ISO 11268 Earthworm Toxicity Testing: Acute and Reproduction Tests for Soil Quality Assessment

ISO 11268 earthworm testing is the internationally recognized standard for evaluating the toxic effects of chemicals, contaminated soils, and waste materials on earthworms, which serve as key bioindicators of soil ecosystem health. The standard specifies controlled laboratory methods for measuring acute mortality and long-term reproductive impairment in Eisenia fetida and Eisenia andrei. This guide explains the scope of ISO 11268, the samples and endpoints involved, the test procedures, and the regulatory applications of the results.

Technical Overview

ISO 11268 is a multi-part international standard published by the International Organization for Standardization (ISO) that specifies laboratory procedures for determining the effects of pollutants on earthworms. Because earthworms represent a large proportion of soil invertebrate biomass, influence soil structure, nutrient cycling, and microbial activity, and are exposed to contaminants through both dermal contact and soil ingestion, they are widely accepted as representative organisms for terrestrial ecotoxicology.

The ISO 11268 series comprises the following parts:

• ISO 11268-1: Determination of acute toxicity to Eisenia fetida/Eisenia andrei, a 14-day mortality test conducted with adult worms.
• ISO 11268-2: Determination of effects on reproduction of Eisenia fetida/Eisenia andrei, a 56-day test assessing cocoon production and juvenile emergence.
• ISO 11268-3: Guidance for earthworm testing beyond the standard laboratory assays, including field-oriented investigations with lumbricid species.

Parts 1 and 2 are closely harmonized with OECD Test Guidelines 207 and 222, allowing data generated under one framework to support submissions under the other. Results are expressed as internationally recognized endpoints such as LC50 for mortality and EC10, EC50, NOEC, and LOEC for reproduction, which feed directly into environmental risk assessment and soil quality evaluation.

Testing Samples

Soil Samples

Natural soils collected from contaminated sites, agricultural land, industrial areas, or remediation zones can be evaluated directly. Field-collected soils are typically sieved, homogenized, and characterized for basic properties such as pH, organic matter content, texture, water holding capacity, and contaminant concentrations before testing.

Test Substances and Chemicals

Pure substances including pesticides, industrial chemicals, metals, pharmaceuticals, and nanomaterials are incorporated into standardized artificial soil at a geometric series of concentrations. Information on purity, water solubility, octanol-water partition coefficient, and stability supports dose selection and interpretation of results.

Waste and Recycled Materials

Composts, sewage sludge, dredged sediments, biochar, and other soil amendments can be mixed into reference soil at realistic application rates to screen for ecotoxicological hazards before land application or waste classification decisions are made.

Control and Reference Soils

Standardized artificial soil, composed of 70% quartz sand, 20% kaolin clay, and 10% sphagnum peat by dry mass with the pH adjusted to 6.0 ± 0.5, serves as the control substrate. A natural reference soil with documented earthworm performance may also be included to strengthen site-specific assessments.

Testing Items

Depending on the test part and study objectives, the following endpoints are measured:

• Adult mortality, recorded at 7 and 14 days, with derivation of the LC50
• Adult body weight (biomass) change over the exposure period
• Behavioral abnormalities such as rigidity, lesions, or surface crawling
• Cocoon production after the four-week adult exposure phase
• Number of juveniles at test termination, the principal reproduction endpoint
• Reproduction toxicity metrics including EC10, EC50, NOEC, and LOEC
• Optional supporting assessments such as soil avoidance behavior and bioaccumulation screening

Testing Methods

Acute Toxicity Test (ISO 11268-1)

Ten clitellate adult worms, generally 300 to 600 mg fresh weight and at least two months old, are placed into replicate vessels containing artificial soil dosed with the test substance or the contaminated soil under investigation. At least three replicate vessels per concentration plus a negative control are used, and a solvent control is included when a carrier is applied. Test units are incubated at 20 ± 2 °C under a 16:8 hour light-dark photoperiod, with soil moisture maintained at 40 to 60 percent of the water holding capacity. Mortality, defined as the absence of a response to gentle mechanical stimulation, is recorded after 7 and 14 days, and the LC50 is calculated using probit or logistic regression.

