onion fly

About the only thing you can do is to cover the leeks with fleece. This stops the female onion fly from laying her eggs on the plants. Leeks are less susceptible to them than onions. The flies hunt by scent so, anything which blocks the smell of the leeks is good, say carrots or marigolds.

Sept 2009
I have been growing leeks for the best part of 50 years and have never had any problems with them. This year for the first time my crop has been decimated by onion fly. They are just rotting in the ground! I am an organic gardener and read in John Seymours wonderful book 'The Self-Sufficient Gardener', which incidentally credits Lawrence D Hills who is my hero, that the best course of treatment is to try to prevent them by dusting the area with flowers of sulphur or soot, or sprinkle with an ounce of parafin to a gallon of water.
Nowadays, most of us grow onions from sets which do not seem to get infected. Perhaps this why they are attacking my leeks. Our local chimney sweep drops off his soot to our allotments so have a word with yours. Soot also helps to deter carrot fly, a good dusting of soot at sowing usually does the trick. Good luck.