Root worms: These are tiny worms that attack the roots of cabbages and radishes..the larvae hatch quickly and then they burrow in the soil heading to the root of the plant.
Root worms are a minor nuisance in radishes and carrots you can cut were they have been and eat the rest....but they will kill cabbage and broccoli. I have seen them kill seedlings if they eat much of the roots.
I have heard that you can slip some heavy plastic around your plant deep near the roots...but this hasn't ever helped me. I usually have to compost the infected plant.