According to the Home Grown Cereals Authority (HGCA) resistance to pyrethroid-based insecticides in UK populations of cabbage stem flea beetle (CSFB) is now widespread. This assertion is based on the results of the organisation’s latest screening results, it says.

Following a pre-harvest call for suspected pyrethroid-resistant CSFB samples to be sent to Rothamsted Research for analysis, 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 tillage crops 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.

“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.

According to HGCA agronomists, this 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.

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 October/early November.