Seed dressings to control slug damage in oilseed rape

Abstract

Aims

Slugs are major pests of oilseed rape that are poorly controlled by conventional bait pellets.

The project had the overall aim of developing oilseed rape seed-treatments containing either molluscicides or feeding deterrents that will give protection against slug damage equivalent to, or better than molluscicidal pellets.

The specific objectives were:

To determine levels of methiocarb, metaldehyde, cinnamamide and 3,5-dimethoxycinnamic acid (DMCA) that provide maximum protection to seedlings, from Deroceras reticulatum slugs, without causing phytotoxicity.

To compare the efficacy of the above four compounds in terms of damage reduction and potential cost.

To compare efficacy in laboratory bioassays of the most promising formulation identified above with conventional molluscicidal pellets.

Results and conclusions

Metaldehyde and methiocarb were not phytotoxic at any doses, whereas all doses of cinnamamide and DMCA were. All compounds reduced slug damage, but metaldehyde and methiocarb consistently performed better than cinnamamide and DMCA. Metaldehyde and methiocarb seed-treatments were compared with baited pellets containing the same active ingredients at recommended field doses. The seed-dressings protected plants from damage by the grey field slug, Deroceras reticulatum, and the dusky slug, Arion subfuscus, as well as, or better than baited pellets.

Implications for levy payers

This was a pilot study to identify if slug damage to oilseed rape can be controlled with seed-treatments. Seed-treatments allow better targeting of the active ingredient, reduce the amount of active ingredient applied, and avoid a separate field-pass for pesticide application. In addition, unlike slug pellets that are broadcast on the soil surface, coated seeds may be drilled below ground, making the active ingredient less available to wildlife. Thus, the cost of slug control to growers and the environment could be reduced. Both metaldehyde and methiocarb show considerable promise as a seed-treatment for controlling slug damage. We therefore recommend that metaldehyde and methiocarb should be field-tested as seed dressings to control slugs in oilseed rape.