Pyrethroid-resistant pollen beetles are an increasing threat to UK oilseed rape crops, but avoiding unnecessary spraying could reduce the risk of the problem getting worse, as Paul Spackman discovers

Do you routinely tank mix a cheap pyrethroid with oilseed rape fungicides “just in case”? If so, it’s likely you are selecting for pyrethroid-resistant pollen beetles and unnecessarily adding to a potentially serious problem, says Jon Oakley of ADAS.

The first resistant pollen beetle in the UK was found in Romney Marsh, Kent, in 2006, but increased monitoring last year detected them as far away as Hampshire and The Wash. Importantly, this new generation of beetles can fly over 100 miles, so could have spread much further last summer, he warns.

“At the moment pollen beetle damage has no economic impact on UK winter oilseed rape crops and needn’t become a serious problem if we ease off on unnecessary insecticide use.”

UK crops typically receive one spring insecticide for pollen beetle, seed weevil or pod midge, but this is not generally needed, as levels of these pests are well below treatment thresholds, he says (see table). “Seed weevil sprays are applied when pollen beetle larvae are feeding on the flowers and are very exposed to treatment. This practice could select for resistant pollen beetles.”

Suffolk UAP agronomist Will Foss says most crops in his region are sprayed for either pollen beetle or seed weevil at the green bud stage each year, but he agrees there is a need to get away from such routine spraying. “You also need to make sure the pest is a field problem and not just the headlands,” he says.

“All the pests tend to be more frequent on the headland, and we urge people to check a transect across the field and not panic if they find a lot at the edge,” adds Mr Oakley. “You can also see high numbers on the odd advanced plant projecting above the canopy.”

Pollen beetle resistance affects all pyrethroids, and although there is no know resistance to OPs, neo-nicotinoids, or spinosad, few are approved for UK use, Pesticide Safety Directorate’s David Richardson says.

“There aren’t any OPs approved in the UK and spinosad’s not registered here either. Biscaya is the only neo-nicotinoid approved, but we don’t want everyone to just use this. Management of any pest species needs at least two modes of action, but fundamentally make sure you only spray where necessary,” he says.

Where an insecticide is required, Mr Foss suggests products such as Mavrik (tau-fluvalinate) or Brigade (bifenthrin) at full-rate appear to be better against resistant pollen beetles than those based on cypermethrin. At about £5-6/ha they are also cheaper than Biscaya, he notes. “Biscaya’s about two to three-times the price of Mavrik or Brigade, but you’re safe in the knowledge it’ll do the job, even where pyrethroids are failing. With the price of oilseed rape as it is, some might be more willing to pay this extra.”

Mr Oakley advises growers that higher rape prices will have little effect on treatment thresholds, which are based on the amount of damage a crop can compensate for before yield reductions start. “Even a doubling of the price would change the threshold by less than 10%.”

To avoid confusion he advises growers to follow the Insecticide Resistance Action Group (IRAG) guidelines (see box).

IRAG advice

Treatment thresholds

Pollen beetle

Seed weevil

Winter OSR
15 beetles per plant in
well-grown crops
Five beetles per plant in
backward crops

One per plant if pod midge risk is low or one per two plant where pod midge risk is high (assuming temperatures above 15C at time of observation)

Spring OSR
Three beetles per plant

No separate threshold for spring OSR

Typical in-field numbers:
Two per plant

0.2 per plant

Note: the weevil threshold takes pod midge into account, as pod midges lay their eggs into seed weevil feeding holes. Pod midge are difficult to find in the field, as they work mainly at night.