Pollen beetle on oilseed rape

Oilseed rape growers need to be monitoring their crops for pollen beetle and assess whether they need to use an insecticide to prevent damaging attacks.

Recent HGCA-funded research shows crops compensate well for pollen beetle damage to flower buds, reducing the need for insecticides.

However, experts say growers need to be fully aware of the latest spray thresholds when examining crops and use insecticide when those limits are reached.

See also:  Half of the UK sees start of pollen beetle migration

1.Understand thresholds

Knowing pest thresholds is key to the control of pollen beetle, says Sam Cook of Rothamsted Research (see threshold table below).

Winter and spring oilseed rape thresholds for treatment (HGCA)

Plants/sq m

Pollen beetles per plant

< 30

25

30-50

18

50-70

11

>70

7

Struggling crops may produce fewer side shoots and green buds, meaning lower pollen beetle numbers could be a threat.

Measuring plant populations can give a good estimate of a crop’s tolerance to pollen beetle damage since it correlates with excess flower buds. Not all the buds produced are thought to be needed to achieve potential yield.

Thresholds may be quite varied because, while some autumn-sown crops look robust, others are backward or thin due to pigeon or autumn cabbage stem flea beetle damage, warns David Parish of crop consultant Niab Tag.Pollen beetle

He suggests these crops may not be able to compensate as well for loss of buds to pollen beetle, so he views them as higher risk.

2. Beetle migration risk predictor

With your threshold clearly defined, use the “Pollen Beetle Predictor” programme’s migration risk maps on HGCA’s or Bayer CropScience’s website to gauge the threat, suggests Dr Cook.

Based on nationwide trapping, the maps highlight regions where pollen beetles are starting to move in, are actively a risk, and are tailing off, respectively.

The risk window is typically from mid-March to the end of April for the winter crops and from late April into May for the spring crops.

“Interpreting the maps with their colour codes is easy – the first warns growers to start looking out for the pest and the third signifies the completion of beetle migration,” she says.

Beetle trapFor about £10, you can buy an Oecos pollen beetle trap to position downwind and confirm the arrival and abundance of the pest, she adds.

3. Monitor pollen beetles

Dr Cook would monitor fields for beetles from the initial fly-in, indicated on the maps by a red dot or three consecutive yellow dots.

She suggests walking 30m in from the headland of each oilseed rape field counting 10 plants every three steps, then averaging the numbers of beetles on each plant.

Mr Parish counts not only pollen beetle populations per plant but relates that to green bud numbers to assess the risk to oilseed rape crops.

“Predicting beetle damage is very difficult, particularly this spring with crops so widely differing in condition. It will be a case of counting beetles and buds every four or five days until flowers start to open and the risk window has passed,” he says.

4. Spray where necessary

Where thresholds are reached, act swiftly and don’t try to save money on cutting application rates, says Mr Parish.

David Parish

David Parish

In recent seasons, resistance to pyrethroid insecticides has become increasingly common, so he recommends using one where there has been no historic problem with control.

“Pyrethroids are typically a quarter to a fifth of the cost of the alternatives so where control has been successful in the past, there is no reason to shelve them. Apply a full rate to minimise the risk of resistance development,” he says.

5. Review the results

Crops are most unlikely to need more than one treatment unless the first spray went on early and the risk window is extended, or unless pyrethroids fail to control the beetles.

Mr Parish recommends reviewing treatment three days after spraying, and if beetles are still active and numbers exceed the threshold, using an insecticide with an alternative mode of action.

Options include the neonicotinoid thiacloprid (such as Biscaya), pymetrozine (such as Plenum) and indoxacarb (such as Rumo). None must be used more than once in a crop, he warns.

For pollen beetle thresholds and fly-in warnings, visit the HGCA’s pest management page or see Bayer’s pollen beetle predictor