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Celery fly
This pest is not as
serious on parsnips as it is on celery, it will still do sufficient
damage to reduce growth significantly, because it attacks and interferes
with the chlorophyll making mechanism of the leaves. The tiny adult
flies lay eggs on the foliage and the small maggots which emerge burrow
into and through the leaves, producing white or brownish blisters as they
go.
Normally good cultivation will
enable the plants to grow back after attack, but in the worst cases the
most badly affected leaves should be removed and the remainder sprayed
with an insecticide. Alternatively you can kill the maggots by squeezing
them, still in the leaf, between your finger and thumb.