Latest results of pollen beetle monitoring in oilseed rape crops have revealed an alarming increase in resistance to pyrethroid insecticides.

“Every year resistance gets worse and the UK is now in a situation where mainland Europe was a couple of years back,” says Kevin Gorman, a research entomologist at Rothamsted Research.

Resistant populations were found in the south, East Anglia and Scotland last year, increasing the risk of damage to flowerbuds and yields.

In tests, a sample from Wye, Kent, was “highly resistant”, Dr Gorman says. Mortality at the field dose was less than 10%. “It showed us that resistance was progressing at quite an alarming rate.”

Four samples taken from Suffolk showed different responses, but there were no susceptible populations. Two were classed as “resistant” and two were “highly resistant”.

“The maximum control people in Suffolk would be likely to get would be 60% in applying the field rate – and it could be lower still,” Dr Gorman notes.

Two samples of pollen beetles taken from spring oilseed rape from Scotland – one treated with pyrethroids and one untreated – were sent in by Andy Evans, of SAC, but Dr Gorman says there was little difference between them.

“There was a strong amount of resistance from a dosage where you would normally expect 100% mortality, but essentially we were getting nothing.”

Pyrethroid resistance is so widespread across the UK that growers should choose products with different modes of action, such as neonicotinoids, indoxacarb or pymetrozine.

The neonicotinoid, Biscaya (thiacloprid) costs three times the price of a typical pyrethroid such as Hallmark Zeon (lambda-cyhalothrin).

“We are in a very privileged position as normally when we come across resistance issues, it’s too late,” Dr Gorman says.

“With pollen beetle, there is only resistance to one chemical class at the moment (pyrethroids) and there are three that are still working.

“Now, more than ever, is the time to move away from pyrethroids and to steer efforts towards these new modes of action and use them all at the same time to make sure that no pressure is too great for any of these compounds.”

Latest results of pollen beetle monitoring in oilseed rape crops have revealed an alarming increase in resistance to pyrethroid insecticides.