I got mites once in a culture I picked up from a petstore. Rather than tap the flies out when I was ready to make a new one I let them climb out and jump out on their own. I had a separate large cup I let them jump into first. Then I tapped that a bunch so they and everything with them would fall to the bottom of the cup. I then once again let them climb up and jump out on their own this time into the new culture. I didn't notice any mites after that, but for the next couple of cultures I made sure to start a new one as soon as the culture started produce while using the above method. Naturally you will also want to completely clean your culture area and house any potential infested cups in a separate spot. Some people like to get mite paper to place under their cups, but I never saw the need. Grain mites are harmless to mantises. They are more annoying than anything else. For fruit flies they can outsource the flies causing the culture to produce less flies and collapse sooner.
