THE exceptionally warm February has led to the earliest start to a pollen beetle migration since records began.

This potential threat to oilseed rape crops – numbers recorded as part of Rothamsted monitoring surveys are currently below threshold level – means that if temperatures rise, then increased activity could coincide with the crop’s green-bud stage, the most damage-susceptible time for the crop.

Sam Cook, a senior research scientist at Rothamsted, said that an integrated pest management (IPM) approach is vital to combatting pollen beetles: “If, in the next few weeks, temperatures exceed 15°C for a couple of days, then it is possible that increased pollen beetle could coincide with the green-bud growth stage. IPM for pollen beetle is really important in order to increase the efficiency of insecticides and reduce their unnecessary use.”

Given this early start to migration, Claire Matthewman, from Bayer, said that a vigilant and integrated approach will be necessary. “Monitoring both the crop and local weather data will go a long way to understanding the risk in your area,” she argued. “Also check plant population and the corresponding pollen beetle threshold, as this serves as a protection against unnecessary crop spraying.

“Due to an increasing resistance to pyrethroid insecticides, in cases where pollen beetle populations exceed thresholds and need treating, use a product with no known resistance issues, such as Biscaya (thiacloprid), which also allows for flexible treatment where crops are uneven.”

Dr Cook added some caution, though: “A joint project between Rothamsted Research and ADAS, funded by AHDB, has shown that most pollen beetle populations rarely exceed threshold. The project also found that spraying below threshold is not linked to yield increase.

“Of far more concern, unnecessary spraying increases selection pressure to insecticides and damages populations of the natural enemies that attack pollen beetles and cabbage stem flea beetles – so I can’t emphasise enough how important it is to only spray when necessary.”

Pollen beetle thresholds

Plants per m² Pollen beetles per plant

<30 25

30-50 18

50-70 11

>70 7

(Source: HGCA Information Sheet 18/spring 2013 ‘Monitoring and control of pollen beetle in oilseed rape’)