Our wheat bran is composed of only wheat. In the mill to process the wheat kernel, the protective outer layer of the wheat seed is removed from the wheat berry and the germ. In the process, the bran is created by using the outer layer, but there are also some particles of endosperm and germ in with the bran. We purchase it from a mill that supplies local bakeries so that we know it has no harmful pesticides or chemicals that might harm our very sensitive insect population. We also sell our worms to humans to eat as food (we actually have an FDA compliant label!) and are extremely cautious about what is fed to our worms.
Having said that, it is not uncommon for there to be an occasional kernel of dried corn that has made it into our silos when the grain truck delivers our feed. It is rare, but after filling our cricket silo (their food contains corn as well as bran), some kernels get left in the chute before he switches over.