6 November 1998

Vegetable pest attacks early drilled oilseed rape

By Andrew Swallow

CABBAGE root fly could be a serious threat to early drilled oilseed rape.

With complete crop loss possible, ADAS is urging growers to check for symptoms now. One Northants grower has already reported serious damage this autumn.

"I suspect it causes a lot more damage than people realise," says Mike Lole, of ADAS Wolverhamptons plant clinic. "Sub-clinical damage is probably fairly widespread."

Early sown crops are most likely to be affected, as flies are attracted to larger plants in late Aug. Eggs laid in the soil within 5cm (2in) of plant stems soon hatch and release larvae to attack the nearby tap root.

Affected plants turn purple, wilt and often die if drought stressed. They are much easier to pull up than healthy plants. In severe attacks the tap root is completely severed.

"It is absolutely devastating," says Craig Pocklington of Station Farm, Horton, Northants. Crops sown in the second week of August were worst hit. "Over half the crop was affected in the worst field. If it hadnt been so wet I think we would have had to re-drill."

Moist conditions have allowed plants to compensate by putting down larger, fang-like adventitious roots. But frost heave or harsh winter conditions could still kill weak plants, says ADAS agronomist Chris Page.

All 174ha (430 acres) of Mr Pocklingtons oilseed rape shows signs of the pest. Some neighbouring crops are affected, too.

The fly, which looks much like a housefly, has similar fat larvae, up to 5-6mm in length. Brassica vegetable crops are protected from attack by dipping roots or drenching modules in pesticide. There is no recommended control in oilseed rape, but Dursban (chlorpyrifos) or Atlas Steward (gamma-HCH) may have some effect, says Mr Page.

"It pays to think if you are going to drill early, is it worth the risk," says Mr Lole. "I have seen an early drilled crop damaged so badly it had to be re-drilled. A safe date depends on the year, but third week August should be late enough." &#42

High levels of cabbage stem flea beetle are also threatening oilseed rape crops. Growers should apply a pyrethroid if threshold levels are breached, says ADAS.

"We recommend growers treat crops if 3-5 larvae, or more, are found per plant. That equates to 60% or more of plants showing scarring on the petioles," says entomologist, John Oakley.

The white larvae, which have a black spot at each end, hatch and invade plants while day temperatures reach 3.2C (38F) from autumn to spring. Yield losses of 0.35t/ha plus are likely if thresholds are breached and crops go untreated.

Little of Craig Pocklingtons Apex has escaped the attentions of cabbage root fly larvae. The damage (inset) would have meant some redrilling of early crops had the weather not been so wet.