As the pollen beetle monitoring window opens, growers are being advised that that numbers seen on UK oilseed rape crops are rarely damaging.
Funded by AHDB and conducted by ADAS and Rothamsted, a three-year project looking at pollen beetle control found that in field experiments, numbers of the pest did not exceed threshold and did not justify insecticide treatment, suggesting that sub-threshold populations of the pest are the norm rather than the exception.
Project lead, Dr Steve Ellis of ADAS says: “If the threshold is anything like 10 beetles or more per plant the chances of reaching that are pretty slim. You can get to the stage where you can almost eliminate certain crops from the monitoring programme and concentrate on crops that require more monitoring and could get to the threshold.”
In the project, an Oecos pollen beetle monitoring trap with an attractive lure was found to be more effective at catching pollen beetles than unbaited yellow sticky traps. Increased trapping efficiency means that the start of migration is more likely to be detected and abundance is less likely to be under-estimated, according to the researchers.
Results indicate that pollen beetle immigration is usually greatest on the north-east side of the field (opposite to the prevailing wind) and this should be the focus for location of traps and crop walking.
The research also found that the Bayer Pollen Beetle Predictor (BPBP) cropscience.bayer.co.uk/tools-and-services/pollen-beetle-predictor accurately predicted the peaks of pollen beetle migration, reducing monitoring effort by about a third compared with weekly in-field assessments and also provided early and accurate detection of when the threshold was exceeded. Monitoring (weekly or via use of the BPBP) resulted in a reduction in insecticide use by about one-third compared with prophylactic treatment. However, the research found that insecticide use did not significantly increase yield in comparison with untreated controls.
The project concluded that current thresholds and monitoring methods provide a good basis for an IPM strategy for pollen beetle that minimises the need for insecticide treatment.
<30 plants/sq.m - 25 pollen beetles/plant
30–50 plants/sq.m - 18 pollen beetles/plant
50–70 plants/sq.m - 11 pollen beetles/plant
>70 plants/sq.m - seven pollen beetles/plant
Pollen beetle migration has got off to a slower start this year compared with last year due to colder temperatures.
However, Bayer says two-thirds of the 83 UK weather stations monitored by the Bayer Pollen Beetle Predictor cropscience.bayer.co.uk/tools-and-services/pollen-beetle-predictor have had weather conducive to suggest pollen beetle migration has started, albeit at very low levels. Migration predictions have been confirmed by pollen beetle catches in Wye, Kent, Starcross, Devon and Welywn, Hertfordshire.
Warmer weather forecast for this week means growers should be alert to migration beginning in their area, warns Bayer.
Rothamsted researcher Dr Sam Cook says this means checking the prediction tool to see if their nearest weather station has turned red. “If so, it’s time to put yellow sticky traps out in fields to monitor local abundance and keep an eye on the ‘new migration’ map within the tool. This tells you when it’s time to get into crops and assess average numbers against threshold.”
Dr Ellis says it is possible the pollen beetle risk could be lower than normal this year. “If the crop gets beyond the susceptible growth stage before the migration gets going, the chance of getting a damaging population will be very low indeed.
“Once flowers open, beetles are attracted to open flowers rather than buds resulting in no damage.”