Plant Doctor Archive

Carrot rust fly

Question LAST growing season I planted parsnip seed. They grew beautifully, but on harvesting them I found each parsnip has large brown, rotted areas just below the crown at ground level and same further down, too. The brown area appears to have been eaten by an insect. How can I get rid of the problem?

Answer  IT sounds like your parsnips have been attacked by the larvae of carrot rust fly. The adult is a small black fly about 4–6mm long which lays its eggs in the soil close to where carrots and closely related plants, like parsnips, celery and parsley, are growing. On hatching, the young, pale, creamy maggot-like larvae feed on the very fine root hairs of seedlings, but eventually they burrow into the main root which can lead to secondary rots, hence the brown areas you describe. The larvae can grow up to 8mm long.

If you want to use an insecticide, granular Diazinon, sold as Soil Insect Killer, and applied when sowing the seed or sprinkled alongside the plants after thinning seedlings, is effective. However, there are other ways of reducing damage that don’t involve the use of insecticides. Try the following:

  • Rotate crops — don’t plant parsnips, carrots, celery or parsley in the same soil year after year. Allow a break of at least one season, preferably two.
  • Don’t leave old damaged plants of the carrot family in the soil — remove them as soon as practicable. And get rid of any carrot-related weeds like wild carrot, fennel and hemlock.
  • Sow your parsnip seeds thinly or thin out closely spaced seedlings as early as possible.
  • Mound soil up around the base of the parsnips as they grow — this is claimed to make it harder for larvae to reach the roots.
  • Shelter your parsnips under a covering of shade cloth or light insect mesh in an attempt to stop the fly from getting to the soil around the plants to lay eggs. Peg the mesh down to the soil about 30cm or so out from the plants.
Weekend Gardener, Issue 179, 2005, Page 30