New information on pollen beetle monitoring and spray control thresholds in winter and spring oilseed rape has been issued by HGCA.

Playing a crucial part in managing resistance to pyrethroid insecticides, the revised thresholds are based on the maximum number of buds each beetle can destroy and the number of excess flowers produced:

• · If there are less than 30 plants/m², the threshold is 25 pollen beetles/plant
• · If there are 30-50 plants/m², the threshold is 18 pollen beetles/plant
• · If there are 50-70 plants/m², the threshold is 11 pollen beetles/plant
• · If there are more than 70 plants/m², the threshold is 7 pollen beetles/plant

Caroline Nicholls, HGCA Research and Knowledge Transfer Manager, stated:"HGCA-funded research, conducted by ADAS, has revealed that the plant population makes a large difference to the pollen beetle threshold. Plants in low plant population crops produce more branches and flowers and can tolerate a higher number of pollen beetles.

"Based on experience in recent years, pollen beetles have rarely been numerous enough to warrant treatment. By preventing 'insurance' sprays and following resistance management advice, such as not spraying after flowering starts, we should be in a good position to counter the resistance threat, which is now widespread throughout the UK, without compromising control."

Pollen beetle thresholds should be used in conjunction with best monitoring practice and findings from a second recently completed HGCA project, led by Rothamsted Research, are helping to provide the tools to help focus monitoring efforts.

Over four years, 178 oilseed rape crops were monitored for pollen beetles across the UK using sticky traps and sampling plants along transects to help test a tool that forecasts pollen beetle migration.

"The tool uses local meteorological data to generate a series of maps indicating whether migration has started, the risk of migration starting over the next three days and the predicted completion of migration," stated Miss Nicholls.

Although the current advice that crops should be monitored when they are most at risk (green-yellow bud stage and temperature >15°C) remains valid, the tool was found to considerably reduce the number of monitoring days required (by 34-53%).

HGCA has reissued its pollen beetle publication (Information Sheet 18) to help the industry prepare sustainable strategies for insecticide applications.

To access the publication, click here.

For information on the pollen beetle forecast tool, visit www.hgca.com/pests