Crop Clinic

Crops in ealry April are backward due to the cold Spring weather. Pollen beetles are unlikely to be active unless temperatures iincrease later in the month. Warm weather could trigger the migration of pollen beetles from their overwintering sites into oilseed rape crops.

There needs to be at least 15 pollen beetles per plant at green-yellow bud to justify an insecticide treatment on winter oilseed rape. If crops are a bit backward because of slug problems in the autumn or pigeon damage, then their capacity to cope with pollen beetle damage is diminished, so a threshold of 5 beetles per plant throughout the crop should be adopted for these crops.

Pollen beetles have been a problem in mainland Europe with some showing resistance to the pyrethroid insecticides that have been almost exclusively used for control for the last decade. Most oilseed rape-growing regions of Europe, have been found to have pollen beetles resistant to the pyrethroids.

The advice to growers is that pyrethroid insecticide use for pollen beetle control should only be used if the treatment thresholds have been exceeded. Remember that once the crop is in flower the beetles can easily reach the pollen they crave without damaging the flower, so no flowering crops should be sprayed for pollen beetle control.

Rape winter stem weevil damage in crops should now be obvious: production of lateral shoots at stem extension. We would welcome any reports and samples of these symptoms so that we can gauge the spread of this pest.