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. 

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. 

• Typically, larvae feed on the outside of leaves first (progressing towards the centre)
• Sometimes, larvae can feed on the leaf undersides (which can become skeletonised)
• They can also start to feed on the edges of leaf ‘shot holes’ (caused by cabbage stem flea beetle)
• The third generation is particularly problematic (as it can coincide with the early growth stages of oilseed rape)

Note: Slugs tend to scrape (rasp) away at tissue and thin the leaf first. 

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 for guidance only).

When damage is severe and the crop is not growing away, consider a pyrethroid spray.

Insecticide resistance

None known.

Amy Catling shows the damage caused by turnip sawfly larvae in winter oilseed rape (filmed in late September 2024). 

Amy Catling, AHDB Senior Knowledge Exchange Manager, talks through the identification, life cycle and crop damage of turnip sawfly larvae. 🐛🌿

To learn more about turnip sawfly biology and impact on vegetable brassicas and oilseed rape, visit: https://t.co/02NP7dFDF2 pic.twitter.com/DD835ILbwa

— AHDB Cereals (@AHDB_Cereals) September 26, 2024