The few days of warm sunny weather has triggered a large migration of pollen beetle into winter oilseed rape crops, with counts exceeding 20 beetles per plant being reported. Crops still at green-yellow bud are at risk from pollen beetle damage to the buds, giving rise to blind-stalks with no flowers or pods forming on them. Any further warm spells when temperatures reach 15°C will trigger further pollen beetle flights into crops. Note that crops in flower will not be damaged by pollen beetle as they can easily access the pollen they crave.
There are updated thresholds for treatment. The plant density needs to be considered to estimate the threat from pollen beetles at green-yellow bud.
- Less than 30 plants/m2 – the threshold is 25 pollen beetles/plant
- Between 30-50 plants/m2 – the threshold is 18 pollen beetles/plant
- Between 50-70 plants/m2 – the threshold is 11 pollen beetles/plant
- Greater than 70 plants/m2 – the threshold is 7 pollen beetles/plant
If crops are backward because of poor overwintering, slug problems in the autumn or pigeon damage, then their capacity to cope with pollen beetle damage is reduced, so a lower threshold should be adopted for these crops.
These thresholds have been revised to help deal with the increased threat from pyrethroid insecticide-resistant pollen beetles in UK crops by ensuring that insecticides are only applied when necessary.
Resistance in pollen beetles to pyrethroid insecticides has been found in Scotland. 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 desire without damaging the flower, so no flowering crops should be sprayed for pollen beetle control.
Any instances of poor control of pollen beetles following a pyrethroid insecticide treatment should be reported to the Crop Clinic to allow us to determine how widespread resistance to this group of insecticides is in Scotland.
With some crops now at early flowering, cabbage seed weevils will be moving into crops, and numbers need to exceed 1 weevil every 2 plants throughout the crop (not just in the endriggs) during flowering to justify an insecticide treatment. Because of the issues regarding pollen beetle resistance to pyrethroid insecticides, only treat for seed weevil if the threshold has been exceeded.
Safety to bees
If any insecticides are going to be applied to a flowering oilseed rape crop, it is essential that local beekeepers are informed of your intention to treat the crop at least 24 hours beforehand. Bees are crucial for obtaining a good oilseed rape yield as they pollinate the flowers. So, safeguard your investment in growing oilseed rape by giving beekeepers enough notice so that they can close their hives on the day of treatment. Try to spray the crop in the early morning or evening when bees are less likely to be foraging, as wild bees and bumblebees need protection against insecticides as well.
Bee Connected is a valuable tool connecting beekeepers with farmers and informing of crop protection activities nearby. This service is brought to you by the Voluntary Initiative, supported by the Crop Protection Association. Bee Connected aims to help communication between beekeepers and farmers. If you’re a farmer or spray contractor, simply register online, enter the details of when and where you’re planning to spray an insecticide that may present a risk to bees (for instance on a flowering crop, or where the field has a conservation buffer strip), and a simple notification will be sent to neighbouring beekeepers registered with the system.
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