The number of cabbage stem flea beetles with resistance to pyrethroid insecticides is rising and becoming more widespread, a Rothamsted Research study has found.
PhD student Caitlin Willis, who is leading the study says: “The state of resistance is pretty much as expected.
"Compared to the 2018 resistance monitoring data, the percentage of resistant cabbage stem flea beetles per sample has increased, an increase now observed across most of England.
"Although only 50 samples were tested in bioassays in 2018, of these samples all bar one sat in the 0-75 per cent level of resistance categories.”
In 2019, over 200 samples of cabbage stem flea beetles have been tested to date.
Of these samples 22 per cent sat in the 75-99 per cent category of resistance, while some displayed 100 per cent resistance to a dose equivalent to the field rate, something not previously observed, Ms Willis adds.
While there are some parts of the country in which resistance levels are clustered, there are also areas where there is a wide variation within a small vicinity.
“It looks like resistance is localised to small areas or farms,” Ms Willis says.
In a separate study led by PhD student Patricia Ortega-Ramos, the CSFB sent in for resistance testing will be assessed to see how well they can be controlled by parasitic wasps.
Studies of the wasp’s lifecycle have revealed that its larvae develop inside the body of the adult CSFB, killing the beetle when leaving its body to pupate.
Rothamsted are still looking for growers in Scotland and the North East and North West of England to submit CSFB samples for resistance and parasitisation testing.