Malathion

NOMENCLATURE
Common name: malathion; maldison; malathon; mercaptothion; carbofos; mercaptotion
IUPAC name: diethyl (dimethoxythiophosphorylthio)succinate; S-1,2-bis(ethoxycarbonyl)ethyl O,O-dimethyl phosphorodithioate
Chemical Abstracts name: diethyl [(dimethoxyphosphinothioyl)thio]butanedioate
CAS RN: [121-75-5]

PHYSICAL CHEMISTRY
Composition: Tech. grade is c. 95% pure. Mol. wt.: 330.3; M.f.: C₁₀H₁₉O₆PS₂; Form: Tech. is a clear, amber liquid. M.p.: 2.85 ^oC; B.p.: 156-157 ^oC/0.7 mmHg. V.p.: 5.3 mPa (30 ^oC). KOW: logP = 2.75; Henry: 1.21x10^-2 Pa m³ mol^-1 (calc.) S.g./density: 1.23 (25 ^oC). Solubility: In water 145 mg/l (25 ^oC). Miscible with most organic solvents, e.g. alcohols, esters, ketones, ethers, aromatic hydrocarbons. Slightly soluble in petroleum ether and some types of mineral oil. Stability: Relatively stable in neutral, aqueous media. Decomposed by strong acids and by alkali; hydrolysis DT₅₀ 107 d (pH 5), 6 d (pH 7), 0.5 d (pH 9) (all 25 ^oC). F.p.: 163 ^oC (Pensky-Martens closed cup).

APPLICATIONS
Biochemistry: Cholinesterase inhibitor; proinsecticide, activated by metabolic oxidative desulfuration to the corresponding oxon.
Mode of action: Non-systemic insecticide and acaricide with contact, stomach, and respiratory action.
Uses: Used to control Coleoptera, Diptera, Hemiptera, Hymenoptera and Lepidoptera in a wide range of crops, including cotton, pome, soft and stone fruit, potatoes, rice and vegetables. Used extensively to control major arthropod disease vectors (Culicidae) in public health programmes, ectoparasites (Diptera, Acari, Mallophaga) of cattle, poultry, dogs and cats, human head and body lice (Anoplura), household insects (Diptera, Orthoptera), and for the protection of stored grain. Typical application rates for agricultural uses 0.5-1.25 kg/ha.
Phytotoxicity: Non-phytotoxic in general, if used as recommended, but glasshouse cucurbits and beans, certain ornamentals, and some varieties of apple, pear, and grape may be injured.
Formulation types: DP; EC; EW; UL; WP.
Compatibility: Incompatible with alkaline materials (residual toxicity may be decreased).

MAMMALIAN TOXICOLOGY
Oral: Acute oral LD₅₀ for rats 1375-5500 mg/kg (pure), mice 775-3320 mg/kg.
Skin and eye: Acute percutaneous LD₅₀ (24 h) for rabbits 4100-8800, rats >2000 mg/kg.
Inhalation: LC₅₀ (4 h) for rats >5.2 mg/l.
NOEL: In 2 y trials on rats, the only effects seen at 500 ppm (29 mg/kg b.w. daily) was inhibition of cholinesterase in plasma and red blood cells.
ADI: 0.3 mg/kg b.w.
Toxicity class: WHO (a.i.) III; EPA (formulation) III (Fyfanon tech. and 5 EC)
EC hazard: Xn; R22

ECOTOXICOLOGY
Birds: Dietary LC₅₀ (5 d) for bobwhite quail 3500, ring-necked pheasants 4320 mg/kg diet.
Fish: LC₅₀ (96 h) for bluegill sunfish 0.1, largemouth bass 0.28 mg/l.
Daphnia: EC₅₀ (48 h) 1.0 ug/l.
Algae: EC₅₀ (72 h) 13 mg/l.
Bees: Toxic to bees. LD₅₀ (topical) 0.71 ug/bee.
Worms: LC₅₀ 613 mg/kg soil.

ENVIRONMENTAL FATE
Animals: In mammals, following oral administration, the major part of the dose is excreted in the urine and faeces within 24 hours. Degradation is by oxidative desulfuration by liver microsomal enzymes, leading to the formation of malaoxon; malathion and malaoxon are hydrolysed and thus detoxified by carboxylesterases. In insects, metabolism involves hydrolysis of the carboxylate and phosphorodithioate esters, and oxidation to malaoxon.
Plants: De-esterified to its mono- and di- carboxylic acids, which are cleaved to yield succinic acid, which is subsequently incorporated into plant constituents.
Soil/Environment: Under normal conditions, it is 99% degraded by hydrolysis within 7 d.