Using thresholds to determine which crops are at risk from pollen beetles

Once crops are at green-yellow bud, there is a risk from pollen beetle damage to the buds, giving rise to blind-stalks with no flowers or pods forming on them. Pollen beetles will only start start moving into rape crops once temperatures reach 15ºC, so warm sunny days will kick-start their migration into crops. Whilst it has been cold and wet over the last two weeks, temperatures have reached 15°C in some areas around 21st-23rd March, and a few pollen beetles have started to fly around.

If beetles do start to appear in crops, note that there are updated thresholds for treatment. The plant density needs to be taken into account to estimate the threat from pollen beetles at green-yellow bud.

• Less than 30 plants/m2 – threshold is 25 pollen beetles/plant
• Between 30-50 plants/m2 – threshold is 18 pollen beetles/plant
• Between 50-70 plants/m2 – threshold is 11 pollen beetles/plant
• Greater than 70 plants/m2 – 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 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 crave without damaging the flower, so no flowering crops should be sprayed for pollen beetle control.

Andy Evans (SRUC) for the Farm Advisory Service