Leather jackets have been a great problem this spring. They have attacked young cabbage plants and young courgettes. They nip the stem at soil level - killing the plant.
There are few chemicals available for use on vegetables which kill leatherjackets. The active ingredient for the job is chlorpyrifos, & Dursban is about the only product on sale. It is water soluble and should be well watered in to make sure it reaches the leather jackets in the soil.