Sowing seeds always leaves you hopeful of a great crop. I always imagine eating my produce when I sow it, and it is always a horrid feeling when something goes wrong. Carrot root fly got to your carrots, I’m afraid. They fly close to the ground and their antennae can detect the smell of carrots from a mile away.
There are a number of things you can do to keep them at bay. Firstly, grow a few spring onions in the same box, it disguises the aroma of carrot. Secondly, since the carrot fly cannot get as high as 60 cm from the ground, build a barrier around your carrots. Better still, buy some horticultural fleece, which comes in three sizes. You want the finest mesh that excludes all insects. Cover the carrots and you will have no further problems. Remember to tuck the fleece into the soil or hold it down so nothing can crawl inside and be especially vigilant when you are thinning your carrots, because this is when the aroma is strongest.