The cabbage root fly likes to lay its eggs at the base of brassicas. The larvae hatch and burrow down to the root. The conventional method of preventing this is to surround the base of each plant with a collar, a disc of around 10cm (4″) of something tough like roofing felt or even cardboard with a slit to allow fitting and a ring of slits to accommodate the growing stem. I’m going to try using some left over damp proof membrane. However, one of my favourite books, “The Half Hour Allotment Book” by Lia Leendertz, recommends sowing a couple of garlic chive seeds in with each brassica seed. Apparently they germinate in around the same time and the smell from the chives helps ward off the pest. Of course, using both methods I won’t know which has worked best!
Potato Wireworm (older)