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Keeping carrot fly and other nasties at bay

The carrot fly poses a serious threat to carrots and can damage up to 90% of the roots.  

It's that time of year again when seeds are sown and seedlings are transplanted out into the veggie garden. It's also that time of year when the nasties appear, and Prof Walker, our perennially cheerful vegetable oracle, is troubled by what's pursuing his carrots.

Effects of carrot flyYou can see here the damage caused by carrot fly. It's not severe, but obviously it wouldn't look good in a shop. If the carrot fly attacks the carrot when it's younger, however, it will kill the carrot.

The carrot fly starts off in October, or a little earlier in the North Island. It lays its eggs near the carrot. These eggs hatch into small, white larvae, like a little white worm, and these make their way down into the carrot. They attack the fine roots first, then the carrot itself.

Carrot fly larvaeThose larvae eventually pupate and turn into another fly. The whole process takes about eight to 12 weeks. You then get more flies hatching and the whole process starts all over again. You can get three or four generations in one year, the last one about May.

Incidences of carrot rust fly, as they're also known, were recorded in England in 1814. In NZ the fly was first found and recorded in Auckland in 1931. By 1952 it was as far south as Palmerston North. By 1980 it had officially spread to the South Island, though growers had been aware of it as early as 1960.

The carrot fly poses a serious threat to carrots and can damage up to 90% of the roots.

Constructing a microclimate cover box - 1One of the methods Prof Walker found most effective is to use pelleted carrot seed (supplied by his commercial friends) which have been pelleted with an insecticide. However, this method is presently unavailable to the home gardener.

Another excellent method is to use a special cover to enclose the whole crop to keep out the fly. Prof Walker says he knows people who have had great success using a microclimate cloth to grow their carrots.  

Constructing a microclimate cover box - 2Not only is it effective against carrot fly, it can be used for all kinds of purposes. Prof Walker leaves it on most of the time for his strawberries, but for other crops he tends to take it off when the plants are coming into maturity. For the carrots, however, he leaves it on all the time.

At this time of the year it can also be used to aid germination of small seeds and to keep the wind off plants. It can also be used as a protection from heavy rain and animals, such as cats.

The material used for the cover is a porous, knitted cloth. It not only lets air and water through, it also keeps out the bugs, such as the carrot fly and other insects.

For anybody wanting to keep carrot fly at bay without using chemicals, this is ideal.

Other Methods to Reduce Carrot Fly

To minimise the damage from carrot fly:

  • When you're thinning, never leave the 'thinnings' about.

  • Be aware that other crops such as parsnip, celery and parsley all host the carrot fly. Don't plant them too close.

  • Practice a rotation as wide as you possibly can.

Reproduced with permission from NZOOM Home and Garden content,
from the previous website of  TVNZ News

The views expressed here are not necessarily those of the RNZIH

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Last updated: September 24, 2004