The butterflies are searching for brassica plants on which to lay their eggs, which in due course hatch out as caterpillars - the ones that can so easily decimate your crop in the space of a day or so.
My main brassica crops are covered with nets which are hopefully keeping the butterflies out, but elsewhere in the garden (as far away as possible from the main brassica crops) I have a group of spare PSB and Cabbage plants, which are unprotected.
I'm not worried if these plants don't make it. I have dubbed them "Sacrificial plants" because they can help to lure the butterflies away from the plants I want to grow. The butterflies are making the most of the opportunity, and I see that the plants are already covered in eggs.
These are the eggs of the Large White, typically laid in groups:
These are the eggs of the Small White, typically laid singly:
The eggs are white when laid, but gradually turn yellow.
I'm not sure what type of eggs these ones are. They are definitely different. Anyone know what they are?
While photographing the butterfly eggs, I noticed a small beetle-type insect on one of the leaves. At first I thought it was a desirable insect, probably a predator of butterfly eggs.
However, closer inspection suggests that this is NOT a beneficial insect. It looks like the Rape Stem Weevil Ceutorhynchus Napi. Furthermore, it was not alone - there were several of them. My photo here is not of the best quality, but I believe it demonstrates conclusively that these weevils eat the central ribs of the brassica leaves:
OK, so I think I have another task ahead of me now - picking off weevils! Perhaps I need to check my main crop plants too, since the nets covering them will evidently not keep out a 3mm weevil.