Evironmental project, 986 – Background report on pre-validation of an OECD springtail test guideline

Background Report on Pre-validation of an OECD Springtail Test Guideline

1 Introduction

This report provides a background report for a draft OECD test guideline for long-term toxicity to Collembola, commonly known as springtails (see Annex A). This report aim at providing a background for the draft guideline and to discuss relevant questions related to this guideline.

Collembolans are the most numerous and widely occurring insects in terrestrial ecosystems. This is the one of the main reasons for why they have been widely used as bioindicators and test organisms for detecting the effects of environmental pollutants and different agricultural (especially pesticide) regimes on biodiversity in soils (e.g. Paoletti et al. 1991; Crossley et al. 1992; Filser 1995; Filser et al. 1995; Frampton 1997; Paoletti and Bressan 1996). A variety of species have been employed in terrestrial ecotoxicological test. For the present draft OECD guideline Folsomia candida and Folsomia fimetaria have been select as previous standard tests are available (for further discussion on this see "results and discussion").

A springtail test employing the species Folsomia candida (ISO, 19993) is already in use for pesticide evaluation within Europe and USA. It is used where Environmental Agencies require an evaluation of the soil quality, for example when evaluating the potential problems related to spreading sewage-sludge or plant protection products on agricultural land. The test is well suited as a standard-test as the test organisms are easily cultured, the test easily accomplished and the demand for equipment is minimal. The springtail is representative for arthropods that is characterised by an aerobiontic cuticle compared to the soft-bodied semi-aquatic organisms viz. the earthworm test. On these grounds the springtail test is recommended for development in the OECD TG in OECD Monograph no. 11 (Detailed review Paper on Aquatic Testing methods for Pesticides and Industrial Chemicals (Paris 1998) ^[1]). The method is also recommended by SETAC ^[2] and in the EU Guidance Document for risk assessment for the soil environment for pesticides ^[3], biocides and industrial chemicals ^[4].

An alternative to the F. candida test is a standard test with the near relative Folsomia fimetaria (Wiles & Krogh, 1998). The two species differ in their habitat choice, F. candida resides primarily in habitats rich in organic matter such as pot-plants and compost heaps, whereas F. fimetaria is common in agricultural soils. This make the toxicity effects estimated based on F. fimetaria more applicable to agricultural land and nature areas. Further, Folsomia fimetaria has a sexual mode of reproduction, which is in contrast to F. candida consist of females only with a parthenogenetic (asexual) reproductive mode. It is expected that when utilising a sexually reproducing species the results will have a broader application, since the majority of species are assumed to have sexual reproduction.

The two species can appear alike from a visual point-of-view, but they differ in size with F. candida being the larger. It is unknown whether this difference is important for their general sensitivity toward chemicals. The literature is inconclusive on whether the two species differ in sensitivity. For example, Krogh (1995) exposed the two species to dimethoate under identical conditions and observed significant (P<0.05) differences in the EC₁₀ values with regard to reproduction. On the other hand, no significant differences were observed for the copper EC₅₀ values for the two species when exposed under identical conditions (Pedersen et al 1999). Form a pure theoretical point-of-view it could be argued that the two species are likely to differ in sensitivity as the surface-to-volume ratio [this is probably important for toxicity of chemicals that have a passive uptake] is related to the size.

As F. fimetaria have a sexual mode of reproduction it may in theory be possible to identify chemicals with sex-determination effects. There are several ways in which to identify a compound that can influence the sex-determination, these includes biochemical assays, morphological tests and sex-ratio tests (deFur et al 1999). Since, the mode-of-action is not known a test of sex-ratio of the juvenile outcome seem the most appropriate test. In addition, a determination of the sex-ratio of the offspring, if successful, could be a simple addition to the overall test design of the draft guideline.

There is at present no clear evidence for which compounds that primarily influence sex-determination of invertebrates. The vertebrate sex hormones have been suggested, but this is uncertain. Testosterone has been reported to have some androgenlike activity in some crustaceans (LeBlanc 1999) and endogenous concentrations of testosterone and other vertebrate-type androgens have been reported in Daphnia magna and Neomysis integer (LeBlanc and MacLachlan 2000, Verslycke et al 2002, 2004). Estradiol has been recorded in the silk moth Bombyx mori (Keshan and Ray 2001). However, neither estradiol nor testosterone affected the sex-ratio in exposed Daphnia magna (Kashian and Dodson 2004). Nevertheless, two chemicals, methyl-testosterone and ethynyl-estradiol, have been suggested as reference chemicals for evaluating potential sex-endocrine-disrupting compounds in invertebrates by the participants of "Workshop on Endocrine Disruption in Invertebrates: Endocrinology, Testing, and Assessment (EDIETA)", held in The Netherlands in 1997 (Ingersoll et al 1999).

1.1 Aims

To support the draft guideline two main activities were pursued.

As the two species differ with respect to ecology, reproduction biology and physiology (size). The importance of this in relation to the guideline will be reviewed and shortly discussed. These issues will partly be pursued via literature review and through laboratory test, with emphasis on possible chemical sensitivity differences due to age/size differences between the two species.
In connection with the test-guideline and a future ring-test it will be necessary to have positive control compounds to ensure the stability of the test-system. Hence, it is an aim to identify the future test concentration ranges for potential positive controls. The selection of the three compounds will be based on previous ring-test programmes.

Footnotes

[1] Danish publication (sponsored by DG ENV, Danish EPA 14th Office and the Danish EPA Pesticide Research Programme)

[2] Cf. ”Test Methods to determine hazards of Sparing Soluble Metal Compounds in Soils” 2002

[3] Cf. EU Guidance Document on terrestrial Ecotoxicology Under Council Directive 91/414/EEC, SANCO/10329/2002 rev.2 final

[4] jfr. EU’s rev. TGD: ”Technical Guidance Document on Risk Assessment in support of Commission Directive 93/67/EEC on Risk Assessment for new notified substances and Commission Regulation (EC) No. 1488/94 on Risk Assessment for Existing Substances and Directive 98/8/EC of the European Parliament and of the Council Concerning the placing of Biocidal Products on the Market” (May 2002)