Influence of seed depth and molluscicide pellet placement and timing on slug damage, activity and survival in winter wheat

In a laboratory experiment, grey field slugs (Deroceras reticulatum) were introduced to trays of clay soil drilled with wheat seeds at 10 or 25 mm depth. Trays were left without molluscicide, or methiocarb pellets were drilled with the seeds or placed on the surface of the soil immediately after drilling. In the absence of molluscicide, there was less damage to seeds drilled at 25 mm than at 10 mm. Methiocarb pellets drilled with seeds at 10 mm depth killed slugs and reduced damage to seeds and seedlings, but the percentage of seeds killed was similar to the percentage of deeper-drilled seeds killed where no molluscicide had been applied. Pellets drilled with deeper-sown seeds did not kill slugs, or reduce damage to seeds and seedlings, whereas pellets placed on the soil surface immediately after sowing were effective in killing slugs and reducing plant damage irrespective of drilling depth. The best protection from slug damage was provided by deeper drilling combined with pellets broadcast on the soil surface. In a field experiment, methiocarb pellets broadcast immediately after drilling at two depths killed slugs and protected seedlings from damage, whereas pellets drilled with seeds were ineffective because seed-bed conditions prevented slugs from reaching seeds and pellets (only 4% of seeds were killed by slugs in the absence of molluscicide treatment). The implications of these results for slug control and for non-target fauna (wood mice and ground beetles) are discussed.