Farmers battling beetle pests in their oilseed rape said the weather and date the seeds were planted could be decisive factors in the success of next year's crop.
The cabbage stem flea beetle (CSFB) has become an increasing problem for arable farmers after neonicotionid seed treatments – previously used to control the pest – were banned by the EU due to fears over their impact on bee health.
Growers in Mid Norfolk said although the region had escaped the worst crop losses suffered elsewhere in the country, there were dramatic contrasts in the leaf damage seen between different fields.
Simon Brock, farm manager at Swanton Morley Farms near Dereham, said a crop planted on August 14 had grown very successfully, while a field of the same hybrid variety, drilled two weeks later on August 28 at Shipdham, was struggling to establish, with leaves full of holes eaten by beetles.
'The crops we drilled in mid August shot away,' he said. 'That was not me being clever – it was drilled in perfect conditions. It was moist and a bit cooler, so it grew like mad and I struggle to find any flea beetle damage. But toward the end of the month it dried and got hot again and that's where the problem was.
'The difference in the later fields is absolutely amazing. Anything drilled after about August 24 has had two insecticide sprays and has classic shot-holing, with lots of small holes. You can see the beetles hopping around on a sunny day.
'The advice was to drill a bit later this year, as they [flea beetles] do the most damage when it is hot and dry and the rape is not growing so well.
'But then we had that hot weekend at the start of September and they really came on.
'I think the crop will recover, but it needs some rain. The lesson from this year is to go back to drilling earlier in a typical year. You get a chance to have another go, and you might hit that perfect drilling window with a perfect seed bed. That is the key.'
Chris Eglington, who farms at nearby Letton, agreed the weather would also be a key factor in the crop's success - with a lack of rain exacerbating the damage from the beetles.
He drilled his fields on August 20 and 21 and some plants are only now starting to emerge.
'The problem is not just slugs and CSFB, it is moisture,' he said. 'The plants desperately need some rain. I have one field that might not make it. It has come through and just been chewed off.
'It is a combination of things. I actually think it is more to do with the weather, but if not for the pests we might have got away with it.'
'It wouldn't be anywhere near as bad if we still had the seed treatments. I would probably need to spray once or twice less.'
'The environmental lobby have the right intention, but the things they do can backfire. We are losing crops and using more chemicals now.'
SPRAYING ADVICE
Oilseed rape growers were advised to consider the effectiveness of their chemical treatments before spraying – and their possible impact on building insecticide resistance in pests.
Teresa Meadows, East Anglia knowledge exchange manager for the Agriculture and Horticulture Development Board (AHDB), said: 'There is still a lot of research that needs to be done, and there are thresholds at which times you are meant to go and treat the crop, depending on how much of the leaf area has been damaged, and monitoring for shot-holing.
'The key thing is thinking about the beneficial insects as well as the flea beetles. In considering how many times you are spraying pyrethroids you should also consider how it will affect resistance. That is why we are working with Rothamsted Research to collect live samples of beetles to assess their resistance to insecticides.'
• The AHDB website has published the latest CSFB research and recommended spray thresholds and resistance guidance about cabbage stem flea beetle.
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