A new spray threshold for treating pollen beetles in oilseed rape crops is set to reduce unnecessary spraying and insecticide resistance development.

A HGCA review looking at all arable pest thresholds revealed that the data and science on which previous pollen beetle guidance was based could not be found.

This fact combined with statistics from the Food and Environment Research Agency (FERA) showing that average pollen beetle numbers had rarely breached threshold levels since the early 80s led to the need for new guidelines.

ADAS entomologist Steve Ellis believes there was a lack of confidence in the old advice, resulting in unnecessary spraying of insecticides.

“We hope growers and advisers might take notice now we can show how these thresholds were developed and the science behind them,” says Dr Ellis.

The oilseed rape crop is susceptible to damage from pollen beetle feeding from green- to yellow-bud stage prior to flowering, after which there is no need for treatment.

Beetles move into oilseed rape crops from winter hosts and bite into and kill buds. Eggs are laid inside the closed buds, and hatched larvae then feed before dropping to the soil to pupate.

The new spray thresholds vary from seven to 25 beetles/plant depending on plant density and covers both winter and spring oilseed rape crops (see table)

The old thresholds advised treatment when 15 beetles could be found on the main stem, which is reduced to five beetles in backward crops and just two in hybrids. Spring crop would be treated with the presence of three.

Crop quality

Dr Ellis explains that the old system didn’t consider the quality of the crop or its growth.

“Plant physiologists have found that the rape plant produces many more flowers than it requires to reach optimum yield, so some can be sacrificed,” he says.

It was also found that spring oilseed rape produced even more excess flowers that winter oilseed rape, which goes against previous perceptions, adds Dr Ellis.

Hybrids were thought to be more prone to pollen beetle damage because they were planted at lower plant populations than conventional varieties, but again the opposite was found.

This evidence suggests that all oilseed rape crops would have some level of tolerance to pollen beetle attack.

Subsequently, work was carried out in pots to establish how many buds a pollen beetle would eat and the potential damage they could cause to a crop.

“Combining this with excess flower data can give a threshold for an individual crop,” says Dr Ellis.

The potential threshold needed an indicator of how many excess flowers a crop was likely to have, as once flowered the crop is no longer susceptible and it would be impractical to get in the crop and carry out flower counts.

A correlation between plant populations and excess flowers was discovered, enabling a prediction of how many flowers the crop was able to sacrifice.

As plant populations rise the new threshold falls because the plants will have a dominant main stem and not compensate for pollen beetle damage by branching lower down the plant.

Dr Ellis believes that the new threshold should dramatically reduce the unnecessary “insurance” spraying that has led to pyrethroid resistance in pollen beetle populations.”With oilseed rape being sown at lower plant populations, the threshold number is generally going to be high. It was rarely breached before, so it is even more unlikely now,” he says.

“We have to be able to justify insecticide use now more than ever, so the less we use, the better,” Dr Ellis adds.

Practicalities

Suffolk agronomist Will Foss with adviser Agrii says the new thresholds will be more time-consuming to use, with more numbers to remember and plant counts to conduct.

“It also assumes even crops, which is rarely a reality. You may have a thick forward crop at one end of a field, with a thin pigeon-damaged crop at the other,” he says.

In these situations it’s possible for areas of the crop to be at the susceptible stage at different times and Mr Foss says that one way to overcome this is to use a more persistent product.

“Despite a higher cost, Steward (indoxacarb) for example, will be effective for longer and deal with pyrethroid-resistant populations.

“It also has a narrow target spectrum, so is better for beneficials,” he adds.

There is a case to argue for spot spraying in patchy crops and Dr Ellis says that once some of the forward plants have begun flowering, they will act as a trap crop.

“The flowers are much more appealing to the beetles than the buds, so will take the pressure off the susceptible patches in the field and reduce the need for treating the backward patches,” says Dr Ellis.

It is also vital to get right out into crops when assessing pollen beetle numbers, says Mr Foss, as the populations are normally much higher on headlands.

“This will also reduce unnecessary insecticide application from over-estimating populations,” he concludes.

Trapping integrated control

A pheromone trap for pollen beetles has been developed and can be used locally to monitor pollen beetle movement on the farm.

It is currently not calibrated to produce spray timings, but can give growers and agronomists an idea of when their crops are at risk from attack.

“The damage-susceptible stage can last up to six weeks and it is impractical to be checking the crop for so long,” says Sam Cook of Rothamsted Research, who was involved in developing the traps.

She says traps should be placed at the edge of the crop, which will pick up any local movement of the pollen beetle populations and alert growers to look in their crop and apply thresholds.

“We can guarantee that if there are no beetles in the trap, there will be none in the crop, as the traps are far more attractive,” says Dr Cook.

Combining the traps with the online decision support tool – Bayer Pollen Beetle Predictor – which can be accessed on the Bayer CropScience website, will further reinforce risk.

The tool gives an accurate forecast of when beetles will be migrating based on weather data from a network of meteorological stations, with warm conditions between mid-March and late April considered high risk.

“Using these decision support systems is key to integrated pest management and can offset the more onerous task of applying the new thresholds,” says Dr Cook.

The pollen beetle traps are now commercially available and cost £8 each. Visit www.oecos.co.uk.

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