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Oilseed rape growers aiming to combat cabbage stem flea beetle should take action to ensure they don’t worsen the problem. ‘Double trouble’ from flea beetle on oilseed rape
  • Weaker stems and higher lodging risk

  • Problem is often widely underestimated

  • Severe lodging reduces oil and yields

Oilseed rape growers aiming to combat cabbage stem flea beetle should take action to ensure they don’t worsen the problem.

Dual damage caused by flea beetle affects stem thickness and increases lodging risk, confirms research by ADAS. Growers can manage the problem – buy need to be careful when doing so.

ADAS entomologist Fran Pickering shared her research results during a BASF Virtual Farm webinar. Flea beetle weakens plant stems, restricts growth and makes crops increasingly prone to lodging, she says.

Lodging is estimated to cost the oilseed rape sector £47-120m annually, according to ADAS figures. The problem is widely underestimated, says ADAS head of crop physiology Pete Berry, who also spoke at the webinar.

New research

Some growers have adapted their crop management to combat flea beetle. In doing so, they have inadvertently increased the potential of lodging – but with careful monitoring  both scientists said they were  confident the risk could be managed.

“It’s long been assumed that flea beetle larvae and their activity affect stem strength,” acknowledged Ms Pickering. “But we wanted to know was by how much? Taking it one-step further, what impact does that have on lodging?”

ADAS field trials took 50 plants at the end of flowering in May from two sites in East Anglia. The team assessed those plants’ stem diameter, breaking strength and the internal damage caused by the flea beetle.

By assessing the percentage area of stem damaged, the plants were sorted into five categories. These ranged from plants with less than 5% damage (minimal) to plants with 76-100% of stem area damaged (severe).

Ms Pickering says: “There was a clear trend showing thicker stems were stronger – this is not news. However, the results also showed that higher damage was associated with the thinner stems. But why?”

The type of damage caused by flea beetle was predominantly mining and browning at the base of the stem. This type of damage restricts the take up of nutrients and water – which in turn restricts growth.

“Plants taken from the second site revealed that for any given stem diameter, more damage makes stems weaker. The larval feeding restricts thickness of the stem and hollows stems, reducing their strength.

“Even low levels of flea beetle damage significantly reduces stem strength,” says Ms Pickering. “On average stems with less than 25% damage were 29% weaker.”

Lodging impact

An ADAS survey of over 2,000ha in 2012 showed that 35% of oilseed rape crops had lodged. Some 99% of fields had some lodging and more than 70% of the crop lodged in 20% of fields.

In another experiment, ADAS researchers mimicked natural lodging in crops to assess the impact at harvest. Where crops lodged at 900 between flowering and early seed fill, nearly 50% of yield was lost.

Where the crop was lodged at 45 degrees, about 20% of yield was lost. Even modest leaning at about 22 degrees reduced yield by 7-16%. Severe lodging also reduced oil content by up to 8%.

Dr Berry says the reason behind this extreme loss was that any leaning or lodging squashed the leaves and the pods together. This impacted on light capture and efficiency of use, he adds.

Growers can take numerous actions to minimise lodging, says Dr Berry. Farmers with large canopies should consider delaying or reducing the first nitrogen split – and using a plant growth regulator, he adds.

Fran Pickering