Carrot fly damage on left parsnip
Carrot fly, also known as carrot root fly (Psila rosae) is a serious and widespread pest. It causes most damage to carrots, but can also affect the roots of related crops such as parsnip, celery and parsley.
Young seedlings can be killed. The first sign of attack on more mature plants is often reddening of the foliage and stunted growth – but there may be no foliage symptoms at all. On lifting, the carrots will have rusty-brown tunnels just below the skin. These are caused by the carrot fly larvae (maggots) which may be visible. Carrots left in the ground until January or later often show the worst damage as some larvae continue to feed right through the winter and can move between plants.
Description of pest
Adult flies are about 8mm long, shiny black with reddish head, orange legs and transparent wings. The larvae are 8-10mm long, creamy-white in colour.
There are usually two distinct generations of carrot fly each season, although in some areas there may be three.
April/May: First generation adult females lay eggs in cracks in soil near suitable host plants. Eggs hatch into larvae after about a week. These enter the roots and start feeding, developing into mature adults within three months.
July/August: Second generation flies lay their eggs. Some of these will develop into adult flies by autumn; others survive as larvae through the winter in carrot roots.
Prevention and control
- Site selection: A windswept site with little protection is ideal. The carrot fly is not a strong flier, and tends to spend a lot of time in sheltered field or garden margins.
- Resistant varieties: Research is being done to develop resistant strains of carrots. Choose ones that show at least some degree of resistance, such as Fly Away F1 and Resistafly F1
- Spacing and thinning: The adult fly locates carrots by a number of means, including scent. Avoid the need for thinning carrots (and thereby bruising the foliage which releases its scent) by using wide spacing when sowing – or sow a tiny pinch of seed at regular intervals.
- Weeding: Carrot fly are attracted by the scent released from bruised foliage. They mainly take to the wing during the day and in bright sunlight. If you have to weed carrots, do it on a dry evening with no wind when the scent of the carrots will not spread so far. Pull carrots for eating in the evenings too, for the same reason.
- Timing of sowing: Select sowing times that avoid the main egg laying period, as indicated in the table below. These times are approximate and will vary from year to year depending on temperature and geographical location.
Month Feb Mar April May June July Aug Sept Oct Egg laying period Sowing times
- Intercropping: There is some evidence to suggest that growing carrots between rows of onions helps minimise damage (probably by masking the smell). The deterrent is only effective when the onions are in active leaf growth. Once the bulb begins to form the effect diminishes. Four rows of onions to one of carrots has been shown as necessary to offer protection.
Barriers: Fleece - use of a non-woven horticultural (crop cover) fleece gives excellent protection if used from sowing time until mid to late June – or until the crop is ready. Check for weeds and diseases regularly under the cover. If the carrots look mildewed, remove the cover for three weeks or so to give the carrots a breather – but it needs to go back on again in early July.
Carrot fly barrier
Lawrence Hills, the founder of HDRA/Garden Organic, advocated the idea of using a vertical barrier to protect the crop. Carrot flies can be deterred by erecting a fence-like barrier around three or four rows of carrots. It should be at least 70cm high, not more than 1m wide. Suitable coverings are fine mesh or polythene.
- Grass mulch: Put a layer of grass mowings 5cm deep between the rows of carrots when they reach 10-15cm high. This should come right up to the base of the plants. Top up with a further 1cm layer at weekly intervals for four weeks. The mulch enables the carrots to make better use of nutrients and water in the soil, encouraging healthy growing conditions and improving their ability to resist attack. It also makes it more difficult for the female flies to lay their eggs in cracks in the soil. A range of creatures will make their home under the mulch, some of which will be predators of the carrot fly such as ground beetles and centipedes. Watch out for slugs and snails which will also thrive in these conditions!
- Ridging: Earth up the carrots with soil to form a ridge at least 5cm high. This may need to be repeated throughout the season to maintain a 5cm ridge. The crop appears to benefit from the extra support given by the earth, and there should be noticeably fewer carrots with green shoulders. Rabbits and slugs also seem to be put off.
- Timing of harvest: Lift main crop carrots by September/October rather than leaving in the ground over winter. Never leave crops known to be infected in the ground, where they could become a source of infection the following year. They can be safely fed to animals, or composted in the centre of a well built heap that will reach a high temperature.