Identification of environmental factors limiting plant uptake of metaldehyde seed treatments under field conditions.

Slugs are serious pests of oilseed rape (canola) and wheat with most damage occurring just after sowing and seedling emergence. As an alternative to the use of bait pellets, molluscicidal seed treatments have been shown to protect seeds and seedlings from slug damage in laboratory and semi-field experiments. However, protection offered to plants in field trials was diminished and shortlived in comparison with laboratory experiments. To determine why field efficacy was reduced, we grew seedlings under a range of environmental conditions, with appropriate controls, that simulated differences between laboratory and field experiments. We then measured the metaldehyde content of plant seedlings using a previously unpublished methodology described herein, which, unlike previous methods, did not first depolymerize the metaldehyde to acetaldehyde. We confirmed that naturally abundant plant-derived acetaldehyde could not interfere with our measurements of metaldehyde, even if depolymerization took place within the column. Our data suggest that reduced field efficacy results from microbial breakdown and/or loss of active ingredient caused by percolating soil water. Once the seedlings had emerged, neither volatalization nor simulated rainwater reduced the metaldehyde content of seedlings. Our findings will help develop superior seed treatment formulations to overcome these constraints.

Seed dressings to control slug damage in oilseed rape.

Slugs are major pests of oilseed rape that are poorly controlled by conventional bait pellets. A series of laboratory experiments investigated the potential of seed-dressings to control slug damage in this crop. Four compounds: metaldehyde, methiocarb, cinnamamide and 3,5-dimethoxycinnamic acid (DMCA) were tested at a range of doses for phytotoxicity and ability to reduce damage by Deroceras reticulatum (Müller). 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-dressings were compared with baited pellets containing the same active ingredients at recommended field doses. The seed-dressings protected plants from damage by D reticulatum and Arion subfuscus (Draparnaud) as well as, or better than, baited pellets. We therefore recommend that metaldehyde and methiocarb should be field-tested as seed dressings to control slugs in oilseed rape.