Slugs are major pests of oilseed rape and wheat. Current chemical control measures often do not give adequate protection to plants, and pose an environmental hazard. Laboratory experiments investigated the potential of seed treatments to control slug damage in oilseed rape. Four compounds: metaldehyde, methiocarb, cinnamamide and 3,5-dimethoxycinnamic acid (DMCA) were tested for phytotoxicity and ability to reduce damage. Metaldehyde and methiocarb seed treatments protected seedlings as well as or better than baited pellets. A broad-spectrum insecticide imidacloprid was tested as a seed treatment to control slug damage to oilseed rape and winter wheat in laboratory experiments. Imidacloprid did not reduce slug damage to oilseed rape at any dose or to wheat at low doses. Higher doses of imidacloprid on winter wheat significantly reduced slug damage. In a field experiment with winter wheat, imidacloprid seed treatments reduced slug damage to a lower level than bait pellets but had inconsistent and short lived activity. The most promising seed treatments, metaldehyde and methiocarb, were tested in semi-field trials and found to have a reduction in efficacy in comparison to laboratory trials. To determine why field efficacy was reduced, seedlings were grown under a range of environmental conditions. The metaldehyde content of seedlings was measured using a novel method of GC-FID analysis. Data suggested that reduced field efficacy results from microbial breakdown and/or loss of active ingredient caused by percolating soil water. Once seedlings had emerged, neither volatalization nor simulated rainwater reduced the metaldehyde content of seedlings. New seed treatment formulations were developed and tested in 3 mini-plot trials on oilseed rape and wheat. In 2 of the mini-plot trials the industry-produced metaldehyde seed treatment protected seeds and seedlings, as well as bait pellets. In the third mini-plot trial, very heavy rainfall at sowing resulted in only bait pellets successfully reducing slug damage.