[chickpeacurry]

I grew carrots last year in large black boxes. Near the end of the crop I had some carrots that I think had been attacked by carrot fly.   Throughout the winter I have been turning over the soil.  I put a compost and sand mix near the top .  Should I be concerned that carrot fly might have overwintering given we have had limited frost. The boxes would cost need a lot of soil and compost to change. Can the carrot fly survive and if so would I have to empty the  boxes completely

[Trillium]

Carrot flies tend to have a flying limit of up to 2 ft. If your boxes are taller than that, it's surprising they got in.

To prevent them you could set up a barrier fence inside of stakes with some fine mesh, perhaps a foot tall would keep them out completely.

You'll find some more good info here: http://www.carrotmuseum.co.uk/cultivation2.html

[sunshineband]

Sometimes the flies get wafted higher by the wind. They can end up on higher shrubs and glide on down into your fenced area unfortunately 

[DD.]

Total enclosure is the only way to go.

Re the soil - get rid. Overwintering carrot fly larvae is one of the reasons it's essential to rotate your carrot sowing.

[solway cropper]

Agree with total enclosure in enviromesh or similar as it's the only way you'll get any protection. I've had 2 foot deep tubs set on a table and the carrots were just as badly affected as if they'd been at ground level.

[harrymac]

Another vote here for barrier protection against carrot fly. I've had limited success relying on height, and for the number of carrots I want to grow, from a practical point of view ground level is the way to go.

Oddly, for the past 2 years I've grown a few inside the polytunnel and they've been free from damage, even though it's not entirely fly-proof. It's quite isolated from other crops so maybe that has something to do with it.