When they are hungry!!!
They are temperature driven, so warm days in winter can see them having a good old munch.

Thanks Jason,

For some reason I thought they did the slug equivalent of hibernating...time to get the gloves on and pick em out

One of my large plants has loads of small and large snails. In london, the temperatures seem to be just about warm enough for them.

So, I would guess it was time to get going.

Now!