Malathion 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).

Malathion (CAS 121-75-5) is a phosphorodithioate organophosphorus pesticide, developed by American Cyanamid Co. It is a non-systemic insecticide and acaricide which acts by contact, ingestion or breathing vapour (Tomlin 1994). Its IUPAC name is diethyl (dimethoxythio phosphorylthio)-succinate, molecular formula is C10H19O6PS2 and molecular weight is 330.3. Malathion is soluble in water to 145 mg/L at 25°C (Tomlin 1994), and has a low log Kow of 2.7. The current analytical practical quantitation limit (PQL) for malathion in water is 0.1 µg/L (NSW EPA 2000).

Uses and environmental fate

Malathion has low mammalian toxicity and has been used in situations of human and animal contact, particularly for control of disease vectors (e.g. mosquitoes), household insects, human lice and other animal parasites in pets, birds and agricultural animals (Tomlin 1994). It also has a wide range of uses against insect pests of flowers, trees and turf and is registered in Australia for around 30 crop types including cereal, fruit, nuts and vegetables (NRA 1997a). Malathion has around 700 registered uses in Australia.

Malathion degrades more rapidly in alkaline water. Its hydrolysis half-life in water is 0.2 weeks at pH 8, compared to 21 weeks at pH 6 (HSDB 1996), but biodegradation may be more significant in acidic waters. Sorption to algae appeared to speed up photodegradation (HSDB 1996). Malathion persistence in river water varied from 52 to 21% over 11 to 14 days (HSDB 1996). Bioaccumulation of malathion was not significant.

Aquatic toxicology

As an organophosphorus pesticide, malathion inhibits the AChE enzyme in organisms. The oxon metabolite is more toxic. Malathion is highly toxic to many fish and invertebrates, although there is a wide variation from one species to another.

Freshwater fish: 24 species, 48 to 96-hour LC50, 4 to 39,600 µg/L. There were wide variations between species. The most sensitive (LC50 in µg/L) were Perca flavescens (263), Micropterus salmoides (250 to 285), Morone saxitilis (18 to 460), Notopterus notopterus (77) and four trout species (4 to 280). The least sensitive were Tinca tinca (16,200), Cyprinus carpio (10,210 to 18,650) and Pimephales promelas (8650 to 25,000). Chronic no observed effect concentration (NOEC) 32-day growth and mortality figures were reported for Gila elegans (990 to 2000 µg/L resp) and Ptylocheilus lucius (1680 µg/L), giving an acute-to-chronic ratio (ACR) of around 1.

Freshwater crustaceans: 10 species, again toxicity varied with different groups. Most sensitive were cladocerans, ostracods and copepods (six species, 48 to 96-hour EC50 immobilisation, 1.4 to 6.2 µg/L). Prawn Palaeomonetes kadiakensis (12 to 100 µg/L) and crayfish Orconectes nais (50 to 180 µg/L) were intermediate. Prawn Macrobrachium lamarrei (1261 to 1687 µg/L) and crab Oziotelphusa senex senex (6000 µg/L) were least sensitive.

Freshwater insects: four species, 48 to 96-hour LC50, 1.1 to 180 µg/L.

Freshwater molluscs: There was a wide variation in the 48 to 96 h LC50 for 5 test species. Thiara (two species) had figures of 31 to 37 µg/L; Viviparus bengalensis (96 hours) had 1510 to 2340 µg/L; Pila globosa; 10,000 to 15,490 µg/L; and Biomphalaria havanensis, 126,270 µg/L.

Marine fish: one species, 96-hour LC50, 22.5 µg/L.

Marine crustaceans: three species, 96-hour LC50, 1.4 to 12 µg/L.

Australian and New Zealand data

Data were available for the waterflea Ceriodaphnia dubia (1.4 µg/L) and the fish Melanotaenia fluviatilis (2090 to 39,600 µg/L). These were among the most sensitive for the cladoceran and least sensitive for the fish.

Factors that modify toxicity

Increases in temperature varied toxicity of malathion in different directions: one-fold decrease for Simocephalus sp. from 10°C to 21°C; four-fold increase for P. promelas from 7°C to 29°C (Johnson & Finley 1980). Hardness did not affect toxicity to fish or invertebrates. Synergy occurred when malathion was mixed with a number of chemicals, including parathion and carbaryl but toxicity was only additive with dichlorodiphenyltrichloroethane (DDT) or toxaphene (Johnson & Finley 1980).

Guideline levels

A freshwater moderate reliability trigger value of 0.05 µg/L was calculated for malathion using the statistical distribution method with a 95% protection level and an ACR of 17.5. There were limited marine data, and a marine low reliability trigger value of 0.05 µg/L was adopted from the freshwater figure. This figure should only be used 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.

HSDB (Hazardous Substances Data Bank) 1996. Micromedex Inc. 31 July 1996.

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.

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