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Using earthworms for pesticide homologation tests: yes, but not just any type

At present, evaluating the risks linked to the use of pesticides with a view to their homologation requires, among other factors, an assessment of their effects on earthworms, and more specifically on the species Eisenia fetida.
This is currently the reference species, even though it is not present in cultivated soils. INRA researchers in Versailles-Grignon have demonstrated that some species that are common in cultivated soils, such as Aporrectodea
caliginosa and Lumbricus terrestris, are more sensitive to pesticides than E. fetida. Overall, their findings show that E. fetida is not the most appropriate species for use in pesticide homologation tests.

Vers de terre (Lumbricus terrestris L.). Individus adultes reconnaissables à leur clitellum.. © INRA, FAYOLLE Léon
Updated on 05/19/2015
Published on 12/05/2012

The pesticides used in agriculture may contaminate air, water and soils, causing ecotoxicological risks to the organisms which live in these environments. Evaluation of these risks is a crucial stage in obtaining a marketing authorisation for these products, and is also important to preventing environmental contamination. The evaluation is based on studies which comply with European standards (Directive 91/414 EEC) and notably includes tests designed to measure the effect of pesticides on earthworms. Very numerous earthworm species exist throughout Europe, and it is E. fetida which is currently used for plant health product homologation tests. However, there is no real consensus regarding the sensitivity of different earthworm species to pesticides.

The INRA scientists in Versailles-Grignon sought to compare the sensitivity of different earthworm species to pesticides. For this purpose, they analysed 1800 scientific publications, from which they selected fifteen studies that were particularly pertinent to their objective. They then constructed a database comprising several variables arising from these scientific publications. Of these, the median lethal dose (LD50) – or the dose at which a substance caused the death of 50% of the population of earthworms under determined experimental conditions – was able to characterise the sensitivity of an earthworm species to a given pesticide.

Thanks to this statistical approach combining the results of a series of studies (or meta-analysis), the researchers were able to show that two earthworm species, Lumbricus terrestris and Aporrectodea caliginosa, are more sensitive to pesticides than E. fetida. The former are found in the agricultural soils that are particularly concerned by pesticide use, while E. fetida tends to prefer decomposing organic matter. It is therefore found in greater numbers in manures or composts, both of which are very little affected by pesticide use.
In light of these findings, it seems necessary to reconsider the choice of E. fetida for the performance of homologation tests. It would appear to be more pertinent to use A. caliginosa, which can easily bred, as the rearing of L. terrestris is more complicated.

Overall, this work has emphasised the usefulness of meta-analyses in ecotoxicology in order to compare the sensitivity of different earthworm species to pesticides. This approach could be applied to other organisms that are also used for pesticide homologation or other ecotoxicological tests. Furthermore, these analyses showed that new homologation procedures linked to the use of A. caliginosa could be defined. Although pesticides have made a major contribution to improving agricultural yields and enabled considerable advances in controlling food resources, their harmfulness to man and the environment nevertheless remains a controversial issue.

Find out more

C. Pelosi, S. Joimel and D. Makowski. 2013. Searching for a more sensitive earthworm species to be used in pesticide homologation tests – A meta-analysis. Chemosphere 90: 895.

http://dx.doi.org/10.1016/j.chemosphere.2012.09.034

Earthworms

As well-known inhabitants of our soils, numerous species of earthworms exist, spread between different soil layers:
• Epigeal: they have a strong preference for dead leaves and plant debris spread over the surface of forest and grassland soils on which they lie as litter. Small in size, these earthworms have a strong, dark red colour, and are called Eisenia fetida, Lumbricus castaneus, Lumbricus rubellus or Satchellius mammalis.
• Endogenic: they live permanently in the soil where they dig horizontal tunnels. Pink, grey or green, but globally weakly coloured, they are of small size and include Aporrectodea caliginosa, Aporrectodea icterica, Octolasium cyaneum or Allolobophora c. chlorotica typica.
• Anecic: they frequent deep soil in which they dig vertical tunnels. They are large – even reaching a length of 1 metre – and red, brown or black in colour, such as Lumbricus terrestris, Aporrectodea giardi or Aporrectodea longa.

Depending on their species, earthworms contribute more or less to the structuring, maintenance and productivity of agricultural, forest and grassland soils.