[Growster...]

I reckon with parsnips, that if they're well away in time, the carrot fly doesn't get as deep as it does with carrots.

Canker can be scraped off anyway, and how much parsnip do you want each time...;0)

Nice question, and no real answer except to protect or grow on top of the Lloyds building..;0)

[LeeR]

You can get Nematodes for carrot fly from various web sites as an alternative control method.