Pest insects in brassica crops: fly pests

The main fly pest affecting brassicas is the cabbage root fly. Find out about its life cycle, ways of monitoring populations and some chemical-free methods of treating infestation.

The main fly pest that infests brassicas is the cabbage root fly (Delia radicum). The roots of brassicas may also be infested by turnip fly larvae (Delia floralis) and bean seed fly larvae (Delia platura), although the latter rarely causes economic damage. Larvae of Pegohylemyia fugax occasionally occur on crops such as cauliflower that are starting to rot. Other pest flies include the swede midge (Contarinia nasturtii) and the leaf miner Scaptomyza flava, which is increasingly common in oilseed rape crops and brassicas such as rocket.

The cabbage root fly can be found all over the UK. It overwinters in the soil in the pupal stage. The first generation of adult flies usually emerge from April to May, and the second generation emerges from July to August. In warmer parts of the UK, a third generation of flies may emerge in late August to September.

‘Late-emerging’ flies

There are some areas of the UK where a proportion of the cabbage root flies emerge later in the spring than would be expected. We call these 'late-emerging' flies, and they are genetically different from 'early-emerging' flies. Relatively large numbers of 'late-emerging' flies appear in some parts of Devon and South West Lancashire. These complete no more than two generations per year. In areas where both ‘early’ and ‘late’ flies occur, there may be continuous pest pressure throughout most of the growing season.

Once they have emerged, female cabbage root flies require time to feed, mate and mature their eggs: this is equivalent to about 80 day-degrees (D°) above 6°C. Eggs are usually laid in the soil close to the base of the plant, and the newly hatched larvae move through the soil to burrow into the roots. Fully fed larvae form pupae in the soil close to the plant roots.

The turnip fly occurs in Scotland and some parts of Northern England, such as South West Lancashire. It has a similar life cycle to the cabbage root fly, but the first generation of adults emerges later than the first generation of early-emerging cabbage root flies. Turnip flies may complete one or two generations per year.

The larvae of cabbage root fly and turnip fly cause damage to brassicas by feeding on them.

Leafy brassicas

In leafy brassicas, slight damage to the root system may not be a problem, but heavy damage may cause the plant to wilt or even die. As the plant grows, it’s more able to tolerate feeding damage, so most transplanted brassicas are only susceptible to significant damage during the first four to five weeks in the field.

Root brassicas

In root brassicas, such as swede, turnip and radish, even slight feeding damage is more problematic due to its effect on the appearance of the root or ‘bulb’. Under certain circumstances, female cabbage root flies may lay their eggs on the aerial parts of brassica plants, including Brussels sprout buttons, broccoli florets and at the base of Chinese cabbage plants. It is believed that this occurs most often when the soil is hot and dry. The larvae cause feeding damage and are contaminants.

You can monitor adult cabbage root flies using water traps or sticky traps – it can be easier to identify them from water trap samples.

Water traps

Water traps are usually yellow. The surface tension of the water is lowered by adding a few drops of detergent. It can be kept fresh by adding metabisulphite tablets. All the insects captured in the traps are scooped out at intervals and taken away for identification. Growers can purchase a proprietary trap that uses a volatile attractant related to the ‘mustard’ chemicals which brassicas contain. This trap is more selective for cabbage root flies than the other types of trap, making identification easier. Cabbage root fly eggs can be sampled from around the base of brassica plants by scooping up the soil and floating the eggs out in water, to which a small amount of antifoam is added. There are no reliable treatment thresholds for cabbage root fly.

Forecasting

The timing of the various stages in the cabbage root fly life cycle can be predicted using a computer program developed with funding from Defra and AHDB. The output from this program is currently available as part of the AHDB Pest Bulletin. This forecast shows when flies of each generation are expected to emerge from pupae and when they are likely to lay eggs.

Female cabbage root flies, in particular, are very mobile insects and may disperse over several miles once they have mated. Oilseed rape is an important host crop for cabbage root fly, and in most cases, it’s impossible to manage local populations through crop rotation and the separation of new crops from sources of flies.

Crop covers

Fine mesh netting has been used widely on swede, turnip and radish crops to exclude cabbage root flies. The netting prevents flies from getting into the crop, but they could still lay eggs through or on the netting where it touches the crop. The hatching larvae may still reach the plants below.

Before adding crop covers:

• Consider the life cycle of the fly first ‒ you’ll need to add the crop covers before egg laying starts
• If growing brassica crops in fairly rapid succession in the same land, ensure flies have emerged from any pupae remaining in the soil before applying net covers
• Note that a complete generation from a newly laid egg to a newly emerged adult cabbage root fly requires approximately 500D° above a base temperature of 6°C
• If the netting is torn, cabbage root flies will get in

The most successful treatment against leaf miner damage in AHDB trials was covering crops of rocket, from emergence to harvest, with insect-proof netting. This significantly reduced puncturing damage from female flies.

Using natural pest enemies

Methods for biological control have been considered for the cabbage root fly, including the use of predators/parasitoids ‒ either beetles or wasps. None of these has yet been shown to be a commercially viable management approach. However, some species do occur naturally in brassica crops at quite high levels (particularly where no insecticides are used) to help suppress the overall population. Using predators is preferable as parasitoids don’t kill the cabbage root fly until it has reached the pupal stage.

Cabbage root fly adults are susceptible to a fungal pathogen that occurs in nature, leading to high fly mortality under certain conditions (usually relatively high temperatures and humidity).

Biopesticides containing the fungus Metarhizium or nematodes have been shown to be effective when applied to brassica modules. However, more tests are needed to make this commercially viable.

If you would like to order a hard copy of the Brassica Crop Walkers' Guide, please contact publications@ahdb.org.uk or call 0247 799 0069.