Cabbage stem flea beetle: The looming larval menace for oilseed rape crops
PUBLISHED: 14:10 07 May 2015 | UPDATED: 14:51 07 May 2015
© Archant Norfolk 2015
While this year’s oilseed rape may look healthy and vibrant, farmers fear a worrying infestation of beetle larvae could spell disaster for next year’s crop.
Ever since an EU restriction on neonicotinoid seed treatments was enforced in December 2013, the argument over whether the ban was justified has continued to rage.
Environmental campaigners point to scientific studies showing that these insecticides have serious, harmful effects on bees and other pollinating insects, while farmers argue there has not been enough data from realistic field trials for a fully science-based decision to be made.
But while that debate continues, farmers’ concerns are now focused on the potential damage which pests can wreak on important crops in the absence of one of the chemicals used to control them.
Last autumn was the first season that neonicotinoid seed treatments were not available to protect emerging winter oilseed rape (OSR) seedlings from damage by cabbage stem flea beetles (CSFB).
Norfolk farmer Chris Eglington, who grows 200 acres of OSR at Letton, near Shipdham, said he had to spray a lot more in the autumn to prevent this year’s crop being destroyed, and he thought he’d done a reasonable job killing off the adult beetles – but not before they had laid thousands of eggs.
Although his plants look healthy above ground, their leaf stems are riddled with cabbage stem flea beetle larvae which will soon move underground to pupate. The adult beetles will emerge in June or July and could be ready to devour the next crop after it is planted in August.
So without seed treatments, and with the beetles developing resistance to pyrethroids, the only other available pesticide sprays, Mr Eglington said it is next year’s crop which he is most worried about.
He said: “Until three or four weeks ago, I had not realised I had a particular problem. It was only when some specialist agronomists came out and were actually looking for them that we found them. So that was not a really happy day for me. On one plant, we found eight larvae in a single leaf.
“We cannot kill them at this stage. We have got to kill the adults. We thought we had done a good job of that, but obviously we had not done a good enough job to stop them laying their eggs.
“We have got strong-looking healthy plants, so I don’t think it is a massive problem for this year’s crop. But we are sitting on a potentially massive problem for the future.
“In my particular farm, where I have grown OSR this year, there will be a huge amount and the adult beetles will be ready to attack the new plants next August.
“If we can’t control them, they will completely destroy the crop. As soon as it comes up, they start attacking it. People are already saying that the amount of rape that’s going to be grown next year is going to be a lot less, and I know two or three farmers who have said this year will be their last year.
“Looking at the plants now, you would not think there is too much wrong with them, but there will definitely be a yield loss – there has to be, because the larvae are sucking goodness out of the ground that should be going into the plant.”
Mr Eglington believes he could be the only farmer in the country who combines subsoiling with precision drilling of oilseed rape, at 21 plants per square metre. The aim of growing the plants more sparsely is to allow more sunlight to reach the leaves rather than reflecting off the yellow flowers, aiding photosynthesis.
After his OSR fields were planted in August Mr Eglington said he had to spray them six times to kill the flea beetles, where we would normally have sprayed it three times.
“That has an effect on our carbon footprint and the amount of chemicals we use,” he said. “I am trying to be a good farmer, but I think the lack of neonics is not helping us at all.”
Guy Gagen, chief arable crops adviser for the National Farmers’ Union (NFU) said: “Sadly I think there will be less oilseed rape grown nationally, in particular in East Anglia, because the risks that farmers are facing are increasing all the time.
“It is not just about seed treatment, it is about general control – and we are starting to see resistant populations as well. The more simple, straightforward insecticide we could have used is now not effective in large parts of East Anglia. That has only emerged in the last couple of years and, again, is stacking the odds against the farmers.
“Those campaigning claiming to be representing the interests of pollinators tend to dismiss agriculture as not being relevant at all. They say farmers should just grow something else.
