Methomyl 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

Methomyl (CAS 16752-77-5), introduced by DuPont, is a commonly used oxime carbamate insecticide and acaricide, which acts by contact and ingestion (Tomlin 1994). Its IUPAC name is S-methyl-N-[[(methylamino)carbonyl]-oxy]ethanimidothioate. Its molecular weight is 162.2 and formula is C5H10N2O2S. It is soluble in water to 58 g/L at 20°C and its log Kow is only 1.24. The current analytical practical quantitation limit (PQL) for methomyl in water is 5 µg/L (NSW EPA 2000).

Uses and environmental fate

Methomyl is used to control a large variety of insects and mites in crops and animal houses, dairies etc. (Tomlin 1994). In Australia, methomyl has up to 57 uses on crops, fruit and ornamentals against Lepidoptera, Diptera, Hemiptera, Homoptera and Coleoptera, as well as mites (NRA 1997a). It is also used against flies on garbage tips and in animal areas.

Methomyl is rapidly degraded in soil and water (DT50 in soil < 0.2 days).

Aquatic toxicology

Freshwater fish: 12 species, 96-hour LC50, 300 to 6800 µg/L. Chronic NOEC Pimephales promelas (31 days mortality), 327 µg/L.

Freshwater crustaceans: five species, 48 to 96-hour LC50, 8.8-920 µg/L (outlying LC50s of 1050 and 3200 µg/L).

Freshwater insects: three species, 96-hour LC50, 29 to 60 µg/L.

Freshwater molluscs: five species, 48 to 96-hour LC50, 870 to 18,000 µg/L.

Marine fish: two species, 96-hour LC50, 340 to 1160 µg/L.

Marine crustaceans: four species, 96-hour LC50, 19 to 410 µg/L.

Factors causing variations in toxicity

The 24% liquid concentrate of methomyl was much less toxic to Daphnia magna than the technical grade but this effect was not noticed for other species. No changes in toxicity were detected for two fish species with 10°C changes in temperature or with pH changes between 6.5 and 8.5 (Johnson & Finley 1980). Ageing of test solutions for 7 days, possibly allowing oxidation of the C–S bond, caused increased toxicity in Gammarus but not in fish (Johnson & Finley 1980). LC50s for crustaceans were higher in hard water.


A freshwater moderate reliability trigger value of 3.5 µg/L was calculated for methomyl using the statistical distribution method with 95% protection and the default assessment factor (AF) of 10.

A marine low reliability trigger value of 3.5 µg/L was adopted from the freshwater figure. This should be used only as an indicative interim working level.


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.

Johnson WW & Finley MT 1980. Handbook of acute toxicity of chemicals to fish and aquatic invertebrates. US Department of the Interior, Fish and Wildlife Service, No 137, Washington DC.

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