The relative toxicity of metaldehyde and iron phosphate-based molluscicides to earthworms Clive A. Edwards a, * , Norman Q. Arancon b , Marcus Vasko-Bennett a , Brandon Little a , Ahmed Askar a a Soil Ecology Laboratory, Department of Entomology, The Ohio State University, 400 Aronoff Laboratory, 318 West 12th Avenue, Columbus, OH 43210, USA b University of Hawaii, 200 Kawili St., Hilo Hawaii 96720, USA article info Article history: Received 1 July 2008 Received in revised form 10 November 2008 Accepted 11 November 2008 Keywords: Molluscicides Earthworms Metaldehyde Iron phosphate Chelating agents abstract Slugs are suppressed by cereal flour-based baits or pellets containing metaldehyde, or containing iron phosphate plus chelating agents, which are also consumed by earthworms and other invertebrates. These studies compared the effects of metaldehyde and iron phosphate alone, with those of iron phosphate plus chelating agents EDDS and EDTA, and of the chelating agents alone on earthworms. OECD artificial soil test: Earthworms (Eisenia fetida) were exposed directly to the molluscicides in arti- ficial soil. The test chemicals were: metaldehyde; iron phosphate; ethyldiaminetetraceticacid (EDTA), ethylenediaminesuccinicacid (EDDS) and mixtures of iron phosphate and these latter two chemicals. LD 50 values were more than 10,000 mg kg 1 for metaldehyde and iron phosphate, 156.5 mg kg 1 for EDTA, 145.6 mg kg 1 for EDDS, 72.2 mg kg 1 for iron phosphate/EDTA, and 83.0 mg kg 1 for iron phosphate/EDDS. Microcosm test: Four mature Lumbricus terrestris were exposed in each microcosm (15 cm diam.  30 cm deep) to pellets containing: no active ingredient; metaldehyde (4%); iron phosphate/EDTA (1%) (Sluggo Ò ); iron phosphate (3%); EDTA (3%); EDDS (3%). Metaldehyde and iron phosphate did not affect earthworm feeding, growth or mortality. Sluggo Ò decreased earthworm feeding and caused loss of weight and mortality. Earthworms consumed fewer pellets containing EDDS or EDTA and lost weight. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Earthworms are considered to be key soil-inhabiting inverte- brates due to their great importance in soil formation, fertility, and nutrient turnover (Edwards and Bohlen, 1996; Edwards, 2004). Because of this, they have been suggested as critical indicators of soil quality (Berry, et al., 1996; Blair et al., 1996), and for the same reason, earthworms were selected by the European Union (EU) and Organization for Economic Cooperation and Development (OECD) to be used as one of a set of critical assays to assess the toxicity of chemicals to the environment (Edwards, 1983, 1984; OECD, 1984). As a consequence, they have been used extensively by commercial companies and registration authorities to screen the toxicity of chemicals including pesticides. Edwards and Bohlen (1992) reviewed the effects of more than 200 biocidal chemicals on earthworms using the OECD/EU test and a range of other test methods and categorized them into four relative levels of toxicity. Molluscs, including slugs and snails, are important pests of a wide range of crops in Europe and USA particularly in wet seasons, and are responsible for very considerable losses in crop productivity. Not many chemicals have been marketed as mollus- cicides and those most commonly used include: metaldehyde and some carbamates such as methiocarb and more recently, iron phosphate plus chelating agents. Because it is difficult to penetrate a mollusc’s coating of slime, most molluscicides are used as poisons in baits based on wheat or barley flour, or as especially formulated flour-based pellets. However, these pellets also attract earthworms and a range of invertebrates such as beetles (Coleoptera), millipedes (Diplopoda), and woodlice (Isopoda), which reduce the pellets’ persistence and effectiveness by removing them from the soil surface within a few days (Bieri, 2003). Metaldehyde was introduced as a molluscicide in 1936 and was first used in slug baits in the early 1940s; it is still the commonest molluscicide used. Methiocarb (Mesurol Ò or Draza Ò ) is a carbamate that has also been used for slug and snail control and which has been demonstrated to affect earthworms (Bieri et al., 1989). Hen- derson et al. (1989) described the molluscicidal properties of some organic complexes of iron or aluminum. These were toxic when absorbed through the crawling feet of slugs. They showed that the metals in experimental milled metal salts could kill slugs exposed in the laboratory on a glass top, but in the field the metals needed to be chelated which increased their mobility and effects. More recently, since 1990s, this has been confirmed for metal-based compounds such as iron phosphate (Bullock et al., 1992; Young, * Corresponding author. Tel.: þ1 614 292 1149; fax: þ1 614 292 2180. E-mail address: edwards.9@osu.edu (C.A. Edwards). Contents lists available at ScienceDirect Crop Protection journal homepage: www.elsevier.com/locate/cropro 0261-2194/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2008.11.009 Crop Protection 28 (2009) 289–294