Watch out for signs of rape winter stem weevil, is the message from experts following reports of the pest in the Eastern Counties.

Despite having the potential to be more damaging to oilseed rape than cabbage stem flea beetle, RWSW often goes undetected, according to Neil Watson, Hutchinsons technical manager.

“It’s not a pest that’s monitored by the Rothamsted insect survey, so we must rely on observations from the field. This season, we’ve primarily seen RWSW appear in crops in the East where it’s established itself as the main problem during the past few years, more so than CSFB,” advises Neil.

“However, we can’t rule out the pest being present in other areas too. RWSW has become resurgent primarily due to the decline in spraying for CSFB adults in the autumn,” he notes.

Adult migrations

One of the main reasons RWSW can go undetected and be so damaging, is because adults often migrate into the crop and start laying eggs without making outward signs of damage, says Neil. “This is unlike CSFB adults where grazing damage is all too evident.”

Additionally, unlike CSFB, adults can fly into crops during a prolonged period, therefore multiple treatments may be required to coincide with migration phases especially when they’re still in flight at this time of the year, he suggests. “Fewer larvae can cause considerably more damage to the stem than CSFB.”

Pyrethroid failure

According to Neil, controlling adults in the autumn is the main way to tackle RWSW because pyrethroids have little or no activity on eggs or larvae.

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

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

Monitoring

Neil says risk should be monitored closely before deciding on treatment, with the only practical way to detect the presence of adults being yellow water traps. “French research shows water traps aren’t a good indicator of the level of damage that’s likely to occur, only the presence within the crop.”

Neil recommends placing a few traps within each crop, just in from the headlands, taking account of the direction of the prevailing wind or nearest neighbouring OSR crop. “There’s no established treatment threshold for RWSW, but if it’s 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, he adds.

“The effectiveness of any treatment is dependent on several factors though, not least the presence of adults in the crop at the time of spraying. Another issue is the level of KDR resistance to pyrethroids within the population,” suggests Neil.

“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,” he concludes.