Science into practice controlling turnip sawfly on watercress

Significant damage to watercress and baby leaf Brassica crops occurred in 2006 caused by turnip sawfly (TSF).

A large-scale immigration of the pest from continental Europe was blamed.

HDC funded project FV 317 to investigate fears that climate change might result in the immigrant population becoming permanently established in southern England and, if so, what control methods could be used.

There was concern that the severity of infestation of watercress was linked to the growing of oil-seed rape crops in watercress-growing areas. These crops were thought to be acting as autumn breeding sites for TSF which, after overwintering in the rape, were a spring source of the pest.

After monitoring of TSF activity in 2007-09 using yellow sticky traps, initial fears appear largely unfounded and there have been few complaints from growers in the past three years. Pest activity was unpredictable on a wide scale, making a national monitoring scheme impractical. Local monitoring using sticky traps could provide early warning of significant activity, especially in areas of southern England most at risk of a mass invasion from Europe.

Growers are advised to monitor TSF activity routinely between June and September (August is usually the peak month). Should significant numbers be detected, control may be necessary, but the options are few. For watercress, there are no approved insecticides that are effective against this pest. The only solution is to deploy insect-proof mesh covers for the duration of the pest activity period. For baby-leaf crops, pyrethroid insecticides could reduce the number of egg-laying females or developing larvae present before TSF problems develop.

Horticultural Development Company

- For details on all HDC activities, visit

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