Turnip sawfly biology and impact on vegetable brassicas and oilseed rape

Turnip sawfly (also known as cabbage leaf sawfly) is an occasional pest of brassicas, but outbreaks can be severe. Adult flies lay their eggs on the leaves of brassicas, which can be quickly skeletonised by larval feeding following egg hatch.

• Warm conditions (>18°C) increase the risk of adult activity
• Favourable winds increase the possibility of mass immigrations from mainland Europe, posing a risk to southern counties
• A third generation may occur after hot summers, which can coincide with the early stages of crop emergence of oilseed rape
• Slow-emerging crops are at increased risk of significant defoliation
• Close proximity to overwintering sites increases the risk to spring-sown crops

Scientific name: Athalia rosae

Adults are around 7–8 mm in length. The abdomen is entirely orange, while the thorax is orange with two black ‘shoulder pads’ (other similar sawflies have an entirely black upper surface to the thorax). Legs are orange with black bands.

Larvae are caterpillar-like and greenish-black with a paler stripe along the side of the body. They have a shiny black head and can grow to 18 mm long.

Jan–Apr: Pupae overwinter in soil.

May–Jun: First generation of adults emerges and lays up to 300 eggs in the margins of host leaves.

May–Jun: Eggs hatch in 6–8 days. Larvae initially feed inside the leaf then externally on the underside of the leaf, which can become skeletonised.

May–Jul: At maturity, the larvae drop to the soil to pupate.

Jul–Aug: Second generation of adults emerges and lays eggs.

Aug–Sep: Third generation of adults emerges and lays eggs.

Oct–Dec: Pupae overwinter in soil. 

Non-chemical control

Situate spring-sown crops away from known overwintering sites (such as autumn-sown oilseed rape), wherever possible.

Protect high-value, sensitive crops from adults with covers, such as insect-proof mesh.

Turnip sawflies are hosts for parasitic wasps and flies.

Monitoring

Adult sawflies feed on pollen and nectar. Their presence in flowering hedgerows can give an early warning of imminent attack.

Use yellow sticky traps and water traps to monitor adult activity in May until September.

Inspect crops for turnip sawfly larvae on damaged leaves or close to damaged plants.

Thresholds

Oilseed rape: One to two larvae per plant.

Please note that this is a German threshold and little information is available on its validity under UK conditions; it is therefore offered for guidance only.

Insecticide resistance

None known.