C. H. Högger. Swiss Federal Research Station for Agroecology and Agriculture, Reckenholz, CH-8046 Zurich   chhoegger@gmx.net, M. Barone, Inst. Zoology, Univ. Bern and C. Oberer, Museum of Natural History, Basel

 Generally slug populations are estimated by measuring their activity by means of traps or baits or combinations of both (Young et al., 1996). Activity of slugs depends on soil temperature and moisture.  Methods using flooding of turves or soil washing result in approximate numbers per unit of soil volume (South, 1992). Expelling soil macroinvertebrates in a defined area offers a compromise on efficiency and labor effort.
Three expellants were compared: 0.6mM allyl-isothiocyanate (=mustard oil), 0.25% formalin (Chabert et al. 1997) and  0.33 % mustard flour (Högger, 1993). Three hundred microliters of synthetic allyl-isothiocyanate were dissolved in 8ml 95% methanol and added to 5 liters of water.  Vegetation was cut at soil level with shears. Steel rings of 15cm height and 56cm diameter(=  1/4m²) were pressed 5cm into the soil and the solution was poured into the rings. After 15 and 30 minutes a second and third pouring were carried out. Most slugs and earthworms appeared after the second pouring.  In an experiment in a wild flower strip mustard oil expelled more small Arion distinctus and A. lusitanicus (<100mg) than formalin. In BAYER® shelter traps the highest slug numbers were similar to those found with mustard oil. The numbers and weights of earthworms produced by the three expellants did not differ significantly from one another, mustard oil being the most efficient expellant, because the usual digging after extraction brought fewest additional worms.
In earlier experiments in summer in a garden with concentrations of 0.2 and 0.4mM mustard oil the snails Bradybaena fruticum, Cepaea nemoralis, and Oxychilus draparnaudi were expelled besides the slugs A. distinctus, Boettgerilla pallens, Deroceras reticulatum and Tandonia budapestensis. The higher concentration yielded more earthworms than the lower one.
  Further field experiments with a range of concentrations and in different habitats are planned. In crop fields this absolute method, which measures abundance and is less weather-dependent than refuge traps, which measure activity (Young et al., 1996), may be useful to forecast the risk of slug damage several days or even weeks before sowing.

Literature:
Chabert, A., Guinot, J., Tisseur, M. 1997. Suivi des populations de limaces au champ.
    Phytoma   No. 497: 16-20.
Högger, C. H. 1993. Mustard flour instead of formalin for the extraction of earthworms in the
    field. Bull. Bodenkundl. Ges. Schweiz.17: 5-8.
South, A. 1992. Terrestrial Slugs. Biology, Ecology and Control. Chapman & Hall, London 428p.
Young, A.G., Port, G.R., Craig, A. D., James, D. A. and Green, T. 1996. The use of refuge traps
    in assessing risk  of slug damage: a comparison of trap material and bait. BCPC Symp. Proc.
    No. 66: 133-140.