large cabbage white butterfly

There is no safe time, the over wintering pupae hatch out in spring, lay eggs, these turn  into caterpillars, pupae and butterflies in the same season and so the cycle continues.

Safest bet is to net.

Then there's the small white to contend with also!

Last year the caterpillars overwintered in the middle of my sprouters and emerged on the first warm day in the Spring. I would think the frosts this year will have finished them?

I find the pea green coloured small white caterpillars are the worst as they hide in the florets of calabrese and caulis so you miss them when picking them off the leaves.

Best solution is to net. The holes need to be 7mm or less with no gaps.