Knowing your oilseed rape plant population density is crucial for making pollen beetle control decisions, according to HGCA.

As pollen beetles tend to start migrating to winter oilseed rape crops during March, HGCA is reminding growers of the current spray thresholds which play a crucial part in pyrethroid resistance management.

In 2013, HGCA published the evidence-based thresholds based on the maximum number of buds each beetle can destroy and the number of excess flowers produced.

Plants in low plant population crops produce more branches and flowers and can tolerate a higher number of pollen beetles.

The current thresholds are as follows:



• 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

Plants/m2 can be estimated by counting the number within a square foot and multiplying by 11.

Caroline Nicholls, HGCA Research and Knowledge Transfer Manager, said: “Based on experience in recent years, pollen beetles have rarely been numerous enough to warrant treatment.

“By using thresholds to prevent ‘insurance’ sprays and following resistance management advice, such as not spraying after flowering starts, we should be in a good position to get on top of the resistance threat, which is now widespread throughout the UK, without compromising control.”

Forecasting pollen beetle migration

HGCA is also reminding growers of the online forecasts of pollen beetle migration which can be used to help to focus monitoring efforts so risk can be assessed.

Miss Nicholls said: “The forecasts use local meteorological data to generate a series of maps indicating whether migration has started, the risk of migration continuing over the next three days and the predicted completion of migration.
“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, our validation work of the forecast tool shows that it can considerably reduce the number of in-field monitoring days required.”

Threshold confidence

HGCA is continuing to invest in pest thresholds research to make sure thresholds are based on the latest knowledge of agronomy and pest biology. In relation to pollen beetle, HGCA is funding work to refine understanding of the management of this pest.

Conducted by ADAS and Rothamsted Research, the work includes simulating concentrated crop damage caused by pollen beetles and pigeons, as well as refining monitoring methods – such as trapping techniques and online migration forecasts.

Based on a simulated single-event attack by pigeons (by mowing the crop) or a concentrated attack to the main stem by pollen beetles (by pruning), early trial results suggest that crops damaged in this way are no less able to tolerate pollen beetle attacks and the current thresholds remain valid.

In relation to further validation of the online migration forecasts, growers with easy access to an oilseed rape crop are being asked to help out. Interested growers, able to check a trap about three times a week, are asked to contact sam.cook@rothamsted.ac.uk (all materials provided).

Further information on pollen beetle management can be found at www.hgca.com/pests