Monitor plants from spring to summer. Any plants which appear to look sickly should be dug up and inspected.
Good practice of yearly crop rotation is a must.
When planting out young vulnerable plants, a Brassica collar can be used around each plant. These can be purchased from a local garden centre or can be homemade. Use thick cardboard or carpet underlay to make your own, cut out a square or circle 15cm across and make a slit to the middle so it can be placed around the plant at soil level. The Cabbage root fly will lay eggs on to this instead of the soil, and the eggs will dry off in the open air, or the birds will eat them.
Young plants may be grown under a fine insect-resistant mesh or horticultural fleece to give protection.
Floating row covers or make-shift plastic bottles can be used to protect young seedlings from having eggs laid on them.
It's also possible to buy root fly mats for plants online or via garden retailers.
If available, you can purchase beneficial nematodes online, which will attack any larvae in the soil. Carefully read the instructions and ensure you are buying the correct species for the target pest.
They are supposedly attracted to soil with high organic matter content.
Likewise, they are believed to dislike rhubarb, so this would be an excellent companion plant for your veg patch.
Be sure to practise crop rotation every year. This will confuse the flies which emerge the following year.