Methyl thiophanate

Microtubulin Polymerization Inhibitors. The ben2imida2oles were first reported to have systemic fungicidal activity in 1964 (29). Prominent examples include thiabendazole [148-79-8] (42) fuberida2ole [3878-19-1] (43) carbendazim [10605-21-7] (44) benomyl [17804-35-2] (45) and thiophanate methyl [23564-05-8] (46). Benomyl (45), the most widely used member of this group is almost certainly inactive as a fungicide until it is converted in plants and soil to carbendazim (44). Likewise, thiophanate and thiophanate methyl (46) are nonfungitoxic until converted to carbendazin (44). 

A subsequent comparison of these ionization techniques for the study of another eight pesticides, this time including three of the five of interest [28], i.e. carbendazim, thiabendazole and thiophanate methyl, showed that ESI gave enough sensitivity to allow reliable determination of the pesticides at concentrations below their respective maximum residue levels. 

The method was then applied to extracts of orange peel but neither benomyl or thiophanate methyl were recovered as the parent compounds due to their well-known conversion to carbendazim during the extraction process - this is an effect that is not encountered when analysing standards. 

Half-lives of two pesticides were estimated using the assumption that the decay was a first-order process. Half-lives were 28 8.5 days and 12 3.4 days for thiophanate-methyl and methiocarb, respectively. Crop volume was not a significant determinant of exposure using a grid count method. This was probably due to a large variation in the results with this method. 

Three pesticides having large variation in recommended dose rate were selected for this study. All were considered chemically stable and non-volatile. Model pesticides that were selected were thiophanate-methyl (CAS 23564-05-08 tradename Topsin M 500 g a.i./L), methiocarb (CAS 2032-65-... 

For extraction of thiophanate-methyl, methanol (250 mL per glove) was added to the gloves, and, for extraction of methiocarb, 250 mL 60/40 (v/v%) methanol/water (per glove) was added to the gloves and adjusted to pH 4.5 with acetic acid. Gloves were then ultrasonicated for 10 min and extracted for 30 min by shaking at 200 strokes per min. 

Extracts of gloves, extracts of foliar dislodgeable residue and absolute foliar residues, and filters containing thiophanate-methyl were analyzed by reversed-phase HPLC and ultraviolet detection at 254 nm (Engel, 1988). The LOD was 50 mg/L for extracts of filters and gloves, and 10 mg/L for leaf samples. Recovery was > 90%, and the "between days" CV of the analytical chemical method was < 5%. 

Note HV = high-volume application LV = Low-volume application Tpm = thiophanate-methyl Meth = methiocarb Aba = abamectin. 

Figure 1 Increase of average DFR of methiocarb (o), thiophanate-methyl (+), and abamectin ( ) due to application with either high-volume or low-volume techniques.

Figure 2 Typical log-transformed decays of DFR for estimating half-lives of methio-carb (o) and thiophanate-methyl (+).

The DFR values were followed over a period of 4 weeks from the high-volume application. The decrease of DFR in the two zones was monitored in six greenhouses (three times after application of thiophanate-methyl and three times after application of methiocarb). Figure 2 shows typical log-transformed decays of DFR (average of two samples for each zone) for methiocarb and thiophanate-methyl. Assuming a first-order decay, half-lives were calculated using Equations (3) and (4) and were found to be 29 8.5 days and 11 3.4 days for thiophanate-methyl and methiocarb, respectively. 

Table 3 Range, Median, GM, and GSD of Assessed Total Dermal and Respiratory Exposure to Thiophanate-Methyl and Methiocarb...

Table 5 Regression Analyses of Total Dermal Exposures to Thiophanate-Methyl (mg/hr) as Dependent Variable to Number of Harvested Flowers and Re-entry Time...

Lrom the regression analyses it is clear that significant relationships were observed between respiratory exposure to thiophanate-methyl and methiocarb (adjusted to the number of flowers harvested) and dislodgeable foliar residues during re-entry. 

Data on the DFR of chlorothalonil on carnation crops in previous studies indicated an increase in DFR of 10 to 20% with solvent extraction (unpublished data). However, in all of the current experiments, no differences were observed between dislodgeable foliar residue and solvent-extracted foliar residue for the pesticides methiocarb and thiophanate-methyl. 

Half-life estimates of approximately 28 days for thiophanate-methyl indicate a very slow decay compared to methiocarb with an estimate of half-life of about 11 days. The application of a model based on a first-order decay process resulted in fairly high R2 and significant fit. The results suggest that both pesticides are relatively stable compared to other compounds under similar environmental conditions (Brouwer et al., 1994). With respect to the objectives of the study and the proposed model, it can be stated that the results confirm the assumption of a linear relationship between application rate (for both application techniques) and the increase of dislodgeable foliar residue. This relationship holds for modeling purposes. The contribution of the crop density or total crop surface area to the process of interception cannot be quantified with the results of the present study. Because the interception factor ranges from about 0.35 to 0.9 (Willis and McDowell, 1987), the... 

Engel, R. (1988) Determination of Thiophanate-methyl by High Performance Liquid Chromatography and UV Detection, Report BT-41, Medical Biological Laboratory TNO, Department of Occupational Toxicology, Rijswijk, The Netherlands (in Dutch). 

Benomyl is used for its protective and eradicant activity against several pathogens of cereals, vines, fruit, rice and vegetables, and is used in postharvest treatments. Like thiophanate-methyl, it undergoes conversion in plants, soils and animals into the methylbenzimidazol-2-yl carbamate, carbendazim, which has a similar spectrum of activity. 

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