The Carrot Fly - Psila rosae
An adult carrot (or carrot root fly) fly is a very small black fly which has been described as "a low flying miniature cruise missile". It is a serious and widespread pest and is really the only carrot pest worth worrying about.
The fly is attracted to the Carrots by smell. It lays its eggs at ground level in the soil adjacent to the Carrots. The eggs hatch and the grubs burrow into the roots. The result is a mess, with grub tunnels all through the crops. It stays in the ground over winter gorging itself on your Carrots, pupates and lays eggs in early spring. Eggs will ideally be laid near to carrots but parsley, (and cow parsley), celery, parsnips and celeriac are also affected. Attacks are particularly bad in old established gardens where the population builds up each year.
There are usually two, sometimes three, generations of carrot fly in a year. After the spring generation have hatched they lay eggs, this generation hatches and matures in enough time to have another frenzy of egg laying. The later generation feeds on carrot roots left in the soil in autumn and early winter. The little white grubs pupate in the soil, and hatch out as the first generation next spring. The first and worst attack occurs mid-May and mid-June; subsequent attacks are in autumn and in winter in mild seasons.
You will not know you have a carrot fly attack until you lift the crop. In severe infestations the first sign is that the Carrot leaves look an orange, reddish, rusty colour. They then turn yellow. On lifting an affected carrot it will be seen that the root end will be black or dark. Close examination of what appear to be good carrots may reveal small holes in the carrot. If put in a bucket of water badly affected carrots will float to the surface. Once the disfiguring damage is done by the grubs tunnelling into carrot roots, this then allows moulds to gain a hold.
The Carrot Fly is low flying and therefore can be prevented from laying its eggs by physical barriers such as polythene. Either grow carrots under fine net film, very similar to mosquito netting, making sure there are no gaps at ground level, or surround the carrots with a 60cm high barrier of clear polythene or fine netting nailed to canes. Make sure there are no gaps at ground level. In dry months water the carrots if they appear to get dry because of the barrier.
You can emulate commercial organic growers method of timing your sowings to miss the egg laying season. This means sowing in February to mid-March and main crop towards the end of May or mid-June. Always sow very thinly to minimise the need for later thinning.
The carrot fly locates its carrots by scent. Crushing the foliage may make them easier to find so leave them alone. If you must weed the carrots, do it on a dry evening with no wind, as the scent of the bruised foliage will not spread so far, and carrot flies take wing in bright sunlight. Harvest carrots for eating in the evenings, for the same reason.
Location can be vital. However unappealing to the gardener, a windswept site with little protection is ideal. Carrot fly adults are weak fliers and tends to lurk round field edges or garden margins.
Mulching with grass cuttings can make it harder for the female flies to find a suitable egg laying site. The crop appears to benefit from the extra support given by the earth and you'll have noticeably fewer carrots with green shoulders. 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. But do watch out for slugs and snails who will also thrive in these conditions!
Probably the best organic way to get rid of pests is to soak the bed once a week with a thin mixture of wood ashes and water using a watering can. Compost and wood ashes will also scare off not only carrot flies but carrot weevils, wireworms, and other carrot pests.
Grow varieties with partial resistance (Nantes, F1 Flyaway and F1 Resistafly) and remember to rotate your crops annually. Most other carrot pests and diseases are also soil-borne and can be controlled by crop rotation.
A mixed crop should sustain less damage than a monocrop.
Inter-planting onions or garlic in the carrot beds may ward off the villainous flies. The idea is that the onion smell fools the female fly, which is otherwise attracted by the lingering foliage scent after carrot thinning. Alas this lovely romantic idea has its limitations as it can only be effective when the onions are in active leaf growth. Best results come from four rows of onions for every one of carrots and the beneficial effects will diminish as the onions start to bulb.
You can also try to reduce this pest by placing a sprig of wormwood around the plants crown. This masks the scent of carrots and the flies should leave them alone. Other plants to try include black salsify (oyster plant), coriander, lettuce, pennyroyal, rosemary and sage.
Grow strongly scented plants around carrot beds such as French or African marigolds.
Mix the carrot seeds with feathery leaved annual flowers when sowing.