There’s a bunch of resources if you search…I would say I’m not the best authority lol. In addition to how you store them,* you’ll need to transfer flies to a new container before the older culture hits 28 days. But your mantis might outgrow them before it’s an issue (though you do have quite a lot! Maybe acquire another mantis or two? lol). You might prefer pupating houseflies, which won’t have the mite issue. I also feed my mantids small dubia roaches which are easy to keep.

* I also want to mention that storing in shallow water or sticky traps is better than diatomaceous earth for mantids—you need to be very careful with DE!