Since the mid 1990s, the use of genetically modified crops has been rapidly adopted worldwide (Qaim 2005). Argentina is the third producer of transgenic crops in the world, with about 15 % of the global surface dedicated to transgenic crops, only surpassed by the USA and Brazil (James 2011). Out of all the crops, soybean is the one that presents the greatest growth in Argentina. Since the 1970s, the surface has grown steadily. While in 1971, only 37,700 ha were occupied with soybean by 2012, the surface dedicated to this crop was 19.7 million hectares (FAO 2012), reaching almost 66 % of the overall cropping surface in Argentina. The rapid adoption of new technologies (i.e., the use of transgenic soybeans resistant to glyphosate and no tillage) contributed to make the soybean exports and its derivatives one of the main sources of the country’s foreign exchange with a contribution of approximately US$2500 million for 2013. By the 2006 cropping season, almost 100 % of the surface sown with soybean in Argentina was already transgenic (Trigo and Cap 2003). The increase in the use of these soybeans resistant to glyphosate-based herbicides, or RR (Roundup Ready®), produced a steady increase in the consumption of this herbicide (Pengue 2000).

The mechanism of action of glyphosate is the inhibition of the 5-enolpyruvilshikimate-3-phosphate synthase enzyme (EPSPs). The inhibition of the EPSPs prevents the formation of enolpyruvil shikimato-phosphate and the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, and tryptophan), which are the precursors of important secondary metabolites in plants, such as lignin, flavonoids, and alkaloids (Pérez-Jones et al. 2005; Wakelin et al. 2004).

Glyphosate deactivates due to its adsorption to soil particles (Sprankle et al. 1975), and its degradation is related to the bacterial activity in the soil, particularly bacteria from the genus Pseudomonas (Gimsing et al. 2004). The USA Environmental Protection Agency has reported that its average lifespan is 60 days (USEPA 1999). Cox (1995) presented data collected from different authors which indicate that depending on the soil’s characteristics and climatic conditions, the persistence of this compound varies from a few days to 1 year. Glyphosate can also be adsorbed by clays and organic matter; Piccolo and Celano (1994) determined that this herbicide can be transported deep into the soil by humic substances. These soil particles can be toxic for organisms that ingest significant amounts of soil (Welten 2000) during their normal feeding, such as earthworms.

Considering the wide rural extensions that are treated annually with this herbicide worldwide, it is important for ecological risk assessment to study not only the acute but also the sublethal, chronic effects that it has on non-target species (Casabé et al. 2007). This is of special interest for those ecological groups that have a relevant importance for soil conservation and fertility, being earthworms one of the most important of such groups.

Earthworm activity modifies both physical and chemical soil properties, and their abundance and distribution are strongly influenced by the environmental conditions and the ecological state of the system (Edwards and Bohlen 1996; Lavelle and Spain 2001; Lee 1985). They also play a key role for soil functioning, and are therefore used extensively in terrestrial ecotoxicity studies.

Eisenia fetida (Savigny, 1826) is an exotic species in Argentina, native to the Nearctic region and it is found in most Argentinean soils (Momo and Falco 2009). In its adult state, it weighs between 400 and 600 mg, and it is capable of ingesting large quantities of cellulosic matter, being able to ingest up to its own weight per day. Based on its feeding and other habits, it is ecologically classified as an epigean species, a mulch eater, which includes all types of decomposing organic matter. This species has a high reproduction rate as long as the environmental conditions are good, and it reproduces year around, placing a cocoon per individual approximately every 10 days. The incubation time varies between 14 and 44 days, producing more than one descendent per cocoon (from 2 to 20 individuals) (Reynolds 1996; Venter and Reinecke 1988).

E. fetida is also an internationally validated species for ecotoxicological tests (ISO 1993; ISO 1998; OECD 2004) due to its cosmopolitan distribution and its easiness to raise and handle. This species is among the most commonly used one in environmental risk assessment and testing with chemicals such as Imidacloprid and RH-5849 (Zang et al. 2000). In a test with ten organic substances, Neuhauser and Callahan (1990), found that sublethal concentrations (lower than the LC50) of Carbaryl and Dieldrin produce a decrease in the growth rate and reproduction of this earthworm. Tests in microcosmos with Cypermethrin showed that E. fetida bioconcentrates this pesticide inside its body by a factor of 2 to 6 and biotransforms 92 % of the absorbed product without presenting adverse effects at the studied concentrations (Viswanathan 1992).

Zoran et al. (1986) also found teratogenic effects that depend on the dosage of the Benomyl fungicide during the regeneration of the posterior segments of these earthworms after amputation.

Toxicology tests are usually carried out as acute (48 or 72 h) or subchronic (7 to 17 days) tests. However, there is an increasing recognition for the need of longer-term tests to determine the chronic and sublethal pesticide effects on non-target populations (Antón et al. 1993; Spurgeon et al. 2004; Venkateswara Roa et al. 2003).

Therefore, the objective of this work was to determine the chronic, sublethal toxic effects of glyphosate in its commercial presentation as Roundup® (Monsanto, SL at 48 %) on populations of E. fetida and to evaluate the ecological importance of those effects on earthworms’ demographic dynamics.