The current threshold for cabbage stem flea beetle (CSFB) larvae of five per oilseed rape plant before the pest is considered to impact yields is based on trial data from the 1980s. Four decades on, with the industry still reeling from the loss of neonicotinoid seed treatments, reports of up to 80 larvae per OSR plant have been recorded.
At a recent BASF and ADAS meeting, the feasible hypothesis that later sowing of OSR – well into September – could markedly reduce the threat of CSFB was explored. While there are many practical workload ramifications during this crucial month, if delaying crop emergence until after the adult CSFB migration (typically in late August), allows a future for this important break crop, then maybe this change in approach is essential.
Speaking at the meeting were ADAS senior research entomologist, Dr Sacha White and plant pathologist, Dr Faye Ritchie, along with BASF’s agronomic services manager, Dr Carol Norris.
Enemy number one
Dr White described CSFB as “grower enemy number one” which at AICC’s estimation of the damage at 11 per cent of the UK crop currently in the ground (which is now likely to be an underestimation), the crop losses would equate to a conservative £79 million.
The late-drilling hypothesis is that later sown crops avoid some important CSFB life-cycle timings (see table over page). A host of different ADAS and BASF trials show that some varieties suffer a lower yield penalty than others when drilled late, which may be more than compensated for by reduced CSFB damage.
“The recommendation in recent years has been for early drilling, the theory being that well established, vigorous crops are better able to withstand adult flea beetle damage by the time CSFB migration occurs in late August.
“What many don’t appreciate is that larvae can have a greater impact on yield than the adult beetle damage,” he suggested.
Drilling later – from the start through to the end of September – is the only agronomic factor that has been found in modelling work to have a consistently positive success rate for pest escape, according to Dr White (see Graph 1 over page).
“September timing avoids the adult flea beetle migration and therefore later arrival and egg laying, together with cooler temperatures means slower egg development and lower levels of larval damage,” he added.
Trap crops and mowing
Other useful cultural control methods combined with delayed drilling that Dr White is investigating are: delayed spraying off of an adjacent field of volunteer OSR crop to act as a ‘trap crop’ studying the palatability of different varieties to CSFB (BASF trials have shown that there are differences between varieties but further trials are necessary to better understand whether some hybrids are better able to withstand adult feeding and larval damage than others), defoliating by mowing or grazing and planting varieties with more vigorous growth habits in the spring and autumn.
The ‘trap crop’ trials showed the benefit of doing nothing for a month post-harvest, with Dr White recording up to 89 per cent fewer adults, 74 per cent less adult damage, 39 per cent more plants and 67 per cent fewer larvae.
“What’s interesting about the ‘trap crop’ is that this capitalises on a biological quirk of adult beetles, which is that once they have migrated, they gradually lose their wing muscles over several weeks, so they won’t migrate again.”
The defoliation trials – mown in December, January and March – showed a reduction in larvae numbers of 31, 43 and 55 per cent respectively, but with a significant yield penalty in the March mown crops.
BASF’s Dr Carol Norris spoke about the impact of delayed drilling on yield and gross output, and cited work that she had been involved in with ADAS using older varieties.
“The crops were drilled on the 9th and 17th of September, what we found was that the yield of the hybrid variety Fencer was least affected by the later drilling. There was an average yield loss of 1.4t/ha for conventional varieties when drilled later but Fencer only lost 0.5t/ha at the later drill date. The hybrids all performed much better than conventional, open pollinated varieties in late-drilled situations.
“The trials also showed that the variety that developed the largest green area index (GAI) in November and December (Fencer) when sown late also achieved the optimum spring canopy and seed set in the spring,” she noted.
Further work at another site with three drilling dates supports this and shows that a very vigorous variety such as Fencer will lose less yield than conventional varieties when drilled well into September (see Graph 2). BASF’s InVigor portfolio now has varieties with even better autumn vigour than Fencer, so these are well suited to drilling in September.
Dr Ritchie added her views on the consequences of later drilling. She said that clubroot and light leaf spot control might be less of an issue, while phoma stem canker could be a greater concern.
“Phoma risk is linked to leaf size. The larger a leaf is, the longer it takes the fungus to grow through the leaf from leaf spots, reach the stem and for cankers to develop. Later drilling will increase the risk so choosing a variety with good resistance to Phoma stem canker will help.”
“It is clear that this year has been particularly bad because we’ve had the perfect storm for flea beetle, a dry summer and autumn during which plants were slow to grow away, followed by a mild winter and spring that encouraged adult beetles to carry on laying eggs,” Dr White said.
With limited, effective chemical control options, including a rise in the incidence of pyrethroid resistance in the east of England, the Midlands and as far west as the tip of Cornwall, cultural options are where we need to be looking for CSFB avoidance he noted.
“I’ve heard of growers spraying pyrethroids up to nine times; my advice is that if the first one, or two, sprays don’t have any affect, then further applications are pointless, and, compounding the problem, they will kill a range of beneficial insects that could help to control the pest.”