Insecticide resistance confirmed in UK flea beetles

Researchers have confirmed the presence of resistance to pyrethroid insecticides in populations of cabbage stem flea beetles in the UK.

The news adds to concerns for oilseed rape growers who, for the first time this autumn, have no neonicotinoid seed treatments to help protect their crops from pest attack.

This leaves foliar-applied pyrethroid insecticides the only method of combating damage caused by flea beetles – and if pyrethroid resistance is widespread, control will be challenging.

See also: Delay oilseed rape drilling to reduce pest and disease threat

Monitoring advice

  • Check numbers of beetles in the previous crop’s harvested seed
  • Use water traps in field to check beetle numbers
  • Assess damage OSR volunteers in stubbles

HGCA research and knowledge transfer manager Caroline Nicholls says the full extent of the problem has not yet been revealed, with only 12 samples tested so far from the east of England.

“We asked for samples to be sent in for testing where control failures using pyrethroids had been observed, so it doesn’t come as a surprise that resistance is present.

“However, it is difficult to say how widespread the resistance is until results from ongoing testing of samples collected elsewhere in the UK are released,” she says.

Turning point

Miss Nicholls explains that the testing work carried out at Rothamsted Research has uncovered varying degrees of resistance in the samples, from beetles that have survived a full dose of the pyrethroid lambda-cyhalothrin (RR), to some that may survive a lower dose (SR) and those that are still completely susceptible (SS).

“It was the same story in Germany and shows us that we are just at the turning point, so it is vital that growers follow resistance management guidelines to slow occurrence of pyrethroid resistance in flea beetles,” she explains.

Research by Adas has shown that oilseed rape crops can tolerate a level of the characteristic “shot-holing” (shown below) caused by flea beetles feeding on young plants, even at the cotyledon stage.

Cabbage Stem Flea Beetle shot-holing

Flea beetle treatment thresholds

Cotyledon stage to two true leaves

  • Consider spraying when 25% of the green leaf area of the whole crop has been eaten.

Three- to four-leaf stage

  • Consider spraying when 50% of the green leaf area of the whole crop has been eaten.

But the most vulnerable time is just before emergence when beetles can damage the growing tip and this can potentially wipe out whole crops before they see daylight.

Foliar sprays cannot be applied to bare soil, so the window for treatment at emergence is very narrow and with no thresholds outlined at this stage, monitoring for pest pressure provides the only indication to whether treatment is required.

“Close monitoring of the pest is crucial to substantiate numbers and a pyrethroid spray should only be applied where it is absolutely necessary to prevent the spread of resistance,” says Miss Nicholls.

She adds that getting the crop off to the best possible start is even more crucial after the confirmation of resistance in the flea beetle population.

“It is important to pay attention to a wide range of factors such as soil moisture, temperature, seed rate, variety and fertiliser choice.”