I think that DE is useful for beetles but I doubt that it is going to do anything to a cabbage butterfly laying eggs or a caterpillar since they do not have exoskeletons to trap the DE particles.
Bt does kill caterpillars but they have to be on the plant first to eat it so won't you have dead caterpillars on your plants? Maybe they only get on the leaves, not the head itself, I don't know because I always use row covers.
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