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The greatest sowing flexibility is the key to taking advantage of the much-improved prospects for oilseed rape with the least possible risk, believes ADAS entomologist, Dr Sacha White at the forefront of industry efforts to manage the cabbage stem flea beetle threat through better understanding.

“Oilseed rape is looking an altogether better bet for many than it did this time last year,” he points out. Establishment conditions favouring the crop more than the pest last autumn and generally good crop development despite the wet winter and cold spring – not to very encouraging crop values – are leaving many who persevered with OSR thankful they did and many who didn’t regretting their decision not to.

“Far less oilseed rape in parts of the country badly hit by CSFB in 2018 and 2019 and generally lower larval populations in our latest national study with Bayer this season are likely to reduce the pest population pressure for the coming one. Our modelling work for AHDB also suggests the very wet conditions of last December won’t have done it any favours either.

“On the other hand, though, this modelling also tells us that cool Mays tend to be associated with greater CSFB problems than warmer ones. And, most importantly, that so much of the cropping risk comes down to conditions in the run-up to sowing and those first critical weeks of establishment.

“As many growers found to their cost in moving away from the crop last year, it’s very dangerous to base decision-making on the previous season’s experience,” warns Dr White.
“With the climate as uncertain as it has become, things can change very rapidly.

“Instead, we all need to be alert to the most critical success – and failure – factors around oilseed rape establishment and flexible enough to adapt our cropping to them. More than anything else, this means knowing when (and when not) to sow oilseed rape.”

What does this mean in practice then?  And how can growers keen to make the most of the  particular rotational as well as financial value of the crop minimise their risk of another season like 2019?

While we are not yet able to predict the timing of egg hatch or the level or duration of adult migration, the past few years have taught us two crucial lessons, in Dr White’s view.

First, that the crops at greatest risk are those emerging into the peak of migration and/or having insufficient ability to grow away from flea beetle grazing. And second, that hot, dry and still weather invariably favours the pest and disadvantages the crop while unsettled, wetter conditions do the opposite.

“It’s crystal clear that the crops most vulnerable to adult CSFB damage are those sown from the third week in August to mid-September,” he stresses. “Earlier drilling can be valuable in getting crops established ahead of the peak of migration and later drilling has also proved a useful escape strategy from larval damage (and often see less damage from adults too if crops emerge after the peak of migration). But both come with their own challenges and, for many, may simply not be feasible workload-wise.

“Equally, the risk of establishment failure – from other factors as well as CSFB – fundamentally depends on conditions around sowing.  So, regardless of when you plan to sow, the most important thing you can do to reduce your risk is only to do so when you are sure of enough moisture to get your crop established.”

Dr White accepts this may mean holding-off on sowing. Or even deciding not to sow the seed you have. But he insists this has to be better than putting time, effort and money into what could all too easily be a lost cause from the start, pointing out that earlier-drilled crops that don’t have enough moisture to get away will be just as vulnerable to flea beetle grazing as those sown in the highest risk period.

As well as maintaining as much drilling flexibility as possible, he insists that establishment essentials like well-structured soils, vigorous fast-developing varieties (though not too fast to guard against the risk of pre-mature stem extension), seedbed moisture preservation, seed-to-soil contact and consistent sowing depth are vital in minimising oilseed rape risk.

“On top of these, there are a number of measures like sowing into long cereal stubbles, companion and trap cropping that growers have been finding useful in reducing adult CSFB damage,” he notes.  “These are progressively more important as moisture conditions become more marginal and the weather and drill timing favour the pest.

“We are understanding more about cabbage stem flea beetle the whole time. Just like resistant black-grass, it’s something we have to learn to live with. As is its insecticide resistance.

“This season is giving our battle with the beetle a welcome confidence boost. But we could easily be back to Square One next season if we don’t maintain sufficient cultural control pressure – the first key to which, I have no doubt, is rotational and sowing flexibility.”


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