Cambridgeshire oilseed rape grower David Walston is very unlikely to use neonicotinoid-treated seed this summer despite being in a flea beetle pest hotspot. He will opt for companion cropping instead.

After comparing the differences between treated and untreated crops, he believes using neonic-treated seed is not a guarantee to keep pests at bay and only has a small effect on flea beetle populations.

The European Union banned the use of three neonicotinoids on flowering crops such as oilseed rape due to their perceived damage to bees, although their limited use was allowed in four counties badly hit by cabbage stem flea beetles last year.

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“Last autumn, I had the chance to use Cruiser-treated seed, as my farm is in one of the four counties covered by the emergency use legislation,” he says.

“As a result, I opted to have about 40% of the seed treated with neonicotinoids, while the rest just received a standard fungicidal seed treatment,” Mr Walston adds.

In one 18ha field, where the variety Campus was being grown, half of the crop was treated with Cruiser (thiamethoxane).

No follow-up foliar insecticides were used, but a companion crop comprising buckwheat, lentils and fenugreek was sown at the same time.

“The companion crop is simply drilled down the same pipe as the oilseed rape and adds about £20/ha to the cost of the seed,” he explains.

Companion crop

Mr Walston was unable to ascertain any effect of the companion crop on flea beetle numbers, having encountered very high numbers of the pest across the whole field.

“However, I did see some differences with the seed treatments. On the non-neonicotinoid-treated area, there were 10.5 larvae/plant, while the treated area had 8.6 larvae/plant. So it appeared as though the treatment had some effect on numbers,” he says.

But the neonicotinoid-treated area had been drilled a week later at a higher seed rate, so the plants were smaller, he recalls.

“When I took that into consideration and did the maths –  comparing 33 plants/sq m to 58 plants/sq m – there were actually more larvae in the neonicotinoid-treated area. The figures came out at 345 v 503 larvae/sq m,” he says.

In another 45ha field of Picto, where no neonicotinoids were used on the seed, he was able to look at the effectiveness of pyrethroid sprays.

“I was advised to spray the field in September, as thresholds had been reached, so I left about one-quarter of it unsprayed as a comparison. I then treated the whole field about a month later, this time for stem weevil,” he says.

Where just one spray had been applied, there were 3.6 larvae/plant, compared to 1.5 larvae/plant after a two-spray approach.

“Although there does seem to have been an effect from the first spray, I’m happy with the decision not to spray all of the field,” he says.

As a result of his investigations, Mr Walston remains unconvinced about the value of neonicotinoids.  

“I won’t be using them this autumn. However, I will be continuing with companion crops,” he confirms.