Cabbage Root Fly and Carrot Fly Control
Cabbage root fly and carrot fly are two of the mot destructive pests in the vegetable garden - and they are very difficult to control. There are no effective chemical controls available to the amateur, not that we would choose that way of controlling them when there are non-chemical alternatives. This leaves us just with physical and cultural control options, but to get the best from these it is important to understand the life cycles of the pests.
Cabbage Root Fly
Attacks all members of the family Cruciferae including leafy, heading and root brassicas, and some ornamentals such as wallflower and stock.
Damage on root brassicas is mainly mining of the surface areas of the swollen root, on other brassicas attacks on young plants will cause wilting and death.
There are normally three distinct generations each season in England and Wales and the milder parts of Scotland, but in cooler areas there may only be two generations.
The pest overwinters in the soil as pupa, emerging as an adult fly over a 4-5 week period from mid April in warm areas, and from mid May in cooler areas.
Before laying fertile eggs the female has to feed on pollen and/or nectar, her favoured food plant is cow parsley (Anthriscus sylvestris) and when cow parsley starts to flower, the first generation of cabbage root fly starts.
This means that controls should be in place at the first sign of cow parsley flowers.
The female fly, similar in size and appearance to a house fly, searches for young host plants and lays her eggs on or just below the soil surface normally less than 2 cms from the host plant stem.
If close-fitting collars are in place she cannot lay her eggs near enough to the plants for the hatched larvae to reach the plants. If collars are correctly applied and in good time, cabbage root fly damage will be minimal. Collars are not generally practical or effective on root brassicas.
Covering young crops with fleece or mesh will prevent the fly reaching the crop at all and prevent any damage. Covers are effective on all brassicas.
The second generation normally starts in mid to late June in the south, July in cooler areas.
The third generation starts in mid August, but by this time there will be some overlap of generations.
Although root crops are subject to damage throughout their lives, on heading and leafy crops significant damage only occurs on young plants, so older plants will not need protection.
Carrot fly
Attacks all members of the family Umbelliferae, which includes carrot, celery, dill, fennel, parsley and parsnip.
Attacks on seedlings will kill plants, but this damage is unusual.
There are two distinct generations each year, the first starts in mid May in the south and in mid June in the north, and the second starts in mid July in the south, in August in cooler areas.
In areas where a lot of host plants occur, particularly close to commercial carrot crops, the generations can be very prolonged and overlap.
The pest overwinters as pupae in the soil or in old host plant roots. The adult flies are about 8mm (?") long and back with yellow legs and irridescent wings.
The female carrot fly does not need to feed before laying fertile eggs.
She flies close to the ground, so vertical physical barriers around the crop about 25cms (1') high will reduce the level of attack.
The female finds host plants by smell, normally in the evening or night, and lays eggs in cracks in the soil adjacent to plants. She is particularly attracted by damaged plants so avoid thinning the crop in the evening and water it immediately after thinning to reduce the smell.
Companion planting with very aromatic plants, eg garlic, basil, marigolds etc, can reduce the females ability to find the host plant, but this is only a partially effective control measure and needs to supplemented with other methods.
Covering the crop with fleece or fine mesh will prevent carrot fly adults reaching the crop, but the edges must be well secured. Avoid growing one susceptible crop after another as there may be carrot fly pupae in the soil
If using a cover it will need to be supported above the crop, using hoops for example, to prevent damage to the foliage.
Although carrots can be grown under fleece throughout their life, celery and parsnips are not suitable for growing in this way.
Covering the crop at the two high risk periods, (mid May to mid June, and mid July to late August in warmer areas, later in the north) will substantially reduce crop damage.
This article was written by Roger Umpleby, if you would like to know more about Roger or would like him to speak at a gardening/horticultural meeting Click here
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