Growers and agronomists are reporting high numbers of cabbage stem flea beetle (CSFB) in oilseed rape crops, with crops just emerging being most vulnerable to attack.
Indigro independent agronomist Luke Wheeler, who covers Leicestershire, Northamptonshire, Bedfordshire and Lincolnshire, says Indigro caught four CSFB in yellow water traps between August 23-29, rising to 150 between August 29 and September 2.
“We had rain over the Bank Holiday weekend followed by warm temperatures which did not drop much in the evenings. From last Friday [September 7} numbers exploded. I think the reason they are so high is temperature and moisture related. Northamptonshire seems to have been worse than anywhere else.”
Growers able to get OSR crops in early seem to have benefited, says Mr Wheeler. “Anything that went in early – from August 10-15 does seem to have got away. If there are two true leaves out they [CSFB] don’t seem to bother it.
“If it was sown from August 15-25 and emerged last week it has been hammered. As soon as the cotyledons have come out it has been destroyed.
“That drilled from August 25 onwards, because it has been so dry, has just sat in the ground and is in a better situation.”
He says some seedbeds have been cloddy, which has not helped, but has been difficult to avoid in some situations because of the dry weather.
Where CSFB numbers exceed the threshold crops can be sprayed with a pyrethroid, but it is not worth repeatedly spraying, he says. “A few have re-drilled rape already and some will have an entire change of crop or re-drill in the next week.”
Falling day and night time temperatures and possible rain mean the current level of CSFB pressure is unlikely to be sustained for more than two weeks, adds Mr Wheeler.
Wiltshire-based grower Martin Smart began drilling OSR a week to 10 days ago and, with crops only just breaking the surface, has so far avoided CSFB;- however, neighbouring farms which drilled earlier are suffering, he says. “With the warm weather they come from nowhere. Anything drilled two weeks before mine is getting a pasting.”
While CSFB has traditionally been associated with the eastern counties, an informal survey of NFU members indicates it is spreading west and north, according to NFU senior plant health adviser Emma Hamer.
“It seems to be a bad year for cabbage stem flea beetle. The drought means there has not been a lot of moisture and plants have not been able to grow away from it.”
Resistance of CSFB to pyrethroids is an issue, with sample testing at Rothamsted indicating resistance levels of more than 50 per cent, according to Dr Steve Foster, research entomologist at the institute. “We are definitely finding similar levels of resistance to those found in the last several years, particularly in the South East.”
Growers can send CSFB samples to Rothamsted for resistance testing. Where there is a high level of resistance to pyrethroids there is no point in spraying, he says. “It is a waste of money, causes damage to the environment and will kill beneficial insects.”
For growers opting to spray, it is important for there to be contact between the insecticide and beetle, so application at dusk or night when the pest is active is recommended, says Mrs Hamer.
With ADAS research showing defoliation could be a method of reducing CSFB larval populations, some farmers are grazing sheep on oilseed rape, says Mrs Hamer. “Sheep eat the petioles and larvae in the plant. They can graze OSR during the winter to December/January before stem elongation so they are not damaging the growing point of the plant.”
Dr Paul Fogg of Frontier has received reports from all over the country of CSFB pressure, even as far north as the Scottish Borders. “It had been thought numbers were on the decline but I don’t think that is the case.”
He advises growers not to be too quick to write off crops. “Some affected crops can go on to do very well. View them on a case by case basis.”