Fenitrothion in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
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).
Fenitrothion (CAS 122-14-5) is a non-systemic phosphorothioate insecticide with contact and stomach action, introduced by Sumitomo Chemical Co and, independently, by Bayer AG (Tomlin 1994). Its IUPAC name is O,O-dimethyl-O-4-nitro-m-tolylphosphorothioate, its formula is C9H12NO5PS and molecular weight is 277.2. It is slightly soluble in water to 21 mg/L at 20°C and has a log Kow of 3.43 (Tomlin 1994). The current analytical practical quantitation limit (PQL) for fenitrothion in water is 0.1 mg/L (NSW EPA 2000).
Uses and environmental fate
Fenitrothion is used for control of insects in cereals, fruit, vegetables and forestry, in stored grain and other products, for control of household insects and for public health control of mosquitoes (Tomlin 1994). Up to 20,000 t of fenitrothion are produced each year (WHO 1992). In Australia, fenitrothion is a key chemical in the control of plague locusts in the inland eastern states, applied both by air and ground spraying.
Fenitrothion is degraded by photolysis and hydrolysis with a laboratory half-life of 200 to 630 days at 15°C and 17 to 60 days at 30°C (pH 5 to 9) (WHO 1992). Degradation was affected by sunlight but not by suspended solids. Its degradation in river water was more rapid, only 1 to 28% remained after 72 hours, compared to 56 to 97% is seawater (WHO 1992). Microbial breakdown was a significant factor. Fenitrothion does not significantly bioconcentrate; bioconcentration factor (BF) values were ≤ 450.
Toxicity of fenitrothion varied widely with different species but some freshwater fish, crustaceans and insects were most sensitive.
Freshwater fish: 26 species, 48 to 96-hour LC50 varied widely with different species, 1.6 to 12,600 µg/L. Most figures were between 1000 and 8500 µg/L (moderate toxicity). Most sensitive species (96 hours) were trout, as well as Barbus ticto (7.6 µg/L), Gambusia affinis affinis (1.6 to 2.6 µg/L) and eel Anguilla anguilla (200 µg/L). Least sensitive were Channa gachua (11,800 to 12,600 µg/L), Cyprinus carpio (12,000 µg/L), Heteropneustes fossilis (12,500 µg/L) and one anomalously high figure for Oncorhynchus mykiss (796,000 to 852,000 µg/L).
Freshwater crustaceans: 14 species, 48 to 96-hour LC50 or EC50 (immobilisation), 0.9 to 30.0 µg/L, additional outlying species were copepod Diaptomus forbesi (1280 µg/L), crayfish Orconectes propinquus (269 µg/L) and crab Oziotelphusa senex senex (300 to 400 µg/L).
Freshwater insects: 11 species, 48 to 96-hour LC50, 1 to 82 µg/L. Additional outlying species were mayfly Isonychia sp. (49 to 164 µg/L), caddisfly Pycnopsyche sp. (137 to 230 µg/L), blackfly Simulium venustum (148 µg/L).
Freshwater molluscs: 13 species, 48 to 96-hour LC50, 7.9 to 25,000 µg/L. Four species had geometric means < 20 µg/L and two species > 10,000 µg/L.
Freshwater annelids: three species, 48 to 96-hour LC50, 2740 to 8500 µg/L.
Freshwater nematode: one species, 96-hour LC50, 1600 µg/L.
Freshwater algae: 10 species, 96-hour EC50 (growth or biomass), 800 to 24,400 µg/L. Chronic no observed effect concentration (NOEC) (14-day growth) for Staurastrum sp. was 100 µg/L giving an acute-to-chronic ratio (ACR) of 8.
Marine fish: six species, 96-hour LC50, 240 to 1370 µg/L.
Marine crustaceans: three species, 96-hour LC50 of 0.1 to 12 µg/L. The 53-h LC50 for Crangon septemspinosa (7 µg/L) was similar. Homarus americanus (lobster) was most sensitive.
Marine molluscs: 96-hour EC50 growth for Crassostrea virginica of 690 µg/L and, for mortality of Mytilus edulis, 15,000 to 18,800 µg/L.
Given the high sensitivity of lobster it was preferable to derive a low reliability trigger value using a high assessment factor (AF) than to adopt the freshwater figure. Low algal toxicity was assumed, allowing for use of an AF of 100.
Australian and New Zealand data
The 96-hour LC50 to the shrimp Paratya australiensis was 1.3 to 2.8 µg/L, similar sensitivity to other shrimp.
Factors that modify toxicity
Variations in temperature (7 to 17°C), pH (6.5 to 9), hardness (12 to 300 ppm) or degradation over 3 weeks, did not alter toxicity of fenitrothion (Johnson & Finley 1980).
A freshwater moderate reliability trigger value of 0.2 µg/L was calculated for fenitrothion using the statistical distribution method at 95% protection and an ACR of 8. A marine low reliability trigger value of 0.001 µg/L was calculated for fenitrothion using an AF of 100. 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.
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.
WHO 1992. Environmental health criteria 133. Fenitrothion. World Health Organization, Geneva.