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Never had a carrot fly attack on my celeriac.Over the years I have found that barrier methods are very effective in reducing damage to carrots. To such a degree that they are used by many plot holders on my site. I think this has produced a virtuous spiral, with fewer and fewer flies for the following generation, they seem much less of a problem than they were some years back. Then, it as well nigh impossible to grow carrots unprotected without the crop being completely ruined. Things seem much better these days. Very encouraging to have a pest control option that doesn't involve insecticide and that works.
Are you sure the local farmers haven't started growing carrots? Most will carpet-bomb the soil with insecticide and create a lure-and-kill-zone for miles around...
All the cow parsley around here doesn't help either.
Cow parsley is related to carrots. See https://www.brc.ac.uk/dbif/invertebratesresults.aspx?insectid=7228which shows the hosts for carrot root fly.Psila rosae = carrot root flyDaucus carota subspecies sativus = carrotAnthriscus sylvestris = cow parsley