Parsnip Disaster

This year about four weeks after my parsnips germinated, I noticed that many of them were wilting and turning yellow. On pulling one up I found it was covered in what looked like brown aphids or some sort of lice. I lost over three quarters of the row to this infestation. What were these creatures - and how can I stop this happening next year. I have grown parsnips for many years and never seen this before.