Serving the farming industry across East Anglia for over 40 years
• Pest reported in eastern region crops • Monitor rape carefully, growers told • Mild weather can prolong problems Oilseed rape growers are being... ‘Hidden threat’ posed by winter rape weevil

• Pest reported in eastern region crops

• Monitor rape carefully, growers told

• Mild weather can prolong problems

Oilseed rape growers are being urged to watch out for signs of rape winter stem weevil – after several reports of the pest appearing in eastern region crops.

Despite having potential to be more damaging to oilseed rape than cabbage stem flea beetle, rape weevil often goes undetected, says Neil Watson, technical manager for agronomy firm Hutchinsons.

“It’s not a pest that’s monitored by the Rothamsted aphid survey, so we must rely on observations from the field,” he says.

“This season, we’ve primarily seen the weevil appear in crops in the eastern counties, where it has established itself as the main problem over the past few years – more so than cabbage stem flea beetle.”

But Mr Watson says he can’t rule out the pest being present in other areas too. “It has become resurgent primarily due to the decline in spraying for cabbage stem flea beetle adults in the autumn.”

Adults often fly into the crop and start laying eggs without making any outward signs of damage. This is a key reason why winter weevils can go undetected and be so damaging, explains Mr Watson.

“This is unlike cabbage stem flea beetle adults, where grazing damage is all too evident,” he adds.

Additionally, unlike flea beetle, adults can fly into crops over a very prolonged period, therefore multiple treatments may be required to coincide with migration phases, especially when they are still in flight at this time of the year.

“Fewer larvae can cause considerably more damage to the stem than cabbage stem flea beetle.”

What to do

Controlling adults in the autumn is the main way to tackle weevils, as pyrethroids have little or no activity on eggs or larvae, says Mr Watson.

“Adults typically emerge in the summer and after a resting phase move into oilseed rape crops to feed and lay eggs in leaf stalks during late September and October. But adult activity and egg laying can continue throughout the winter as long as conditions remain mild, as is the case this year.”

After hatching, larvae develop in petioles then tunnel into stems and feed throughout the winter until exiting in March and April to pupate in the soil before a new generation emerges after flowering.

Risk should be monitored closely before deciding on the need to treat, and the only practical way of detecting the presence of adults in the crop is by using yellow water traps.

“French research shows water traps are not a good indicator of the level of damage that is likely to occur, just the presence within the crop.”

Growers are advised to place a few traps in each crop, just in from the headlands, taking account of the direction of the prevailing wind or nearest neighbouring oilseed rape crop.

“There is no established treatment threshold, but if it is easily found in traps, then you know you have a potential problem.”

If treatment is required, it should ideally be applied just before egg laying, which is usually 10-14 days after adult RWSW have been first found in the crop. “It might mean multiple applications being linked to multiple migration phases into the crop.”

Treatment

Effective treatment depends on several factors though, not least the presence of adults in the crop at the time of spraying, says Mr Watson.

Another issue is the level of KDR resistance to pyrethroids within the population, he adds.

“Resistance has been confirmed in France, so we can only assume it’s the same in the UK, although no data has been collated here.”