Joanna - those critters are in the fly family, right? Bti should work on them. Make sure the dunks you buy do contain Bti - there is another type out there that doesn't contain Bti. I would make a solution from crumbled up dunks, strain it and put it in a sprayer. Spray the surface of your worm compost. Maybe open it up first and let the worms go hide out from the light before spraying just to make sure. I did read about using Bti for critter problems in the worm bins. You may have to spray a couple of times since the Bti works by inhibiting live offspring, NOT by outright killing off the mature flies.
