OECD 207 Earthworm Acute Toxicity Test: Principles, Methodology, and Regulatory Applications

OECD Test Guideline 207 is an internationally standardized laboratory method for evaluating the acute toxicity of chemicals to earthworms, a key sentinel organism for soil ecosystem health. This article explains the test principle, applicable sample types, endpoints, experimental procedure, validity criteria, and typical regulatory applications of the OECD 207 earthworm acute toxicity test. It is intended for regulatory specialists, environmental scientists, and laboratory professionals involved in chemical and pesticide risk assessment.

Technical Overview

OECD Test Guideline 207, titled "Earthworm, Acute Toxicity Tests," was adopted in 1984 and remains one of the most widely applied international standards for assessing the effects of chemical substances on soil-dwelling organisms. Earthworms play a central role in terrestrial ecosystems: they drive the decomposition of organic matter, improve soil structure and aeration, and form the base of many terrestrial food chains. Because they ingest soil and are exposed to contaminants through both dermal contact and ingestion, earthworms are considered representative indicators of soil contamination.

The guideline describes two complementary test designs: a filter paper contact test, which provides a short-term screening assessment of dermal toxicity, and an artificial soil test, which simulates a more realistic exposure scenario in a standardized soil matrix. The primary endpoint of the artificial soil test is mortality after a 14-day exposure period, from which a median lethal concentration (LC50) is derived. The filter paper test serves mainly as a preliminary or range-finding tool.

Results from OECD 207 studies are used in hazard classification, terrestrial ecological risk assessment, and regulatory decision-making under frameworks such as the European REACH regulation, plant protection product authorization schemes, and national chemical notification programs. The method is technically compatible with ISO 11268-1, which is harmonized with the OECD guideline for the artificial soil exposure pathway.

Testing Samples

Industrial Chemicals and Pure Substances

Single-ingredient chemicals, including industrial intermediates, solvents, metals, and metal salts, are the core scope of the guideline. Test substances are characterized for purity, water solubility, vapor pressure, and stability before dosing, as these properties determine the spiking procedure and the achievable concentration range.

Pesticides and Agrochemical Formulations

Active ingredients and formulated plant protection products, such as emulsifiable concentrates, wettable powders, suspension concentrates, and granules, are routinely evaluated using OECD 207. Testing both the active substance and representative formulations allows regulators to assess formulation-related toxicity differences.

Contaminated Soils, Wastes, and Soil Amendments

Although OECD 207 was originally designed for chemicals, the artificial soil procedure is frequently adapted for environmental samples such as contaminated site soils, composts, sewage sludges, biocidal treated materials, and dredged sediments. In these adapted designs, the test soil is typically diluted in a standardized clean or artificial control soil to establish a concentration-response relationship.

Nanomaterials and Substances of Unknown or Variable Composition

Nanomaterials, polymers of low concern, and UVCB substances (substances of unknown or variable composition, complex reaction products, or biological materials) can be assessed with modified dispersion and dosing procedures, provided that exposure characterization is documented and the test validity criteria are met.

Testing Items

The OECD 207 study delivers the following core measurements and endpoints, reported together with full study documentation:

• 14-day LC50 in artificial soil (median lethal concentration, with 95% confidence limits where data permit)
• 48-hour (optionally 72-hour) LC50 in the filter paper contact test
• Mortality counts in all treatment groups and controls at the 7-day and 14-day observation points
• Behavioral observations, including abnormal movement, lethargy, darkening, lesions, or avoidance of treated soil
• Body mass (fresh weight) changes measured at the start and end of exposure as a supporting sublethal indicator
• NOEC and LOEC values for mortality when a full dose-response relationship cannot be established
• Limit test result at 1000 mg/kg dry soil when no toxicity is expected at the maximum regulatory concentration
• Physical-chemical characterization of exposure media, including pH, moisture, and test substance concentration checks where analytically feasible

Testing Methods

Test Organism and Preparation

The recommended test species is the compost worm Eisenia fetida (also spelled Eisenia foetida) or its close relative Eisenia andrei. These species are used because they are easy to culture in the laboratory, have well-documented biology, reproduce rapidly, and exhibit sensitivity to a broad range of soil contaminants. Adult, clitellate worms at least two months old with a fresh body weight of approximately 250 to 600 mg are selected from a healthy, synchronized laboratory culture. Before dosing, worms are conditioned in untreated artificial soil, typically for at least 24 hours, and any injured or atypical animals are excluded.

Filter Paper Contact Test

In the contact test, glass vials are lined with filter paper that is treated with 1 mL of the test solution or suspension. Water is used as the carrier; poorly water-soluble substances may be dissolved in a volatile organic carrier such as acetone, which is fully evaporated before the paper is re-wetted with clean water. At least five to six concentrations in a geometric series, together with water and carrier controls, are tested, generally with ten earthworms per concentration. The vials are incubated in the dark at 20 ± 2 °C for 48 hours, with an optional extension to 72 hours, after which mortality and visible abnormalities are recorded. This design assesses direct dermal toxicity and is primarily used as a screening or range-finding step.

Artificial Soil Test

The artificial soil test is the definitive exposure pathway of the guideline. The standardized substrate consists of 70% industrial quartz sand, 20% kaolinite clay, and 10% sphagnum peat on a dry weight basis, with the pH adjusted to 6.0 ± 0.5 using finely ground calcium carbonate. The water content is adjusted to approximately 40 to 60% of the maximum water holding capacity. The test substance is mixed homogeneously into the soil, either as an aqueous solution or suspension, on a solid carrier such as quartz sand, or via a volatile solvent that is allowed to evaporate before worms are introduced.

