Determination of Abundance and Activity of Slugs, Snails and Earthworms with Mustard Oil and Shelter Traps

In biodiversity studies slug populations are estimated often by measuring their activity by means of traps or baits or combinations of both (Young et al., 1996). However, the activity of slugs and snails depends on moisture and temperature over night. Methods using flooding of lawn turves or soil washing result in approximate numbers per unit of soil volume, i.e. abundance (South, 1992). While expelling earthworms with irritants such as formalin or mustard flour suspension in a defined area, the emergence of slugs and snails too can be observed. Mustard oil (allyl-isothiocyanate) is the spicy active ingredient in mustard flour and is also contained in leeks and garlic. The synthetic substance has the advantage of defined composition and concentration. Trapping methods for determination of activity and expelling methods for determination of abundance of slugs and snails and/or earthworms were compared below. A food coloring dye showed the penetration depth of the expellant. Three methods were compared in eight habitats.

All concentrations of mustard oil expelled more Arion lusitanicus and A. distinctus than formalin or mustard powder (Fig. 1). The differences between the expellants and between the mustard oil concentrations were however not significant (Duncan's Test; P > 0.05). The numbers and weights of Deroceras reticulatum expelled with mustard oil were negligible. During digging no slugs were found. Some may have been overlooked because soil particles stuck to their bodies, in contrast to the earthworms, which were well visible.

Fig. 1: Populations and Activities of slugs in a wild-flower strip, measured with expellants and shelter traps, respectively.

The Bayerâ and Grimm traps caught distinctly more small Deroceras reticulatum, which were active over night, than were expelled from the defined area on the same day. The catch was removed from the trap and the soil surface. It was released again in the plot. In the two week observation period the number of Deroceras reticulatum decreased significantly (P < 0.05), whereas the number of the snail Trichia hispida remained nearly constant (Fig. 2). Overall the Grimm board trap caught significantly more Deroceras reticulatum and Arion spp. than the Bayerâ mat. This may be due to the better insulation effect of the thicker material (Grimm et al. 2000).

Fig. 2: Activity of slugs and snails in a wild-flower strip, measured with Bayer and Grimm traps

The numbers and weights of earthworms expelled by the three types of expellants did not differ significantly from one another (Duncan's Test; P > 0.05; Fig. 3). Digging contributed 40 to 55% of the total numbers and 22 to 40% of the total weight. Depending on the aim of an investigation the additional effort of digging to approach absolute numbers may be worthwhile. Large earthworms (mostly Lumbricus terrestris) were recovered more completely by expelling alone.

Fig. 3: Numbers and weights of earthworms expelled with 3 expellants alone and with digging

A concentration of 0.6mM mustard oil was chosen for further work on expelling slugs, snails and earthworms simultaneously. Although mustard oil is a lacrimant in the undiluted form, it poses fewer health hazards than formalin. It has the advantage that the concentration of the synthetic active ingredient is defined in contrast to the undetermined one in natural mustard flour.

Mustard oil determines slug and snail populations in defined areas, i.e. population densities; whereas traps determine activities in undefined areas. The activities depend on temperatures, moisture and availability of other shelters.

In three plots where the annual crops winter rapeseed, winter barley and winter wheat had been harvested the penetration of the mustard oil solution in the soil pores was made visible by the food coloring agent Vitasyn^® Blue AE 85 [Clariant GmbH, Frankfurt (Main), Germany] (Flury et al. 1994). This dye, also called E 133, Food Blue 2, or C. I. No. 42090, was added to the 3^rd application of mustard oil. A stock dye solution was prepared by dissolving 20g of Vitasyn^® Blue crystals in 100ml water and carried to the field in screw capped containers. This amount of the stock solution was added to 5 liters of mustard oil solution giving a 0.4% final concentration (Foto 1 and Foto 2). One half hour after the dye solution had penetrated into the soil a trench was dug across the dyed surface about 0.8m deep. The soil profile and the bottom of the pit were examined.

Depending on the soil conditions the wall was dyed blue in the top layer (Foto 3 and Foto 4) and then in single channels, i.e. root channels, soil cracks or earthworm burrows (Foto 5). At 0.8m depth at the bottom of the trench cross sections of earthworm burrows were dyed. This means that the mustard oil reached earthworms and slugs at least to this depth and may have brought them to the surface. Since no slugs were found during digging, their extraction with mustard oil may have been complete and indicate their true population density. In contrast, earthworms were not extracted completely with any expellant.

In eight locations the concentration 0.6mM of the expellant allyl-isothiocyanate (mustard oil) was compared with the two shelter traps, the Bayer mat trap and the Grimm board trap. There habitat types were wildflower, meadowstrip, meadow, and hedge. The hedges 2 and 3 and the corresponding meadowstrips 2 and 3 were side by side. On a given day the slugs and snails were expelled with mustard oil and collected for identification and counting in the laboratory. The traps were baited with cucumber and left over night. On the following day slugs and snails were collected and identified as well. For each method there were 6 replicates in each location.

The shelter traps tended to give higher numbers of slugs and snails than the mustard oil (Figs. 4 and 5). However, expelling and trapping resulted often in different species in the same habitat.

Fig. 4: Abundance and activity of Snails and Slugs determined by three Methods in eight Habitats, Part 1

Fig. 5: Abundance and activity of Slugs and Snails determined by three Methods in eight Habitats, Part 2

Fig. 6: Biomass of D. reticulatum determined by 3 Methods in 4 Habitats

The number of species detected by different methods in a given habitat was often very close (+/- 1) (Fig. 7). But these species were not always the same. The hedges 2 and 3 as older habitats had often higher and more diverse populations than the adjacent, younger meadowstrips 2 and 3.

Fig. 7: Number of Mollusk Taxa determined by 3 Methods in 8 Habitats

The comparison of the three methods shows that the results are not independent of the method and the choice of the method must be based on the purpose of the investigation. So far there is no universal all-purpose method. The Grimm board trap tends to catch more slugs than the Bayer mat trap, probably because of the better insulation effect. The advantage of the use of mustard oil is the immediate and reliable result expressed as population density, whereas the traps determine activities over longer periods of time in an undefined area. Arthropods are observed as well.

I thank my colleagues Th. Frank, B. Grimm and B. Speiser for commenting on an earlier version of the manuscript.

Literature:

Chabert A., Guinot J. & Tisseur M. 1997. Suivi des populations de limaces au champ. Phytoma No. 497: 16-20.

Grimm B., Paill W & Kaiser H. 2000: Daily activity of the slug Arion lusitanicus Mabille. J. Moll. Stud. 66:125-130.

Flury M., Flühler H., Jury L. A. & Leuenberger J. 1994. Susceptibility of soils to preferential flow of water: A. field study. Water Resource Res. 30: 1945-1954

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. Chapmann & Hall, London 428p.

Voss M., Hoppe H.-H. & Ulber B. 1998. Estimation of slug activity and slug abundance. J. Plant Dis. Prot. 105 (3): 314-321.

Young A.G, Port G.R, Craig A. D, James D. A & 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.

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