Azinphos-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).

Azinphos-methyl (CAS 86-50-0), also known as guthion, is a phosphorodithioate organophosphorus (OP) insecticide and acaricide introduced by Bayer AG. A related chemical, azinphos-ethyl (CAS 2642-71-9) has similar aquatic toxicity (Tomlin 1994). Both are non-systemic and act by contact and ingestion. The IUPAC name for azinphos-methyl is S-(3,4-dihydro-4-oxobenzo[d]-[1,2,3]-triazin-3-ylmethyl)O,O-dimethyl phosphorodithioate, formula is C₁₀H₁₂N₃O₃PS₂ and molecular weight is 317.3. It is only slightly soluble in water to 28 mg/L at 20°C and has a log K_ow of 2.96. The current analytical practical quantitation limit (PQL) for azinphos-methyl in water is 0.1 µg/L (NSW EPA 2000).

Uses and environmental fate

Azinphos-methyl and azinphos-ethyl have similar uses against chewing and sucking insects in fruit, vines, nuts, vegetables and a variety of other food crops (Tomlin 1994). Azinphos-methyl is also used against spider mites. Both chemicals have low mobility in soil.

Aquatic toxicology

Freshwater fish: 15 species, 96-hour LC50, 0.36 (Esox lucius) to 4270 µg/L (Carassius auratus). Most figures were below 500 µg/L although four species were less sensitive and some figures for Pimephales promelas were also high (geometric mean of 268 µg/L).

Freshwater amphibian: one species, Xenopus laevis, 96-hour LC50, 420 to 2940 µg/L.

Freshwater crustaceans: seven species, 48 to 96-hour LC50, 0.1 to 7.5 for five species and 21 to 56 µg/L for Asellus brevicaudatus and a crayfish Procambarus sp.

Freshwater insects: three species, 48 to 96-hour LC50, 0.37 to 88 µg/L.

Marine fish: three species, 96-hour LC50, 3.5 to 45 µg/L.

Marine crustacean: one species, Penaeus monodon, 96-hour LC50, 120 µg/L.

Australian and New Zealand data

Hardersen and Wratten (1996) reported 48-hour LC50 values for two species of New Zealand damselfly: Xanthocnemis zelandica (31.0 to 33.6 µg/L) and Austrolestes colensonis (88 µg/L).

Factors that affect toxicity

Degradation in water over 1 to 3 weeks increased toxicity between 1.3 and 2 times (Johnson & Finley 1980).

Changes in hardness did not significantly affect toxicity of azinphos-methyl. Increased temperature generally increased toxicity of azinphos-methyl to fish. A two-fold increase occurred for Oncorhynchus mykiss between 2°C and 18°C and for bluegills P. promelas between 12°C and 22°C. A much larger increase in toxicity, 17 times, occurred for yellow perch, Perca flavescens, between 7°C and 22°C.

Guideline

A freshwater moderate reliability guideline figure of 0.02 µg/L was derived for azinphos-methyl using the statistical distribution method with 95% protection and the default acute-to-chronic ratio (ACR) of 10. The 99% protection level is 0.01 µg/L and is recommended as the trigger value for slightly to moderately disturbed systems because the 95% figure does not give an adequate margin of safety for protection from acute toxicity.

In the absence of adequate marine data, the freshwater value of 0.01 µg/L could be adopted as a low reliability trigger value, to be used only as an indicative interim working level.

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.

Hardersen S & Wratten SD 1996. The sensitivity of the nymphs of two New Zealand damselfly species (Odonata: Zygoptera) to azinphos-methyl and carbaryl. Australasian Journal of Ecotoxicology 2, 55-60.

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.

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.