A combination of cabbage stem flea beetle larvae infestation and heavy frosts have knocked oilseed rape crops back in recent weeks.
And despite the sub-zero temperatures seen in mid-February, CSFB populations are unlikely to have been affected due to the pest’s ability to survive extended spells of cold weather, says Frances Pickering, crop protection consultant at ADAS.
“Studies show that CSFB larvae are happy to survive for several days at -5degC continuously. The larvae are probably as present now as they were before the cold snap. However, as cold conditions reduce egg laying and egg hatch, the recent cold weather is likely to have reduced late larval invasion.”
In the autumn, earlier drilled crops generally showed the most promise, but now the picture is more mixed, Ms Pickering says.
“Larval numbers this year have been more variable than in recent years, but some crops have very high larval loads. Much higher pressures are being reported in early drilled crops compared to those drilled in the second half of September.
“We have found that if you have 10 larvae per plant you may get yield reductions of around 0.5 tonnes/hectare, with losses increasing with increasing larval load. However, the yield response is quite variable and some crops are able to achieve reasonable yields with high infestations. We do not understand what determines this variable tolerance and this is something we are investigating in a new AHDB and industry-funded project.”
Although there is little that can be done to mitigate yield losses, recent work on stem strength by ADAS found that lodging risk is increased if a stem has been damaged by CSFB.
“If growers have a crop that is looking fairly leafy and big, it might be worth using a PGR to reduce the risk of lodging going forward.”