A range-finding test, for example at 1, 10, 100, and 1000 mg/kg dry soil, is typically performed first. The definitive test then uses a geometric series of at least five concentrations, with a spacing factor generally not exceeding 2.2, bracketing the expected lethal range. Ten earthworms are added to each test container, usually in three to four replicates per concentration, together with an untreated control and a solvent control where applicable. The containers are incubated at 20 ± 2 °C under a defined light regime, commonly continuous illumination of approximately 400 to 800 lux or a 16:8 hour light:dark cycle, for 14 days.

Environmental Conditions and Validity Criteria

Throughout the exposure period, temperature, moisture, and pH are monitored and maintained within the specified tolerances. Mortality is assessed at day 7 and day 14 by emptying and hand-sorting each container; a worm is considered dead when it shows no response to gentle mechanical stimulation of the anterior region. The study is considered valid when cumulative mortality in the control group does not exceed 10% at the end of the 14-day exposure. Additional quality requirements include the use of healthy worms from a synchronized culture, homogeneous distribution of the test substance in the soil matrix, and documented culture history.

Data Evaluation and Reporting

Mortality data are analyzed using appropriate statistical models, such as probit or logit regression or maximum likelihood estimation, to calculate the LC50 with 95% confidence intervals. If the data do not support a robust LC50 estimate, the NOEC, LOEC, or the highest tested concentration causing no mortality is reported, for example as an LC50 greater than 1000 mg/kg in a limit test. The final study report documents the test substance characterization, preparation of dosing solutions, soil composition and spiking procedure, biological observations, analytical verification where performed, statistical methods, and a GLP compliance statement when the study is conducted for regulatory submission.

Applications

OECD 207 data are requested or accepted across a wide range of regulatory and scientific contexts:

• Chemical registration under the European REACH regulation, where earthworm acute toxicity supports the terrestrial compartment hazard assessment and risk characterization
• Plant protection product authorization in the EU and other jurisdictions, where acute earthworm toxicity (OECD 207) is evaluated together with reproductive toxicity (OECD 232) for risk quotient calculations and persistence trigger evaluation
• National new chemical notification programs, including registration schemes in China, Japan, Korea, North America, and elsewhere that require terrestrial invertebrate toxicity data as part of the environmental fate and effects dossier
• Biocidal product evaluation, where soil organisms are among the non-target organisms of concern for products released to the terrestrial environment
• Contaminated land and site risk assessment, where adapted OECD 207 and ISO 11268-1 procedures support soil screening values, remediation target derivation, and site-specific ecological risk assessments
• Scientific research, including comparative toxicity studies, mixture toxicity evaluation, QSAR model development, and environmental safety assessment of novel materials such as nanomaterials and biodegradable polymers

FAQ

Why is Eisenia fetida the recommended test species?

Eisenia fetida and Eisenia andrei are epigeic compost worms that are straightforward to culture year-round, have short generation times, and respond sensitively to many soil contaminants. Their extensive use across decades of research has generated a large reference database, which makes them the internationally recommended sentinel species for standardized earthworm toxicity testing under both OECD and ISO frameworks.

What is the difference between the filter paper contact test and the artificial soil test?

The filter paper contact test exposes earthworms directly through their skin to the test substance on moist filter paper for 48 hours and functions mainly as a rapid screening or range-finding tool. The artificial soil test exposes worms for 14 days in a standardized soil matrix containing defined proportions of sand, clay, peat, and moisture, providing a more environmentally realistic exposure pathway. The artificial soil LC50 is the endpoint that carries primary weight in regulatory risk assessment.

How long does a complete OECD 207 study take?

The definitive artificial soil exposure itself lasts 14 days. Including worm acclimation, range-finding tests, definitive exposure, mortality sorting, statistical analysis, and report finalization, a complete study is typically completed within four to six weeks, depending on the number of test substances and the complexity of the dosing procedure.

When is a limit test appropriate?

A limit test at a single concentration of 1000 mg/kg dry artificial soil is appropriate when existing information, such as structural analogs, physical-chemical properties, or screening data, indicates that no toxicity is expected at or above this concentration. If no mortality occurs at the limit concentration, the LC50 is reported as greater than 1000 mg/kg.

Can OECD 207 be used to test contaminated field soils?

The guideline was written for soluble and insoluble chemical substances, but the artificial soil procedure is routinely adapted for field-collected soils, wastes, and amendments. Adaptations typically involve diluting the test soil with clean reference soil or artificial soil to create a concentration series, and the study should then be reported in line with ISO 11268-1, which explicitly covers soil sample testing.

How does OECD 207 relate to ISO 11268-1?

ISO 11268-1 is the international standard for earthworm acute toxicity testing and is scientifically harmonized with OECD 207, particularly for the artificial soil design. The two documents share the same species, soil composition, exposure duration, and core endpoints, with minor procedural differences. Data generated under either standard are generally mutually accepted by regulatory authorities.

Does the study need to be conducted under GLP?

When the study is submitted to regulatory authorities for chemical or pesticide registration, it must generally be performed in compliance with Good Laboratory Practice (GLP) principles, including full documentation of the quality system, study plan, raw data, and archive retention. Non-GLP screening studies are acceptable for internal research, range finding, and early development decision-making.

What if chronic or sublethal effects are the concern?

OECD 207 is an acute mortality study. For long-term and sublethal effects, the OECD 232 earthworm reproduction test should be used, which exposes Eisenia fetida or Eisenia andrei for eight weeks and derives a NOEC or ECx for reproduction. Regulatory schemes for persistent substances or repeated-application scenarios typically require both acute and chronic earthworm data.