Well they will pick on onions, if they can't get at the leeks, but those symptoms can be other things. I find that the pests in the leaves rarely get into the bulbs, you can cut off the worst foliage if you feel it necessary, but the drying out of the onions tends to solve the problem... that's why they prefer leeks, they are in the ground longer, allowing the wee beastie to complete it's life cycle... as the onion foliage will die off in the next few weeks any baby beasties will die with it.. seems to be my experience... onions going mushy is usually white rot...