Oilseeds and vegetable brassicas
Identification and symptoms
Pollen beetle adults are small (approximately 2.5 mm), metallic greenish-black and have clubbed antennae. Females bite slits in the base of oilseed rape buds and lay their eggs inside.
The larvae are creamy white, with a black head, three pairs of legs and dark brown spots and short bristles along the back. They grow to 3–4 mm long. Adults and larvae attack buds and flowers, resulting in withered buds and reduced pod set.
- Adults overwinter in sheltered spots in leaf litter, mainly in deciduous woodland
- When temperatures exceed 15°C (usually, from mid-March and throughout April), adults migrate into oilseed rape crops and feed on pollen inside buds or in open flowers
- Eggs are laid in closed buds. Damage to buds declines as the flowers begin to open and pollen becomes more easily obtainable
- On hatching, larvae feed on pollen (within the buds and in flowers) throughout May before dropping to the soil to pupate
- A new generation of adults emerges in June–July. This generation feeds on pollen from a wide range of flowers, including spring oilseed rape
In oilseed rape, adult and larval feeding can lead to bud abortion and reduced pod set. This damage rarely results in reduced yields for winter crops. Spring crops are more vulnerable, as the susceptible green/yellow bud stage often coincides with beetle migration.
Adults are also occasional pests of cauliflower and broccoli, as they feed on the curds or florets in mid-summer.
- Crops are usually most at risk when the weather is dry and warm (above 15°C)
- The damage-susceptible stage of the oilseed rape crop is green to yellow bud
- Oilseed rape will usually compensate for early damage by producing more and larger seeds on lower racemes
- Backward crops and those suffering from pigeon damage are at greater risk
- Once the crop is in flower, it is no longer at risk
- In vegetable brassicas, the risk of infestation is higher if horticultural crops are close to fields of oilseed rape
Monitoring (oilseed rape)
Although classed as a pest, pollen beetles are rarely abundant enough to warrant treatment. With resistance to pyrethroid insecticides widespread throughout the UK, chemistry needs to be used cautiously. Monitoring pollen beetle numbers is essential.
The number of pollen beetles per plant should be monitored throughout the damage-susceptible stage of the crop (green-yellow bud). Sample at least 10 plants along a 30 m (minimum) transect, from the middle of the headland towards the centre of the crop. Calculate the mean number of beetles per plant.
Baited monitoring traps (Oecos) can be used to detect local movements and focus monitoring efforts.
Thresholds (oilseed rape)
Treatment thresholds for winter and spring oilseed rape are based on the maximum number of buds each beetle can destroy and the number of excess flowers produced by different plant populations – low plant population crops produce more branches and, therefore, more flowers.
The threshold number of beetles per plant at the green to yellow bud stage are:
- 25 – If there are fewer than 30 plants/m2
- 18 – If there are 30–50 plants/m2
- 11 – If there are 50–70 plants/m2
- 7 – If there are more than 70 plants/m2
Plants/m2 can be estimated by counting the number of plants within a square foot and multiplying by 11. Ideally this should be done at several positions within a field. It is easiest to count plants at the 5 to 6 leaf stage after the risk of slugs reducing the population has passed. However, if there is winter plant kill, a spring plant count should be done at the same time as the pollen beetle assessment.
Any treatment should only be applied during the susceptible green-yellow bud stage. Treatments should not be applied after flowering starts.
Monitoring (vegetable brassicas)
The pollen beetle forecast (currently available on the AHDB Pest Bulletin) predicts when adult pollen beetles will emerge from oilseed rape crops and migrate into vegetable brassica crops and other locations.
Thresholds (vegetable brassicas)
Pollen beetle larvae are attacked by parasitic wasps. They are relatively poor dispersers, so planting oilseed rape crops close to where the crop was grown in the previous year can help ensure parasitism. The parasitic wasps may not be affected by insecticides applied against pollen beetle at green bud, as they arrive in crops during flowering. Where insecticides are used extensively, however, levels of parasitism can be considerably decreased. Minimum tillage following oilseed rape will enhance survival of the parasitic wasps. Trap cropping with turnip rape can attract more parasitoids into the crop and can often reduce populations of pollen beetle. Natural enemies also include spiders, ground beetles and rove beetles.
Latest information on insecticide resistance can be accessed via ahdb.org.uk/knowledge-library/IRAG
Pollen beetle on an oilseed rape bud
Pollen beetle damage to cauliflower