Aphids

I stopped growing brassicas because of the white fly. Never had them in the greenhouse since.

They're only really a problem on brassicas you eat the leaves of i.e. kale, which they seem to adore (and kalettes which are a kale x sprout cross). Which is one of the reasons why I've stopped growing full size kale and just harvest them as baby leaves for salads (also I don't really like big kale leaves, too tough and chewy). We harvest the asian greens they're on in the greenhouse regularly so they don't get time to build up numbers and cover the underside of whole leaves with their nymphs. Otherwise they're just a bit of a nuisance when they fly in your face. I find the mealy cabbage aphid much worse because they can badly stunt the growth of plants they feed on and they absolutely will go for the bits of sprouts and kalettes that you harvest as well as the leaves. It does make you wonder how much insecticide farmers must use on brassicas to keep them so clean.

Right with you there on commercial crops and insecticide, how on earth do farmers keep away carrot fly and, as you say, all the nasties that attack brassicas.