Large numbers of adult turnip sawflies are appearing in brassica and other crops, in the south east of England, prompting advisers to warn oilseed rape growers to be on their guard this autumn.

The pest is about the size of a housefly and yellow and black in colour. Its black larvae can strip an oilseed rape crop bare of leaves if enough get into the crop.

“We are seeing very high numbers of adult flies in brassica and game cover crops in Kent, Lincolnshire and the south west of England,” says UAP’s technical manager, Chris Bean. “And reports are starting to come in from other areas of the country as well.”

Third generation of sawfly

Mr Bean says that emerging rape crops in the autumn of 2006 were particularly badly hit by sawfly, causing yield losses similar to that of high levels of pigeon damage.

Adult sawflies emerge in May and June and egg-laying begins a few days later. A second generation appears in July and August. However, under favourable conditions, a third generation occurs, and it is these larvae that cause the damage to oilseed rape crops.

“Last year the problem was not so widespread as in 2006, and it is not clear as to why numbers are currently high,” he says.

“The warning is there. People must be on their guard and be ready when their oilseed rape crop emerges this autumn if yield loss to sawfly is to be avoided,” advises Mr Bean.

Seed treatments like Chinook (beta-cyfluthrin + imidacloprid) have given some control, he says, but clothianidin (as in Deter) is better. Bayer hopes to have a new seed treatment for rape based on this active available this autumn.

Pyrethroids also control the pest very effectively, says Mr Bean, and spraying could coincide with early flea beetle control. Treatment thresholds remain unclear, but sawfly larvae are voracious feeders and should be treated once leaf-stripping is seen, he adds.