Cabbage Root Fly

Key Points

Cabbage Root fly larvae can cause severe losses in many brassica crops
The first generation is the most damaging
Applications of Dursban WG can be made to peat blocks and modules
Applications of Dursban WG can be made as an in field drench treatment

Introduction

Cabbage Root fly — Delia radicum — is an extremely destructive pest, often causing serious damage to UK crops of cabbage, cauliflower, Brussels sprouts, Chinese cabbage, radish, swede and turnip. Plants are attacked at any growth stage, the most serious damage being caused to young plants, and can be killed if attacked in the seedbed or soon after transplanting.

Damage is caused by larvae attacking plants, the worst damage resulting from larvae of the first generation of flies, occurring during late April and throughout May. The second generation of larvae often attacks brassica plant beds and transplants during July. Brussels sprouts can be rejected by processors because of attacks to the buttons in autumn.

Biology

There are two generations of Cabbage Root fly each year, occasionally three generations in certain areas. Cabbage Root flies overwinter as pupae in the soil, giving rise to the first generation of flies in late April and early May. The second generation can be found from the end of June and, where it occurs, the third from mid-August onwards. In intensive brassica growing regions flies will be present throughout July, August and September.

Egg laying by females is predominantly on or just below the soil surface, close to the main stem of a host plant. Occasionally eggs are laid on the foliage of plants, or on the buttons of Brussels sprouts. Egg laying begins in the south of the UK from late April onwards and from the middle of May further north. Weather conditions at this time of year seem to favour rapid build up of injurious infestations and damage is more severe from the larvae of first generations than it is from later ones. Eggs are white, oval and finely ribbed, about 1mm in length. Hatching occurs relatively quickly, after three to seven days, depending on temperature.

Larvae reach 8mm in length when fully grown. They are white or cream in colour, legless and lack a distinct head. Feeding mainly takes place on root tissue, occasionally on aerial parts of the plant including sprout buttons. When mature, after feeding for about three weeks, larvae move from the plant and form a reddish-brown overwintering pupa several centimetres in to the soil.

Product Choice

Crops of broccoli, Brussels sprouts, cabbage, calabrese, cauliflower and Chinese cabbage can be protected in field situations using a drench treatment of Dursban* WG. A drench should be prepared using no more than 60g of Dursban WG in 100 litres of water. This diluted preparation should be applied at the rate of 70ml to the base of each plant, or at the rate of 5.0 litres per 30m of row. Only one drench application can be used per crop. Treatment should be made within four days of transplanting or at seedling emergence for direct sown crops after the third week of April.

Treatments of Dursban WG can be made as a peat block or module drench to plants of Brussels sprouts, cabbage, calabrese and cauliflower prior to planting out. Only one treatment can be made prior to planting out and must be carried out at the 3 – 4 leaf stage of the crop.

Peat Blocks

The leaves should be moistened with a light spray of water immediately before treatment.
Dursban WG should then be applied at the rate of 60g per 5000 blocks in as dilute a solution as possible.
The drench volume should be determined by estimating the uptake of water by one block and then multiplying by 5000 to calculate the total volume required per 5000 blocks.
This volume should be at least 25 litres per 5000 blocks.
Immediately after treatment, the spray should be washed off the leaves of the treated crop using clean water.
The volume of water should be sufficient to clear the leaves of spray deposit but not so much that Dursban WG is leached from the block.
Any such leaching will reduce levels of control achieved.

Modules

The process is similar to that explained for peat blocks, see above.
Water volumes quoted are for modules of 11 – 13ml capacity.
Larger volumes can be used with larger modules.
Leaves of the plants should be wetted with a light spray of water immediately before treatment, using 2.0 litres of water per 5000 plants.
Dursban WG should then be applied at the rate of 30g in 5.0 litres of water per 5000 plants.
Immediately after treatment leaves must be thoroughly washed with clean water, using 5.0 litres of water per 5000 plants.