Thiamethoxam virescens

Thiamethoxam has been reported to yield 7V-desmethyl thiamethoxam as well as clothianidin, the latter representing the dominant product 19). The metabolic conversion of thiamethoxam to clothianidin has been implied for its toxicology. To address this claim, we conducted an analytical study in which a 1000 ng (1 il dose of 1000 ppm) dose of thiamethoxam was injected into tobacco budworm Heliothis virescens 5 instar larvae and symptoms monitored at time points of 13, 30, 60 and 120 minutes after exposure. Symptoms of thiamethoxam toxicity were reliably observed 30 minutes after injection thus this time period was chosen for further experiments. Symptoms recorded were uncoordinated locomotion and rapid mandibular motion at a frequency of 3-4 jaw movements per second. 

Figure 4. Formation of clothianidin from thiamethoxam in H. virescens. A mass of1000 ng thiamethoxam was injected into 5 instar larvae. In Figure B, the concentration of clothianidin from Figure A is shown at an enlarged scale.

Next, we focused on whether the amounts of clothianidin produced as a metabolite of thiamethoxam at 30 min could induce the symptoms observed. Therefore known amounts of clothianidin, namely 50, 100, 500, and 1000 ng were injected into four replicates of H. virescens larvae to determine symptoms... 

There is a clear mismatch between the mass of clothianidin produced by metabolism of thiamethoxam, namely 23 ng, and the 100-500 ng of clothianidin needed to exhibit toxicity symptoms in H. virescens after 30 minutes. Therefore, it is extremely unlikely that the mechanism of action of thiamethoxam is propesticidal via clothianidin, as claimed 19). The most parsimonious hypothesis is that thiamethoxam is biologically active in its own right. 

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See also in sourсe #XX -- [ ]

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