Dimethoate 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).

Dimethoate (CAS 60-51-5) is a phosphorodithioate OP pesticide with contact and stomach action (Tomlin 1994). Its IUPAC name is O,O-dimethyl S-methylcarbamoylmethyl phosphorodithioate, formula is C5H12NO3PS2 and molecular weight is 229.2. It is soluble in water to 23.8 g/L at 20°C and has a low log Kow of 0.7. The current analytical practical quantitation limit (PQL) for dimethoate in water is 0.1 µg/L (NSW EPA 2000).

Uses and environmental fate

Dimethoate is used for control a variety of insects, including aphids, and mites in a range of food crops, cotton and tobacco (Tomlin 1994).

Dimethoate is hydrolysed in alkaline solution with DT50 of 12 days at pH 9 (Tomlin 1994). It has 50% loss from soil in 7 to 16 days.

Aquatic toxicology

Dimethoate has very high toxicity to a few species of fish and invertebrates but moderate to low toxicity to others.

Freshwater fish: 16 species, 96-hour LC50. There was a wide variation in toxicity between species: three species had figures 2.3 to 65.0 µg/L, five species had 3100 to 7800 µg/L and the seven species 4570 to 70,800 µg/L (Poecilia reticulata had an outlying figure of 560,000 µg/L). A chronic 7-day no observed effect concentration (NOEC) (mortality) for Brachydanio rerio was 3100 to 5300 µg/L.

Freshwater crustaceans: six species, 48 to 96-hour LC50 or EC50 (immobilisation), 2.0 to 2.6 for three species, 200 to 6400 µg/L for three species.

Chronic NOEC for Daphnia magna: for growth (16 days), 29 µg/L, for immobilisation and mortality (20 to 23 days) it was 32 to 170 µg/L; for reproduction it was 100 µg/L.

Freshwater insects: one species Pteronarcys californica (stonefly): 96-hour LC50, 43 µg/L.

Freshwater molluscs: four species, 48 to 96-hour LC50, 2.9 to 36.0 µg/L, plus 5360 for Viviparus bengalensis.

Freshwater algae: one species, 96-hour EC50 (growth) 480,000 µg/L.

Freshwater amphibians: one species, 96-hour LC50, 7.8 µg/L.

Marine fish: one species, 96-hour LC50, 700 µg/L.

Marine algae: 96 hours (photosynthesis), 9 to 11 µg/L.


The freshwater data were distinctly bimodal (2 to 70 µg/L and ≥ 3000 µg/L) and only the more sensitive group was used.

A freshwater moderate reliability trigger value of 0.15 µg/L was derived for dimethoate using the statistical distribution method at 95% protection and the default acute-to-chronic ratio (ACR) of 10. Given the very limited marine data, this figure should be adopted as a low reliability marine guideline. This should be used only as an indicative interim working figure.


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.

NSW EPA 2000. Analytical Chemistry Section, Table of Trigger Values 20 March 2000, LD33/11, Lidcombe, NSW.

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.