Demeton-S-methyl in freshwater and marine water

Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals' of the ANZECC & ARMCANZ (2000) guidelines.

The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.

Description of chemical

Organophosphorus pesticides are derivatives of phosphoric, phosphonic, phosphorothioic, or phosphonothioic acids, comprising many chemicals with a wide range of uses (WHO 1986). They exert their acute effects in insects, fish, birds and mammals by inhibiting the acetylcholinesterase (AChE) enzyme, but may also have a direct toxic effect (WHO 1986).

Demeton-S-methyl (CAS 919-86-8) is a phosphorothioate OP pesticide, first introduced by Bayer AG. It is a systemic insecticide and acaricide, which acts by contact and ingestion (Tomlin 1994). Its IUPAC name is S-2-ethylthioethyl O,O-dimethylphosphorothioate, molecular formula is C₆H₁₅O₃PS₂, and molecular weight is 230.3. It is moderately soluble in water to 22 g/L at 20°C and its log K_ow is low at 1.3 (Tomlin 1994).

Uses and environmental fate

Demeton-S-methyl is registered in Australia for over 50 uses, mainly against aphids, mites and flies in around 10 food crops, including fruit, cereal and vegetables, plus cotton, ornamental plants, tobacco, and various pastures (NRA 1997a). Demeton-S-methyl hydrolyses rapidly, particularly in alkaline conditions and is rapidly broken down in soil (Tomlin 1994).

Aquatic toxicology

Data (48-hour LC50) were only available for four species of freshwater molluscs (snails), 3700 µg/L for one and 2800 to 35,000 µg/L for three.

Guidelines

A freshwater low reliability trigger value of 4 µg/L was calculated for demeton-S methyl using an assessment factor (AF) of 1000. In the absence of marine data, this was adopted as a marine low reliability trigger value. These figures should only be used as indicative interim working levels.

The nature of OP pesticides indicates that toxicity to some species of crustaceans and insects could occur at concentrations lower than those that caused mortality in molluscs. Any concentrations of demeton-S-methyl above 4 µg/L should be examined more closely.

References

ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.

NRA 1997a. Database extraction of selected pesticides: Registered uses in Australia, National Registration Authority, July 1997, Canberra.

Tomlin C 1994. The pesticide manual: A world compendium. 10th edn, British Crop Protection Council & Royal Society of Chemistry, Bath, UK.

WHO 1986. Environmental health criteria 63.Organophosphorus insecticides: A general introduction. World Health Organization, Geneva.