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EXTOXNET I'll'-MI:TI IIDATTIION ii//ace.orst,edu/cgi-bin/m f's/O l/pips/meth idat.ht m?�13 t ) Ecological Effects: i • Effects on birds: Methidathion is highly toxic to birds following acute exposure. The reported oral LD50 values for the compound are 23 mg/kg to 33 mg/kg in mallards, 8.41 mg/kg in Canadian geese, 33.2 mg/kg in the ring-necked pheasant, and 225 mg/kg in the chukar partridge [8,6]. 1 Effects on aquatic organisms: The compound is very highly acutely toxic to aquatic organisms (both vertebrate and invertebrate); the reported LC50 values of the compound are 10 to 14 ug/L in ` rainbow trout and 2 to 9 ug/L in bluegill sunfish [13,72]. Tests on lobsters indicated that the combination of methidathion and another organophosphate insecticide, phosphamidon, was more toxic than either compound separately or than would be expected if the toxicities were added together [131. Studies with bluegill sunfish indicate that there is only a slight potential that the compound would accumulate in fish tissues [6]. Maximum levels of the residues of the pesticide after 1 month of exposure to verylow concentrations in the water, 0.05 u /L were 1.0 ug/kg p gin the edible tissue, 3.9 ug/kg in nonedible tissue, and 2.4 ug/kg in whole fish. These concentrations indicate a low bioconcentration factor of 46 for whole fish. After 2 weeks in water without methidathion, the concentration in whole fish fell by nearly 80% [2]. 4 • Effects on other organisms: Methidathion is slightly toxic to bees [131. Environmental Fate: i, • Breakdown-in soil.and.groundwater, Methidathion,is of0low.persistence in,the,sol environment; ` .reporteeld d fi .hal f-iiv_es are 5ao 23 days_, with a representative value of about 7 days [191. Breakdown of the compound in soil occurs through the action of soil microorganisms [83]. Under a alkaline conditions, methidathion is rapidly degraded by chemical action [8]. Methidathion and its breakdown products are poorly bound by soils, and so may be mobile [19,81]. However, they have f not been detected in any groundwater sources. This is probably due to the short half-life of the compound and its degradates. • Breakdown in water: No data are currently available. • Breakdown in vegetation: In plants, methidathion is rapidly metabolized [13]. Oranges sprayed with Supracide at a rate of nearly 2 pounds per acre had residues of the compound of about 0.1 ug/ml [81]. Within 2 days over 60% of the compound was removed from the outside of the fruit, and within 1 week, less than 1% of the compound remained [81]. Physical Properties: M t t • Appearance: Methidathion is a colorless crystalline compound at room temperature [13]. • Chemical Name: S-2,3-dihydro-5-methoxy-2-oxo-1,3,4-thiadiazol-3-ylmethyl 0,0-dimethylphosphorodithioate [13] • CAS Number: 950-37-8 • Molecular Weight: 302.33 • Water Solubility: 240 mg/L @ 20 C [13] • Solubility in Other Solvents: v.s. in octanol, ethanol, xylene, acetone, and cyclohexane [13] • Melting Point: 39.5 C [131 • Vapor Pressure: 186 mPa @ 20 C [13] • Partition Coefficient: 4.7243 [8] • Adsorption Coefficient: 400 (estimated) [19] Exposure Guidelines: i E3 of 4 1/26/00 8:16 ANI