Knowing oilseed rape density 'crucial'

Knowing your oilseed rape plant 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 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.

HGCA’s Caroline Nicholls 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.”