Reproduction Test (ISO 11268-2)

The reproduction test quantifies sublethal chronic effects over 56 days. Ten adult worms per replicate are exposed for four weeks, during which they receive a defined food ration such as finely ground dried manure. At day 28, surviving adults are removed, counted, and weighed; cocoons are counted, and the soil is incubated for a further four weeks to allow juvenile emergence. At day 56, juveniles are recovered by hand sorting or heat extraction and counted. The number of juveniles per replicate is the primary endpoint, analyzed to derive ECx values or NOEC/LOEC through appropriate statistical methods. Eight replicates per treatment are typically applied to ensure robust statistical power.

Test Validity and Quality Control

• Control mortality does not exceed 10 percent in either test
• Mean juvenile production in the reproduction test control meets the defined threshold with acceptable variability
• Temperature, moisture, and pH remain within specified tolerances throughout the study
• Test organisms originate from synchronized, healthy laboratory cultures with no recent chemical exposure
• A reference substance such as chloroacetamide for acute testing, or boric acid or carbendazim for reproduction testing, is run periodically to confirm assay sensitivity

Applications

ISO 11268 test data are used across regulatory, industrial, and research contexts:

• Environmental risk assessment of industrial chemicals under REACH and equivalent chemical control regulations worldwide
• Registration and monitoring of plant protection products, supporting risk characterization for terrestrial non-target organisms
• Evaluation of biocidal products and veterinary pharmaceuticals with potential soil exposure
• Ecotoxicological screening of contaminated land, brownfield redevelopment sites, and mine-impacted soils
• Quality control of composts, sewage sludge, dredged sediments, and other materials intended for land application
• Verification of remediation effectiveness through pre-treatment and post-treatment comparison
• Research on emerging contaminants, including microplastics, nanomaterials, PFAS, and pharmaceutical mixtures
• Scientific support for deriving soil screening values and environmental quality standards

FAQ

Why are Eisenia fetida and Eisenia andrei used as test species?

These species can be reared reliably in the laboratory year-round, have comparatively short generation times, and possess an extensive historical toxicity database that enables comparison across studies and laboratories. Although they are compost-dwelling species rather than deep-burrowing earthworms, their sensitivity profile is considered adequately protective for terrestrial risk assessment purposes.

What is the difference between ISO 11268-1 and ISO 11268-2?

ISO 11268-1 is a short-term acute test that measures adult mortality over 14 days and yields an LC50. ISO 11268-2 is a long-term chronic test lasting 56 days that measures reproductive output, reporting endpoints such as juvenile numbers, EC10, EC50, and NOEC. Reproduction tests are generally more sensitive and are typically required when acute data indicate potential hazard or when chronic risk assessment is mandated.

How long does testing take?

The acute test requires 14 days of exposure plus soil preparation and organism acclimation, and studies are usually completed within three to four weeks. Reproduction tests require 56 days of exposure plus preparation, extraction, and statistical analysis, and are generally completed within nine to twelve weeks.

How much sample is needed?

For a definitive test with five to seven concentrations plus controls, approximately 5 to 20 kg of soil on a wet weight basis is recommended, depending on the number of replicates and vessel size. Smaller screening studies can be performed with less material, and the testing laboratory should be consulted for study-specific requirements.

Can ISO 11268 tests be performed under GLP?

Yes. Accredited ecotoxicology laboratories conduct ISO 11268 assays under Good Laboratory Practice to support regulatory submissions to authorities such as ECHA, EFSA, the US EPA, and national agencies. Final reports include complete method descriptions, raw data, validity criteria verification, and statistical derivations.

How are the results interpreted?

Acute LC50 values and chronic EC10 or NOEC values are compared with predicted or measured environmental concentrations to calculate risk quotients or to classify soils according to ecotoxicological hazard schemes. Lower endpoint values indicate greater toxicity, and results falling below regulatory trigger values may necessitate refined testing or risk mitigation measures.