“But OSR is a very important crop economically, and for the whole agricultural system. It is very versatile and produces an oil used in a multitude of products from mayonnaise and salad cream to biofuel. Also, some people forget that most of what is harvested is used as animal feed, at a time when we import a lot of the animal proteins the we use.”
The flea beetle menace
Cabbage stem flea beetles are widespread in the UK and northern Europe. They migrate into OSR crops soon after emergence, chewing through leaves and causing “shot-holing’ symptoms, which can result in stunted growth and, in severe cases, can even kill the seedlings before they emerge. Their main natural predators are wasps, which eat the larvae in the spring.
Caroline Nicholls, research manager for the Home-Grown Cereals Authority (HGCA), said the East and the South East were the areas worst affected by oilseed rape losses last year – but the extent of any future crop damage will be largely affected by the weather conditions.
“If the weather is optimal, we could end up with lots of larvae developing into adults, and then it depends on what the weather is like when the new oilseed rape crop is establishing itself,” she said.
“Last year, conditions were already dry and the crop was slow to establish, so that meant the beetles were able to eat it quicker than it was able to grow.
“In the East area (comprising Norfolk, Suffolk, Cambridgeshire, Bedfordshire, Hertfordshire and Essex) we lost 10pc of the crop, but 3pc was successfully re-planted later in the autumn.
“So overall there was a 7pc loss, and the restriction on the use of neonicotinoids could potentially have affected the crop losses that were recorded, because of the limited control options to reduce the damage.
“If you have only got the same chemistry you have now, which is pyrethroids – and we confirmed resistance to that last autumn – it could end up with a situation where you lose control of that pest, with more resistance developing.”
Neonicotinoids: The debate continues
Recent studies have reignited the war of words between the farming industry and environmental campaigners about the effect of neonicotinoid pesticides on bees.
A study by Newcastle University found that bees may be attracted to food sources containing banned neonicotinoids, even though the consumption of these pesticides caused them to eat less food overall.
Meanwhile a Swedish field trial, also published in the science journal Nature on April 22, found that use of the neonicotinoid seed coating reduced the density of wild bumble and solitary bees in flowering oilseed rape fields.
Friends of the Earth campaigner Paul de Zylva said the studies added more weight to the calls for the temporary ban to be extended.
He said:“There has been a concerted campaign by the NFU and Crop Protection Association (CPA) variously claiming that the restriction is leading to large losses of crops. Those organisation also make wild claims that the restriction is politically-motivated and not based on science, when the restriction is based on the most thorough reviews of evidence ever carried out – more thorough than any studies or tests carried out by the pesticide companies or by the NFU and CPA themselves.
“It is important to recognise that reduced crop yields can be affected by a number of factors especially poor growing conditions or bad weather. It is unscientific to automatically attribute any reduced yield of a crop to any single factor unless proper studies have been carried out and factors ruled out.
“The two-year duration of the restriction should have been used by the government, research institutions and indeed, the NFU and CPA and others, not to blame the ban for crop losses that have not occurred, but to get behind researching safer ways to produce and protect crops which can give us all genuine assurance that their use does not harm our bees, our water and our soils.”
Nick von Westenholz, chief executive of the CPA, said: “The latest studies in Nature must be seen in the context of an ongoing campaign to discredit neonicotinoid pesticides, regardless of what the real evidence shows. One of the studies, again conducted in the laboratory rather than under realistic field conditions, makes the claim that bees have a slight preference for foraging on crops treated with these pesticides and implies they are more at risk. What’s important is not whether bees show a slight preference for these crops, but that there is no effect on their health when field-realistic amounts of these pesticides are used.
“It is a shame that the debate around the use of these important technologies appears to be increasingly politicised, with anti-pesticide activists consistently promoting their agenda under the auspices of independent research. Meanwhile, the only effect of the restriction on neonicotinoids in Europe so far has been a steady stream of reports from farmers that their crops are suffering serious losses.”
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