Can't tell you for sure, but I do know that whenever I plant a brug in the ground, it peters out and dies from nematode infestation. So the poisonous component doesn't affect the nematodes.
I also put any fallen leaves and flowers back on top of the soil under the brug plants. Slugs and snails eat them with no apparent ill effects. I had a green caterpillar last year that was feasting on a few leaves.
