Abstract: Slugs are an important pest of grass seed production on marine loam soils. Significantdamage occurs from the start of germination until tillering, the main slug attack occurring atemergence and shortly after. Slug attack causes plant loss resulting in open spots in the fieldwhich in severe cases may cause significant yield loss. Predominantly baited molluscicide pelletsare used to control slugs in Dutch grass seed production however occasionally being applieduntimely resulting in repeated application or even unsatisfactory control. Four field trials in threesubsequent years and one semi field trial were carried out to test the efficacy of grass seedtreatments to protect the crop against slug damage. In the field trials treatments were comparedwith both an untreated control and a reference treatment consisting of broadcast applications ofmetaldehyde slug pellets at a rate of 448g a.i. per hectare. A field trial starting in autumn 2003together with a field trial in 2004/2005 and an additional semi field trial in 2004 resulted in twofield trials testing metaldehyde at three application rates and thiacloprid at one rate, in 2005/2006.Tested on heavy marine loam soils both metaldehyde at 360g and thiacloprid at 154g a.i. per kgseed resulted in excellent crop protection, reduced application rates however showing efficacy aswell. Plant numbers as well as crop development figures were equal to or significantly higherthan the results of two applications of metaldehyde baited pellets. Results are discussed withparticular focus on the variation in results and possible variation in mode of action betweenmetaldehyde and thiacloprid.