The small, yellowish maggots of this fly feed on the roots of carrots, celery, celeriac, parsley and parsnips, and over winters as pupae in the soil.
The adult flies emerge in early June, and lay their eggs in the soil near carrots and other host plants, and the maggots hatching from these feed on the roots for about a month.
The damage is done when these maggots tunnel into carrot roots.
Moulds gain hold, resulting in young plants wilting and dying, and the roots of more mature plants become spoiled by the extensive tunnels.
A second generation of adults often appears in August.
The numbers of over wintering pupae may be reduced by destroying all infested plants in the autumn, preferably by burning.
Usually you will not know if your crop has become affected until you lift the crop.
In severe infestations the first sign is the Carrot leaves turn an orange, reddish, rusty colour, then finally they then turn yellow.
On lifting an affected Carrot, the damage will be seen as a series of dark, sometimes black patches on it.
Closer examination may reveal small holes.
A test for severely damaged carrots, i.e. those not worth keeping / storing is;
Place the suspect carrots in a bucket of water if they rise to the surface the chances are that it is badly affected.
This however does not mean that those which do not float are unaffected, you will only find this out when you come to prepare them for eating.
It is worth considering using varieties of carrots that are reputed to be fly resistant.
Prior to sowing, treat the seed with a suitable seed-dressing, and if necessary, water a Jeyes fluid solution into the soil around susceptible plants.
It is thought that the smell from these products confuses the fly and they move on to lay their eggs elsewhere.
Repeat twice at ten day intervals.
Apply the first treatment in mid-May on early carrots, in July on main crop.
Another application can be applied after thinning to mask the smell of newly exposed carrots.
Carrot flies only fly in bright sunlight, so if weeding is required,do it on a dry wind free evening, to prevent the scent of the bruised foliage spreading.
Pull carrots for eating in the evenings too, for the same reason.
For those people who do not use chemicals a covering with agri-fleece can help the situation.
Lift all edible roots before the winter and place into store.
Carrot fly screening
It is said that the flies do not fly higher than 0.5m (18") above the soil so some people sometimes surround their crops with a 60cm (24") barrier to protect their crops.
Sadly however these crops sometimes become affected by infestations to the amazement of the grower, hence the advice above to completely cover the crop with fleece.
On the occasions when such infestations occur it is thought that the flies are carried above / over the barrier by thermal air currents and can't get out again.