I got tired of feeding the pests, and took action.
The first action was to add slugbait around the plants. No success; the next morning, a couple more plants were missing leaves.
I should have been more scientific about this, changing only one variable at a time, but I was tired of losing plants. So I did two things:
- I tore all the straw mulch out from around the garlic (it didn't need it any more anyway!), and
- I added a bit of diatomatceous earth around the victimized plants.
Success! They've stopped being devoured. I found at least one slug among the composting straw, so that could have been our culprit. But I've left the diatoms out anyway.