A small fly whose larvae are a widespread pest of carrots, burrowing into the roots.
Psila rosae, family Psilidae.
The same treatment will keep the carrot fly away from that crop....
Plant it among the cabbages and with onions and carrots to repel carrot fly.
I leave sowing carrots until June to avoid the first attack of carrot fly, but children will eat them at any size and a row sown now will be far enough on before the roots have become woodwormed by the larvae.