Pest: Pollen beetle

Identification
The adult beetles are up to 2.5 mm long, metallic greenish to blue / black. The larvae are creamy white, 3 - 4 mm long, with a brown / black head and three pairs of short brown legs.  The uppersides of the body of the larvae are lightly coloured with dark brown spots and short bristles. 

Symptoms
Both the adults and larvae feed on the developing buds and flowers in search of pollen.  Holes can be found in developing buds where the beetles have eaten through the sepals to reach the stamens.  This can lead to bud abortion and reduced pod-set.

Life cycle
Adults emerge during late March through to May. Initial feeding begins close to   the overwintering sites and migration into crops occurs on warm days, they fly well when temperatures reach at least 15°C.  Populations are commonly more numerous on the headlands of oilseed rape crops and movement into the main body of the crop will occur with favourable weather conditions.  Eggs are laid in flower buds and these hatch after 7 - 10 days. When fully developed the larvae pupate in the soil and the adults emerge in mid-summer.   Autumn flowering brassicas are frequently damaged by a 2nd generation.

Importance
Pollen beetles are widely distributed throughout the UK and feed on the pollen of a wide variety of plants.
Invasion into winter oilseed rape is often too late to cause serious damage and yield loss.  Damage at green - yellow bud is more likely to be serious, particularly on backward crops, compared to later feeding on open flowers. 

In contrast, spring OSR begins to flower when pollen beetle activity is peaking and is potentially more susceptible to attack. Accordingly, in conventionally bred spring varieties the threshold of beetles per plant is about 1/3 of that for WOSR.  Some resistance to pyrethroid insecticides has recently been found in the UK, particularly in southern and eastern areas,   and severe resistance to pyrethroid insecticides is common on the continent.

Threshold
In winter oilseed rape the threshold for treatment is 15 beetles per plant and 5 beetles per plant for slow growing backward crops.  In spring oilseed rape a threshold of 3 beetles per plant has been advised.  These thresholds are currently under review.

Once the crop is in flower it is no longer at risk.  The beetles will be attracted to the flowers in preference to the buds and so can help in pollination.

Pest Traits

Season: Spring

Crop: OSR

Damage: Flower

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