In article , Steve Simpson
writes

I have an allotment and want to grow carrots this year. All the other
allotment holders have been telling me of the menace that is "carrot fly".

Anyone familiar with planting onions near them. I heard the carrot fly
doesn't like the smell.

That is true, and onion flies don't like the smell of carrots.

Carrot flies are attracted to the aroma from carrot leaves, they can
smell them a mile off if the leaves are crushed or bruised, i.e. during
thinning, hoeing or hand weeding, so try to avoid touching the leaves.
They cannot fly very high though, so a physical barrier around the
carrots can help to deter them.
--
Alan & Joan Gould - North Lincs.