Look out for unexpectedly poor control of pollen beetles by pyrethroid insecticides used on oilseed rape this season and be prepared to report it.

The message, from ADAS, comes against the background of widespread insecticide resistance among the pests in France and Germany.

“We have been checking a number of apparent control failures in the UK,” says entomologist Jon Oakley.

“It is probably only a matter of time until we find resistant populations in our crops.

“The problem is that we grow a wide mix of brassica species.”

Where these are close together, the beetles can migrate from winter rape to spring rape and then on to horticultural crops, says Mr Oakley.

So while winter rape growers may be the first to treat against the pest, by the end of summer the same populations may have been subjected to two or three further sprays on other brassicas, increasing the resistance pressure.

“It doesn’t help that both rape and vegetable growers use the same group of pyrethroids for control.

Pollen beetles can fly quite a distance – often more than 10 miles.

It is not impossible that thunderstorm activity could carry the pest still further, maybe even across the Channel.”

Pollen beetle levels in the UK have risen in recent years, he adds.

Three years ago it was rare to find populations exceeding threshold levels.

Now 20-30 beetles a plant are common.

Temperatures must reach 16C (61F) for the pests to fly and so far few have come out of hibernation.

But when the weather does warm, they will reach crops quickly and their arrival will probably coincide with a spurt in growth, the anthers of early emerging flowers providing food, warns Mr Oakley.

Spraying should be threshold-driven, with the triggers varying according to crop stage, he advises.

On well-established, healthy crops of winter oilseed rape at green or yellow bud, growers should apply an insecticide when there are 15 or more beetles a plant.

In poor or pigeon-damaged crops treatment of as few as five beetles a plant at the green bud stage is advisable.

For spring crops the threshold is only three per plant and it is likely that two sprays will be needed.

The resistance on the other side of the Channel is thought to be have been caused by over-use of pyrethroids.

German growers in particular often have to contend with stem weevils, which means their crops regularly need a further insecticide.

The problem was first spotted in France in 2002 when control from certain pyrethroids, for example lambda-cyhalothrin and cypermethrin, fell dramatically.

The exception to this was with tau-fluvalinate (as in Mavrik), notes Elizabeth Spence, development manager with supplier Makhteshim.

Farm observation at the time and subsequent French field and laboratory tests show it remains 30% more effective than other pyrethroids, she says.

“It seems to have a different more stable esterase-based formula compared with other pyrethroids, which the pest struggles to break down. This makes it less susceptible to metabolic resistance.”

As a result, Mr Oakley’s advice is to mix and match treatments.

“If you had to use repeat sprays, one at least ought to be Mavrik.

“But growers shouldn’t panic.

It is a case of being aware of the situation and keeping an eye out for it.”

Rothamsted Research has developed a test for pyrethroid resistance in pollen beetle and last season checked more than 20 populations for DEFRA without detecting any, says Ian Denholm.

“But given the status of the pest in mainland Europe growers should be vigilant, and we would be interested to hear from anyone who thinks they have a problem via email (ian.denholm@bbsrc.ac.uk).

andrew.blake@rbi.co.uk