The problem is caused by adult flies laying eggs (March to April and October to November) you must rotate the crop as some times some pupae may end up in the soil, especially where plants have rotted off.
No pesticides are available to the gardener as yet - I would burn the crop - and move the next years crop to another location - were you can protect it with a fleece or insect fine mesh protection - Ken

Leeks

Thanks Ken. I have noted what you say and will plant in a different bed next year and use fleece. Pat

http://www.growsonyou.com/scrumpygraham/blog/13335-the-onion-blog

That was my experience with it on onions.
I tried netting last year as Ken said and it worked.
I wouldn't use fleece.
To answer snoopy, I used netting around my carrots this year and no root fly at all. If you did have root fly, best thing is to not cover and let the larvae grow into flies. They'll then fly away looking for fresh carrots at some other poor souls place. So in theory, you could use the same carrot bed if you sow at the right time.

http://i938.photobucket.com/albums/ad225/oldshobdonion/carrots_zpsb89e5cd8.jpg

probably by luck then.
Trouble with sowing end of June is maincrop carrots don't have time to develop so you are restricted to quick growing crops that don't store well and are obviously a lot smaller.
Growing in raised beds with netting is the best way....still got at least 60 carrots left to harvest, doesn't sound a lot, but one does 2 meals for 3 people.