Rapid crop development in spring is one of the areas attracting interest in the battle against cabbage stem flea beetle. Teresa Rush reports.
This autumn will be the third time growers have faced the prospect of planting oilseed rape without a seed treatment to control cabbage stem flea beetle (CSFB).
According to a recent NFU analysis, the combined cost of managing pest-damaged winter OSR crops and yield loss amounted to £41 million last year.
Since the suspension of neonicotinoid seed treatment use, growers have looked to a range of techniques to limit the damage, with mixed success.
Independent agronomist Andrew Blazey, along with his Prime Agriculture colleagues, advises across 17,000 hectares of OSR in Norfolk, Suffolk, Essex, Cambridgeshire and Hertfordshire – counties which host some of the most severe CSFB damage hotspots.
He calculates CSFB control in autumn 2014, the first year of the ban on neonicotinoid seed treatment use, cost his growers an extra £62/ha on average, comprising three foliar pyrethoids and a foliar neonicotinoid spray for turnip yellows virus (TuYV) control.
While an AHDB/ADAS national survey put crop loss at 2.7%, the average figure in his region was nearer 10% that autumn, he says.
In other responses, Mr Blazey and his growers cut their OSR area – from more than 2,700ha in 2014, to below 2,000ha this year – and adopted a range of agronomy approaches in an attempt to limit the impact of the pest. These have included switching from hybrid to conventional varieties, early drilling, increasing seed rates, aiming for early removal of volunteer OSR from the previous crop, improving seedbeds and using a starter fertiliser.
Last autumn saw much easier conditions, says Mr Blazey. “Slightly cooler temperatures, average rainfall, a bit of moisture when you needed it and the most important thing was the crop got away and had sufficient moisture.
“My losses were somewhere less than 2% and these were late August/early September drillings.
“The CSFB adult feeding was as bad, but the growing conditions were better. Thresholds were triggered on most fields and a similar level of foliar insecticide was used on non-neonic crops.”
But despite the kinder autumn, high levels of CSFB larval invasion were seen this spring, contributing to further crop loss, and this came after growers had already committed more than 50% of crop variable costs and spent more than £200/ha on establishment and application costs, says Mr Blazey.
He does not, however, believe the neonic seed treatments provided all the answers when it came to CSFB control. He highlights trials on one farm where larval invasion levels in spring where higher where a seed treatment had been used, compared to where a three-spray insecticide programme was used in autumn. “But the crop did get up and away more quickly in the spring where the neonic was used,” he says.
This ability of OSR crops to grow quickly away from larval damage early in spring – assuming they have got to that point – is attracting increasing interest as growers seek out new approaches to help tackle the CSFB threat. And the focus is on how individual varieties perform in this respect.
The latest research and development work from Dekalb indicates OSR hybrids which are faster to develop before winter and earlier to commence stem elongation after winter can limit damage from CSFB larvae.
National Institute of Agricultural Botany OSR expert Simon Kightley also believes fast development early in spring may enable OSR crops to grow away from CSFB larvae damage. Early flowering is an indicator of varieties likely to show early stem extension, he adds.
During this season’s slow, cold spring, continued larval activity was apparent in the rosette leaves of OSR plants, says Mr Kightley. But while early-flowering types, with rapid stem extension appeared to grow away from this activity, slower-developing, later-flowering types remained at the rosette stage for longer.
And according to Bayer seeds and traits manager Sarah Middleton, growers with crops of the company’s hybrid variety Harper – an early flowering variety with good spring vigour – were reporting fast, strong recovery from pest and winter damage.
The Dekalb study, carried out with a Cambridge breeding trial significantly affected by flea beetle, showed some of the more than 30 varieties grew to the standard, untreated protocol, losing 40% or more of their main stems to the pest and showing serious stunting at flowering. In contrast, other varieties lost no stems whatsoever and showed little or no stunting.
Dekalb technical specialist Anders Christensen says: “Contrary to popular wisdom, higher levels of main stem loss tended to result in a slight decrease, rather than any increase, in oilseed rape branching.
“They also resulted in clear delays to flowering.
“We recorded obvious differences between varieties too, with those developing more rapidly before winter losing markedly fewer main stems and suffering noticeably less stunting from flea beetle larvae than those developing less rapidly. The faster autumn developing varieties also tended to be better branched at flowering.
“Main stem losses and stunting tended to be lower in varieties, moving into stem elongation earlier after winter than in those taking off less rapidly.”
Dekalb says the study showed its varieties DK Extrovert, DK Exalte, DK Exception and DK Exentiel were better able to tolerate larval feeding from the early winter as well as adult flea beetle damage at establishment.
This ability to grow away from flea beetle larvae damage appeared to be enhanced by inherently earlier stem elongation.
Rapid development before winter and early stem elongation after winter should also be valuable in providing tolerance to slug and pigeon problems, Mr Christensen adds.