Answer: The most common type of maggots found in a worm bin are grey brown and about 1/2" long. These are the larvae of the soldier fly, a large pretty, blue/black fly. These larvae are attracted to compost piles and to the worm bin, and will not harm you or your worms. In fact, they are good decomposers and, like the redworms, will produce a high quality casting.
If you haven't added animal proteins, and don't have any foul odors in the bin, then in all likelihood the maggots you are seeing will be soldier flies. Once your bin has soldier flies, it can be difficult to say goodbye to them. Your best tactic is to simply allow them to grow out of the larval stage (which they do quickly) and fly off. If you really can't stand them, you'll have to harvest the worms and get rid of all your vermicompost material (put it in an outdoor compost pile, or bury it in the garden). Then put your worms back into fresh bedding.
Fortunately, there are several things you can do to keep their numbers way down. First, don't bring them in. Fruit flies come into the home as cocoons in the peels of fruit we buy. Their tiny eggs are planted into the skins. One way to kill the eggs, if you want to go to this length, is to drop the skins and peels into hot water, microwave a couple of minutes or steam them. The eggs aren't buried deeply, so you shouldn't have any problem reaching them.
In the worm bin, always bury your food waste several inches deep in the bedding. In addition, you can place a couple layers of damp newspaper flat over the surface of the bedding. Each of these helps to keep adult fruit flies from accessing the buried food, where they'd love to lay their eggs.
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