody, that may help with future problems.
The fruitfly cultures came from xxxxxxxx. I mean the culture is loaded full of them.They are only in the produceing culture,I haven't seen any in the starter culture.
Unfortunately, I hear that a lot when a place produces a lot of fly cultures. I would be willing to bet they will show up in the new, non-producing cultures too. They would have been treated the same way. You are better off looking for a culture to start with from a local you trust. Then culture all your own. Always control...keep the on mite paper. When you start new cultures, dust the flies you are using heavily and then just use the top flies. (feed your frogs with the lower ones) This helps knock some of the mites off of the flies.
Thanks Doug,what should I use to dust with.I don't know of anyone local who is produceing/selling flies.I will send them a email today about the culture,I am not mad just alittle upset to start off with mite infested flies.
Just a superfine powder. It's the dust you are looking for in this case, not the nutrition or vitamins it supplies. The finest dust I know of is Repashy Calcium Plus. When my repashy Calcium Plus gets old, I save it for just this purpose. Oh, another mite control method is this. Mix up your culture with everything EXCEPT the top sprinkle of yeast and the flies. Microwave it till really hot. I go about 6 minutes for 5 cultures. This kills any mite eggs.
edit: Looks like Doug beat me too it!