Phorate
PESTICIDE NAME: Phorate
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Trade name(s): Thimet, Agrimet, Geomet
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Manufacturer(s): American Cyanamide Co.
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Ag. Res. Div.
P.O. Box 400
Princeton, N.J. 08540
I. Basic information
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A. Molecular structure: C7H17O2PS3
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B. Chemical name: O,O-Diethyl S-[(ethylthio)methyl]phosphodithioate
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C. Derivatives: partially oxidized to sulfoxide then to sulfone
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D. Molecular weight: 260.4 g/mole
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E. Solubility in water: 17.9 mg/l
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F. Common physical appearance: clear, mobile liquid
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G. Oral LD50(rat): 1-5 mg/kg
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H. Pesticide classification: organophosphate insecticide
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I. Restricted use list (N.Y.): yes
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EPA priority pesticide list: no
J. Crop use: corn, beans, potato
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II. Text
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Phorate is an organophosphate insecticide used on vegetable crops
in New York. It is oxidized to the sulfoxide then to the sulfone in
soils. The degradation presumably follows first-order kinetics. The
persistence of phorate is influenced by soil type; the half-life has
been found to range from lwk to 17d.
Phorate is considered to be immobile and the adsoption isotherm is
Freundlich. Adsorption is reversible; however, hysteresis has been
found.
III. Soils information
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A. Degradation and transformation
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A discussion of degradation of phorate in soils must include the
transformation of the parent material to the metabolites. The sulfoxide
is formed quite rapidly which is then rapidly transformed to the
sulfone(8,9). Sulfoxide is the major metabolite in submerged soils
while sulfone is predominant in non-flooded soils(8). Degradation
follows first-order kinetic reactions and the half-life of phorate
varies from a few days to a week in sandy loam(12,13), 6d in silt loam
under surface application and 30d in silt loam under incorporated
application(11). The parent phorate decreases to <1-10% in 3-7wks in
sandy loam soil(12,13) or 14wks in peaty loam(12). In subtropical silt
loam, 4% of the applied phorate was recovered at 6wks and 0.4% at
12wks(14). The sulfoxide of phorate has been reported to reach its
maximum concentration at 1wk in sandy loam and 3wks in peaty loam; both
soils show a 99% disappearance of sulfoxide at 4mo(12). In this same
experiment, the sulfone maximum was reached at 4wks in sandy loam, 7wks
in peaty loam and remained constant (20-30% sandy loam, 30-40% peaty
loam) at 34wks.
Degradation appears to be slower in sterile than non-sterile
soils. The major reaction in sterile soils is the oxidation of phorate
to the sulfoxide with the sulfone not appearing until microbes are
introduced(3). Persistence of the chemical and the conversion to
metabolites is greater in muck soils than soils of lesser organic matter
content(7). Degradation slows as temperatures at 10cm soil depth
<6-7deg C(12).
The following tables present data concerning phorate degradation
in soils. The reference is given in parentheses at the end of each
title.
Recovery of phorate and its metabolites (percent of applied) in loam
and sand(5)
Species Loam Sand
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phorate 2.4% 3.7%
sulfoxide 24.0% 12.1%
sulfone 25.1% 3.5%
other 1.2% 1.7%
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Persistence of phorate (percent of applied) in silt loam as a function
of surface or mixed application method(11)
Month Surface(%) Mixed(%)
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0 97.6(phorate) 56.0(phorate)
0.25 44.2(sulfox) 77.1(sulfox)
1 19.2(sulfone) 49.3(sulfone)
2 6.1 " 37.6 "
3 7.2 " 28.9 "
5 4.5 " 18.7 "
12 2.1 " 12.1 "
16 1.4 " 4.3 "
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Percent phorate lost under irrigated and non-irrigated conditions (1)
Days % Phorate lost
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0 irrigated 0
non-irrig. 0
10 irrigated 6
non-irrig. 5
30 irrigated 69.6
non-irrig. 70.0
45 irrigated 100
non-irrigated 98.6
60 irrigated -
non-irrig. 100
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Percent of applied phorate as residue in sand and muck(7)
SAND: phorate 15% at 1wk, 1% at 4wks
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sulfoxide 16% at 1wk, %o.m.
sl 2.32 0.94 264 3.27 0.88 372
ls 5.48 0.91 298 6.89 0.86 375
l 9.62 0.92 211 12.65 0.89 277
sil 16.14 0.88 246 19.23 0.86 294
l 73.79 1.01 233 96.16 1.02 304
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IV. References (*denotes key reference)
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*1.Agnihotri, N.P., H.K. Jain, S.Y. Panday, R.S. Dewan, A.N. Sexena,
and K.M. Peshwani. 1975. Ind.J.Ent. 37. 68-71.
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2.Chapman, R.A. and C.M. Cole. 1982. J.Environ.Sci.Health. B17.
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487-504.
*3.Chapman, R.A., C.M. Tu, C.R. Harris and Carol R. Harris. 1982
J.Econ.Ento. 75. 112-17.
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*4.Felsot, A. and P.A. Dahm. 1979. J.Ag.FoodChem.1979.27.557-63.
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5.Fuhremann, T.W. and E.P. Lichtenstein. 1980. J.Ag.FoodChem. 28.
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446-52.
6.Getzin, L.W. and C.H. Shanks, Jr. 1970. J.Econ.Ento.63.52-8.
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*7.Harris, C.R. and R.A. Chapman. 1980. Can.Ento. 112. 641-54.
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8.Kahn, S.U. 1980. Pesticides in the Soil Environment. Amsterdam:
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Elsevier Press.
*9.Leistra, M. 1978. J.Environ.Sci.Health. B13. 343-60.
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*10.Lichtenstein, E.P. 1975. PureAppl.Chem. 42. 113-18.
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*11.Lichtenstein, E.P., T.W. Fuhremann, K.R. Schulz and T.T. Liang.
1973. J.Econ.Ento. 66. 863-6.
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*12.Suett, D.L. 1971. Pestic.Sci. 2. 105-12.
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*13.Suett, D.L. 1975. Pestic.Sci. 6. 385-93.
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14.Talekar, N.S., L.T. Sun, E.M. Lee and J.S. Chen. 1977.
J.Ag.FoodChem. 25. 348-52.
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*15.Visalakshi, A., M.R.G.K. Nair and R.S. Aiyer. 1979. Pl.andSoil.
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51.571-6.
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