The management of pollen beetle in oilseed rape
Although classed as a pest, pollen beetles are rarely abundant enough to warrant treatment in oilseed rape. With resistance to pyrethroid insecticides widespread, it is essential to use chemistry cautiously.
What do pollen beetles look like?
Pollen beetle (Meligethes spp.) 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.
Does pollen beetle damage oilseed rape?
Yes. In oilseed rape, adult and larval feeding can lead to bud abortion and reduced pod set. However, 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.
When does pollen beetle feed on oilseed rape?
As the name suggests, pollen beetle feeds during the flowering period.
Pollen beetle life cycle
- Adults overwinter in sheltered spots in leaf litter, mainly in deciduous woodland.
- When it is dry and temperatures exceed 15°C (usually, from mid-March–May), adults migrate into oilseed rape crops and feed on pollen inside buds or in open flowers.
- Eggs are laid (April–June) in closed buds (green-yellow-bud stage). 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–June before dropping to the soil to pupate.
- A new generation of adults emerges in July–August. This generation feeds on pollen from a wide range of flowers, including spring oilseed rape.
How to monitor for pollen beetle
During the green-yellow bud stage, sample at least 10 plants along a 30m (minimum) transect, from the middle of the headland towards the centre of the crop. Calculate the mean number of beetles per plant. Use baited monitoring traps (Oecos) to detect local movements.
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. In vegetable brassicas, the risk of infestation is higher if horticultural crops are close to fields of oilseed rape.
Pollen beetle treatment 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.
Estimate plants/m2 at several positions within a field. Count plants at the 5-to-6 leaf stage, after the main slug-risk period. If many plants die over the winter period, recount plants in the spring.
Based on the number of beetles per plant at the green-to-yellow bud stage
|Fewer than 30 plants/m2||25|
|More than 70 plants/m2||7|
- Oilseed rape usually compensates for any 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
- Do not apply treatments after flowering starts
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.
First detected in the UK in 2006 and now widespread, pyrethroid resistance in pollen beetle means it is essential to avoid the use of sprays, where possible, and to exploit other modes of action. The latest resistance management information is available from the IRAG page.
Pollen beetle on an oilseed rape bud
Pollen beetle damage to cauliflower