Oh dear Ruth this sounds ominous!
Here are photos sent by Portzed:
Also he says:
This year everyone on our 3 acres of allotments (Portishead) has been suffering from an infestation of caterpillars on the parsnip leaves.
It started late July and was very visible by 10th Aug. We have not knowingly had this problem before. I have not found any textbook or internet reference to our precise problem, but there seems to be a
similar caterpillar in the USA.
Can you help us to answer the following
How do we get rid of them?
How do we prevent them in future years?
Will they spoil the root crop?
As a matter of interest Celeriac and carrots are unaffected and we have avoided carrot root fly so far…
A spray of bacillus thurigensis has been recommended here, see other posts in this thread. Maybe it means mesh over parsnip leaves from about May next year?
It must affect the roots but does anybody know how long they continue eating?