Allotment Growing

The Cabbage Root Fly - Delia radicum brassicae

What is Cabbage Root Fly?

The Cabbage Root Fly is a serious pest affecting most brassicas including cabbages, cauliflowers, broccoli, calabrese, Brussels sprouts, kale, swedes and turnips caused by the fly's maggots eating the roots of brassicas.

Symptoms of Cabbage Root Fly

Affected plants tend to wilt and droop initially, showing a blueish tinge later as if nutrient deficient. Young plants die although older plants may well survive cabbages often fail to heart and cauliflowers form a tiny head. To confirm it is the cabbage root fly you need to examine the plant roots and look for tunnels and or white maggots eating the roots, which blacken and die.

Life Cycle of Cabbage Root Fly

• In April and May with the advent of warmer weather the first generation of flies emerges from pupae of the latest generation of the previous summer, which have over-wintered in the soil.
• The female flies lay their eggs close by the stems of the brassicas. These then turn into maggots that make their way into the stem and roots creating tunnels where they feed.
• After about three weeks the maggot is fully grown and leaves the damaged plant behind moving a short distance into the soil and becomes a small brown pupa, about 1cm long.
• A week or so later, another generation emerges and flies off in search of more of the cabbage family to cause the grower problems. This means that, unlike the carrot root fly with defined danger periods, the cabbage root fly is a problem all through the season.

Preventing Cabbage Root Fly Damage

Because the fly is about for most of the spring and summer it is difficult to avoid. Starting plants in modules and pots can help as a stronger root system has developed before the attack and earthing up affected plants may enable them to produce more roots to compensate for damage.

There is no chemical insecticide answer available to the home gardener so we must rely on organic prevention methods. For stemmed brassicas; cabbages, cauliflowers, broccoli etc the best method is to use a disc or square of barrier material. You can buy these or easily make your own by cutting a piece of carpet underlay about 15cm square, cutting a line to the centre and placing it around the stem of the plant. You may need to peg this to stop it blowing off. It is time consuming but effective.

To keep the fly away from turnips and swedes the best way is to grow under tents of horticultural fleece.

Digging over in the autumn and winter will expose the pupae to the birds, which will help reduce the problem for next season.

Natural Enemies of the Cabbage Root Fly

All immature stages of the cabbage root fly have natural enemies in the soil. The diet of small beetles may include their eggs, maggots and pupae.

More on Brassicas

• Coping with Club Root - Plasmodiophora Brassicae
• Brassica Pests & Problems
• Cabbage Root Fly
• Growing Cabbages
• Growing Cauliflowers
• Growing Brussels Sprouts
• Growing Swedes
• Growing Brocolli
• Growing Calabrese
• Growing Turnips
• Growing Radishes
• Growing Romanesco