Management of rape winter stem weevil on oilseed rape

Rape winter stem weevil is an occasional pest, where damage is caused by larvae feeding within the stem, which can cause stunting and lateral shoots to be produced in spring. Larvae can be distinguished from cabbage stem flea beetle by the brown head and lack of legs.

• Risk increases risk near wooded areas

Scientific name: Ceutorhynchus picitarsis

Adult beetles are 2.5–4.0 mm long, metallic black, with elongated snouts, elbowed antennae and reddish–brown leg tips.

Larvae hatch white, plump and legless, with an orange-brown head. They can reach 5 mm in length.

This brassica specialist has one generation a year.

The weevil is a locally and sporadically important pest of winter rape. However, unlike other weevil pests of oilseed rape, it lays eggs in the autumn and early winter.  Its life cycle is similar to that of the cabbage stem flea beetle.

Sep–Oct: Adults migrate from aestivation sites to feed on the leaves of autumn-sown oilseed rape crops.

Sep–Nov: The main egg-laying period. Eggs are laid in batches in punctures or crevices at the base of leaf petioles. Egg laying continues in mild winters.

Dec–Feb: The larvae bore into the petioles to feed and later invade the main stem and developing crown.

Spring: Mature larvae descend to the soil to pupate (at depth of 3-4 cm).

May–June: New generation adults emerge and aestivate (enter a resting phase) in leaf litter in woodland margins.

The effect of larval feeding can vary widely, depending on the number of eggs laid on the plant. The larvae can destroy the terminal shoot. This either kills the plant or results in the development of secondary shoots. Surviving plants may be stunted with a rosette-like appearance. Attacked crops tend to be irregular and patchy, with uneven flowering and ripening. Damage symptoms may not be visible until spring.

Survey data in England shows that there was a general decline in the incidence of this pest during the years in which neonicotinoid seed treatments were used (2006–14).

(Source)

Non-chemical control

Natural enemies include spiders, ground beetles, rove beetles and several parasitoid species.

Monitoring

Monitoring can be challenging because adults and larvae are difficult to find. Inspect the crop for young larvae from late October, especially in fields with a history of attack by this pest.

Thresholds

None established.

Insecticide resistance

None known in the UK.

Note: Pyrethroids can control this pest. A pyrethroid can be applied as a tank mix with a light leaf spot fungicide treatment, if the timing is appropriate. Ideally, adults should be sprayed prior to egg laying.