[Lardman]

This years onions weren't looking too health anyway with a good 50% of them rotting in the ground.

I noticed today that a large proportion of what was left had been chewed off at ground level leaving most of the leaves flat on the floor severed from the bulb.  I can see no damage anywhere else just at the base.

No sign of slugs or snails and pellets are untouched and they're covered with debris netting to keep the birds off.

The Shallots planted next to them look perfectly fine at the moment.

[skywalker]

Rabbits?

[PennyS]

The ones I planted last summer got rabbitted. 
Well I guess they were the culprits because I now have rabbit proof fencing and this year's onions haven't been razored like the other ones did.
So that would be my guess too.   

[Lardman]

I've not see any rabbits or any signs of them (I think the rest of the wildlife around here would have seen them off) Im growing in the garden and I think Id have seen them by now.

The debris netting is on a wooden caged around them and weighed down with bricks, I've not seen any sign of damage to the net either.

I have seen tree rats - surely they'd nibble the leaves rather than just slice them off at the base. ?

[JayG]

This is a tricky one unless you catch the culprit "at it".

I've got all the usual suspects in my garden, but they don't always behave the same way every year; I've had sparrows peck the new shoots on my clematis half to death one year but not the next, ditto summer raspberries, and although there is a family of very acrobatic grey squirrels round here up to now they have never decimated my sweetcorn, which has happened to others apparently ( )

I know it's not much help but because onions aren't usually too troubled by "the usual suspects" and that you have them netted it must be one of the "less usual suspects" and I can only suggest you double-check your netting to make sure nothing can get in.

[moose]

I had a similar problem and discovered it was a squirrel that had found a way in.

[rusty100]

I discovered it was a moth caterpillar eating mine, not ants as per my previous post. I caught it in act then identified it using the internet. It had all of my spring onions away.

[bigben]

I lost a tray of leeks last year just before they were due to go in. All cut off clean at the surface of the compost they were growning in. Luckily I had another next to it that was not touched - still dont know what it was but suspect birds.

[whistler]

My money is on the fluffy, wuffy bunnies, I've actually caught them at it!  Mind you it must be the courting season 'cos they've left the garlic alone.