It seems France could be to blame for this autumn’s explosion of turnip sawflies, which are causing significant levels of damage in southern and eastern oilseed rape crops.

“They are mainly migrants from France,” ADAS entomologist Jon Oakley said today [2 October]. “I’ve found that large numbers were recorded coming across.”

Warm September southerly winds helped bring the pest across the channel, he believed. “Once they were here they’ve found our conditions highly conducive.”

Large numbers of adults had been recorded in ground-placed water traps in oilseed rape crops as far north as Shropshire, while larval damage, caused by feeding on leaves, had been reported in several southern counties.

“I was in north Wiltshire yesterday and in places they had taken the crop out, so it is pretty severe.”

The pest could be controlled using pretty much any pyrethroid with approval for cabbage stem flea beetle, Mr Oakley thought. “The suggestion is from France and Germany that a threshold of 1-2 larvae per plant should be used, although we don’t have any work on the validity of this in UK conditions.

“My concern is if we don’t take them out now, come next May, when it re-emerges, we could have an awful lot around, which could do a lot of damage to flowering crops.

“So I’m edging towards saying if they are around the threshold level take them out now.”

The larvae look like little black caterpillars and grow to around 18mm, while the adults are yellow and black in colour.