Carrot Fly
Dealing with Pest No 1 - The Carrot Fly
The carrot fly, Psila rosae is a serious and widespread pest and is really the only pest worth worrying about. The damage is done by the grubs tunnelling into carrot roots, disfiguring them and allowing moulds to gain a hold.
How do you know you have a Carrot Fly attack? Basically you will not know 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 Carrots are put in a bucket of water badly affected ones will come to the surface. This however does not mean that those which do not float are totally unaffected.
Covering the crop with Enviromesh or similar is the best method of fly control. 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. Pull carrots for eating in the evenings too, for the same reason.
You will often see the suggestion that growing onions in between the carrots keeps the carrot fly at bay. 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 rarely works in practice. Onions 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.
Other strongly scented plants such as French or African marigolds, pennyroyal and garlic have all been suggested in the past, but have insufficient data to back up their claims. Mix carrot seed in with a packet of mixed annuals and sow as usual. A mixed crop should sustain less damage than a monocrop.
You can also emulate commercial organic growers method of timing your sowings to miss the egg laying season. This means sowing in February or early March, or waiting until Mid June.
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. Watch out for slugs and snails who will also thrive in these conditions!
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.
A couple of varieties offer some resistance to the problem. If the problem is serious in your area these will probably be attacked with the rest. Best results are obtained if no other variety is grown alongside. Autumn King types are carrot fly’s favourites, so avoid sowing these if there is a known problem.
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, onion family, pennyroyal, rosemary and sage.
How do I detect Carrot Fly?
Basically you will not know 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 dark or black. Close examination of what appear to be good Carrots may reveal small holes in the Carrot. If Carrots are put in a bucket of water, badly affected ones will come to the surface. This however, does not mean, that those which do not float are totally unaffected.
An adult carrot (or “rust”) fly is a very small black fly which has been described as “a low flying miniature cruise missile”. The fly is attracted to the Carrots by smell. It lays its eggs 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 carrot.
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 and Parsnips, are also affected. After the spring generation have hatched they lay eggs in June and July and this generation hatches and matures in enough time to have another frenzy of egg laying August / September time.
This insect pest also loves parsley, parsnips, celery and celeriac. Attacks are particularly bad in old established gardens where the population builds up each year. Most carrot pests and diseases are soil-borne and can be controlled by crop rotation.
Adult flies, attracted by the smell of carrots, lay their eggs at ground level so the little white grubs can hatch out and tunnel into the carrot roots. Therefore vulnerable young seedlings die and the foliage of large seedlings turn bronzy red and the plants weaken.
There are usually two, sometimes three, generations of carrot fly in a year. The first and worst attack occurs mid-May and mid-June; subsequent attacks are in Autumn and in Winter in mild seasons.
The Carrot Fly is low flying and therefore can be prevented from laying its eggs by physical barriers such as polythene. Surround the carrots with a 60cm high barrier of clear polythene film or fine netting nailed to canes, making sure there are no gaps at ground level. These barriers should be no more than 100cm wide. In dry months water the carrots if they appear to get dry because of the barrier.
The later generation feeds on carrot roots left in the soil in Autumn and early Winter. Grubs pupate in the soil, and hatch out as the first generation next Spring.
Other practical tips include sowing early carrots before mid-March and main crop towards the end of May or mid-June. Sow very thinly to minimise the need for later thinning. You can try growing carrots under fine net film, very similar to mosquito netting, making sure there are no gaps at ground level.
Inter-planting onions or garlic in the carrot beds may also ward off the villainous flies. Mix carrot seed with feathery leaved annual flowers when sowing.
Compost and wood ashes will also scare off not only carrot flies but carrot weevils, wireworms, and other carrot pests. 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.
Some chemical control can be achieved through dusting the rows before sowing in Spring with bromophos, phroxim or pirimphos-methyl dust. Protection of late crops can be done by spraying with half strength liquid pirimphos-methyl in early August.
Grow varieties with partial resistance.
Tags: Garden pests-weeds
