Cabbage root fly biology and management in brassica crops

Cabbage root fly larvae feed on the roots of vegetable brassicas, with damage being dependant on the crop type, growth stage and growing conditions. Newly transplanted or recently emerged crops are most at risk as the root systems are less developed.

• Newly emerged crops and recently transplanted module-grown plants are at a greater risk. These lack the large root systems needed to tolerate moderate-to-high levels of infestation
• Control is particularly important in crops where the marketable part is damaged (for example, root crops, Brussels sprout buttons, broccoli florets, Chinese cabbage)
• In oilseed rape, crops that have emerged before the end of August are most at risk of attack, but it is generally considered a minor pest

Scientific name: Delia radicum

Adult flies are greyish–brown and resemble small houseflies. The male is 5–6 mm long and the female 6–7 mm.

Eggs are white and about 1 mm long.

Larvae are fairly transparent, tapering and legless with a reduced head. The mouthparts look like black hooks. Fully grown larvae are 5–8 mm long.

Pupae are dark brown and barrel-shaped, with smoothly rounded sides.

Nov–Mar: Pupae from late second and early third generations overwinter in soil.

Apr: Adult flies emerge and feed/mate.

May: Eggs are laid, mainly in the soil around the stems of brassica plants (first generation).

May–Oct: Larvae feed primarily on the roots of brassica plants, although they sometimes feed on the aerial parts. Plants may wilt or die. When mature, larvae drop to the soil to pupate.

Jun–Oct: Most pupae reside 8–12 cm below the soil surface.

Jul–Sep: Two to three overlapping generations occur. Adults emerging in August and September can infest oilseed rape.

The life cycle is driven by temperature, and activity is earlier in warm years or locations. In the UK, there are generally two to three generations of adults in the South and only two generations in the North.

In some areas (for example, parts of Devon and southwest Lancashire), a proportion of cabbage root flies emerge later in the spring than would be expected. These are called ‘late-emerging’ flies and they are genetically different from ‘early emerging’ flies.

When plants have large root systems, damage may not be apparent until harvest. Even at this stage, damage to aerial parts may be difficult to identify.

Non-chemical control

Fine mesh netting (crop covers) may prevent egg laying by female cabbage root flies on susceptible crops (such as swede). They can lay eggs on or through netting that touches the crop, but the incidence of this is low. Other physical and cultural approaches (companion planting, vertical fences and trap crops) are not as effective as insecticides or netting.

Natural enemies contribute to control. These include two parasitoids, spiders, ground and rove beetles, and predatory flies (Muscidae).

To date, biological control with predators or parasitoids has not been commercially viable. There is evidence that the use of insect-pathogenic nematodes or insect-pathogenic fungi may be effective in certain situations.

There is no host–plant resistance available at present.

Monitoring

Capture adult flies in yellow water traps or more specialised traps that release a semiochemical (related to the distinctive chemical compounds produced by brassica).

Sample the soil around plants to determine the presence of eggs.

Use weather-based forecasts to predict the time of egg laying.

Thresholds

None established. Larvae are very difficult to control, so prophylactic treatment is often necessary.

Insecticide resistance

None known.