Resistance to pyrethroids in UK populations of cabbage stem flea beetle (CSFB) is widespread according to the latest screening results.

Following a pre-harvest call for suspected pyrethroid-resistant CSFB samples to be sent to Rothamsted Research for analysis (as part of HGCA Project 214­–0019), knock-down resistance (kdr) to pyrethroids has now been confirmed in every UK sample tested so far.

HGCA is now calling for more growers and agronomists to send suspected pyrethroid-resistant CSFB samples to Rothamsted Research to build an even more robust picture of resistance in the UK.

Caroline Nicholls, HGCA Research and Knowledge Transfer Manager, said: “The researchers at Rothamsted developed a genetic test to look for the mutation associated with kdr resistance previously reported in Germany.

“The results found kdr-RR pyrethroid-resistant beetles in all samples tested so far.

“This is a cause for concern as this type of resistance is likely to cause control problems with all pyrethroids applied at recommended field rates.

Testing results (as of 15 September 2014)

SS = Homozygous susceptible beetles. SR = Heterozygous susceptible beetles (carries kdr but does not confer resistance to pyrethroids). RR = Homozygous resistant beetle (carries kdr which confers a moderate resistance to pyrethroids applied at recommended field rates).

“In fact, the frequency of kdr-RR individuals was surprisingly high – almost 60% of all beetles tested were classified as kdr-RR.

“Samples were taken where resistance was suspected, so there is likely to be a bias in the results but the data confirm that resistance is widespread.”

Further to the genetic test, live beetles were also placed in glass vials coated with lambda-cyhalothrin – the pyrethroid which is found in a number of insecticides authorised for use in oilseed rape.

A range of doses, including the equivalent of recommended field rate, was tested. Once again, resistance was detected in all samples.

More adult beetle samples required

Miss Nicholls continued: “We have been contacted by a number of growers concerned about CSFB in emerging oilseed rape crops wanting to send in samples for analysis.”

“Unfortunately, there isn’t a tried-and-tested approach for collecting live samples of CSFB in the field at this time of year.

“We found the best way to catch live beetles is to go out at night with a torch to find beetles. Once located, beetles can then be trapped using a jar.

>>> see the sampling video

“We are urging growers who feel they are experiencing CSFB control issues to follow the sampling guidelines on the HGCA website (www.hgca.com/neonics) and to send CSFB samples off for analysis by Rothamsted Research.”

Only apply pyrethroids in response to risk

At this stage, the evidence reinforces the point that any decision to apply pyrethroid insecticides must be based on risk.

Failure to do this will place additional selection pressure on resistance mechanisms and could result in further control issues in years to come.

If a pyrethroid application is deemed necessary, it should be applied at full recommended field rate.

There are reports of beetles being easier to find at night which may aid monitoring and possibly make for a better spray target but there is no research to confirm the effectiveness of this strategy.

Emerged oilseed rape plants will now be less vulnerable to damage to the precious growing point. However, CSFB will lay eggs at the base of the crop and, if conditions are mild (>3˚C), larvae may enter plants to feed from October to early April.

Research suggests that larvae are worth controlling if more than 35 beetles are caught in yellow water traps or if two or more larvae per plant are found in late Oct/early Nov.

HGCA’s revised CSFB publication (Information Sheet 24) provides further information on assessing the need to spray larvae as well as adult beetles.

HGCA is also making people aware of NFU’s call for reports of cabbage stem flea beetle damage to be sent to them. To find out more, click here.

www.hgca.com/pests