Do you keep the boxes outdoors? Do you have problems with any other mealworm predators getting in?
I have mine in my barn in a closed room, that said, there are mice in there which I have heard like to snack on meal worms, I keep mine in a Tupperware with no lid, but they are on top of an upside down 5 gallon bucket. I haven't had any problems with anything bothering the worms except for a tiny spider that caught one of the smaller (1/4 inch or smaller) worms and hauled it up into its web somehow, that was more funny than anything....