Growers are being advised to watch out for signs of rape winter stem weevil (RWSW), following reports of the pest in eastern counties.
According to Hutchinsons technical manager Neil Watson the pest has the potential to be more damaging than cabbage stem flea beetle (CSFB) but often goes undetected. “It’s not a pest that’s monitored by the Rothamsted aphid 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 has established itself as the main problem over the past few years, more so than CSFB. 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.
One reason that RWSW can be so damaging is that adults will fly into the crop and lay eggs, without any signs of damage to the crop, this is unlike CSFB, where grazing damage is evident.
Also, adults can fly into the crop over a prolonged period, meaning that multiple treatments may be required to coincide with migration phases. “Fewer larvae can cause considerably more damage to the stem than CSFB,” he warns.
Mild weather prolonging egg laying
Neil notes that controlling adults in the autumn is the main way to tackle RWSW, as pyrethroids have little or no activity on eggs or larvae.
“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. However, adult activity and egg laying can continue throughout the winter as long as conditions remain mild, as is the case this year.”
After hatching, the larvae will develop into petioles which then tunnel into stems and feed throughout winter. They will then exit in March or April to pupate in the soil before a new generation emerges after flowering.
Before deciding to treat, the risk should be monitored closely, and Neil explains that the only practical way to detect the presence of adults 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.”
He recommends placing the traps just in from the headland, taking account of the prevailing wind or the nearest neighbouring oilseed rape crop.
“There is no established treatment threshold for RWSW, but if it is easily found in traps, then you know you have a potential problem.”
If treatment is necessary, it should be carried out just before egg laying, which is usually between 10 and 14 days after adults have been found in the crop. The effectiveness of these treatments is dependant on several factors, not least the presence of adults at the time of spraying.
Another is the level of KDR resistance to pyrethroids within the population. “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.”
