So you have planted out your brassicas and remembered to net them to keep the pigeons off; and you limed the soil well to help avoid clubroot!
Butterflies and caterpillars
If you used butterfly netting you also protected them from caterpillar attack - if you make sure there are no holes or the butterflies will find a way in!
Cabbage Root Fly
Did you remember to protect them from Cabbage Root Fly?
The usual way is to put a small mat around the plant so the fly lays its eggs on an impervious surface and they dry out or are eaten. Mats are available from the Trading Hut but most people make their own from cardboard or carpet or anything that comes to hand.
Try Ant Powder
An alternative, not strictly approved of by the powers that be, is to use ant powder round each plant. Ant powder contains the same pesticide that used to be in 'Cabbage Root Fly Preventer Powder' before it was withdrawn as not complying with the new regulations. (The garden chemical companies didn't make enough money from it to make it worthwhile paying for the necessary testing.) However, ant powder has been used by one of our 'expert' allotmenteers for some time without any Root Fly attack.