Thiamethoxam

EC Decision on the view of the possible inclusion of thiacloprid, forchlorfenuron, thiamethoxam m... 

Neonicotinoids are potent broad-spectrum insecticides that exhibit contact, stomach and systemic activity. Acetamiprid, imidacloprid, nitenpyram, thiamethoxam and thiacloprid are representatives of the neonicotinoid insecticides (Figure 1). The mechanism of action is similar to that of nicotine, acting on the central nervous system causing irreversible blocking of postsynaptic nicotinic acetylcholine receptors (nAChR). Neonicotinoid insecticides are often categorized as antagonists of the... 

The definition of crop residues for thiamethoxam and thiacloprid includes the parent and its metabolite fV-(2-chlorothiazol-5-ylmethyl)-fV -methyl-A"-nitroguanidine (designated the guanidine compound) for thiamethoxam, and 3-(6-chloro-3-pyridylmethyl)-l,3-thiazolidin-2-ylideneaminocarboxamide (designated the amide compound) for thiacloprid (Figure 3). 

Amide compound of thiacloprid Guanidine compound of thiamethoxam Figure 3 Stmctures of metabolites of thiacloprid and thiamethoxam... 

Acetamiprid Imidacloprid Nitenpyram Thiaclopiid Thiamethoxam Imidacloprid... 

Analyte Acetamiprid Imidacloprid Nitenpyram metabolite Thiacloprid metabolite Thiamethoxam metabolite Imidacloprid... 

Ministry of the Environment, The Official Analytical Method for Thiamethoxam of the Ministry of the Environment, Japan, Ministry of the Environment, Tokyo (2001) (in Japanese). 

Zhou, Q., Y. Ding, and J. Xiao. 2006. Sensitive determination of thiamethoxam, imidacloprid and acet-amiprid in environmental water samples with sohd-phase extraction packed with multiwalled carbon nanotubes prior to high-performance hquid chromatography. Anal. Bioanal. Chem. 386 1520-1525. 

Guzsvany, V., Kaddar, M., Gaal, F., Bjelica, L. and Toth, K. (2006), Bismuth film electrode for the cathodic electrochemical determination of thiamethoxam. Electroanalysis, 18(13-14) 1363-1371. 

Thiamethoxam is a new broad-spectrum systemic insecticide used for control of aphids, leafhoppers, scales, thrips, whiteflies, and others. Its oral LD30 in rats is >5000 mg/kg. 

Most soils have a pH ranging from 4 to 9. The degradation of pesticides such as organo-phosphates and carbamates is affected by the pH of the soil. Most organophosphates are hydrolyzed under alkaline conditions, but diazinon is unstable in acid soils. Carbamates such as carbofuran are also hydrolyzed under alkaline conditions. The persistence of neonicotinoids is primarily determined by the pH. Imidacloprid and thiamethoxam are hydrolyzed under alkaline conditions, whereas thiacloprid and acetamiprid are less stable... 

Karmakar, R., Singh, S.B., and Kulshrestha, G., Persistence and transformation of thiamethoxam, a neonicotinoid insecticide, in soil of different agroclimatic zones of India, Bull. Environ. Contam. Toxicol, 76,400, 2006. 

Figure 6 A proposed activation of thiamethoxam in insects and plants.

The toxicokinetics of the thianicotinyl thiamethoxam is similar to that of imidacloprid. When applied orally to rats, goats, or chickens, thiamethoxam is rapidly and almost quantitatively absorbed. Its excretion, predominantly in urine, is fast. Accumulation in tissues is negligible. Thiamethoxam itself does not bind strongly to the neonicotinoid binding site of the nicotinic acetylcholine receptor but it is reported to be converted to clothianidin, a neonicotinoid with high affinity for the insect receptors, in insects and plants (Figure 6). It is possible that this activation proceeds via formation of an N-desmethyl thiamethoxam intermediate, another... 

Green, T., Toghill, A., Lee, R., Waechter, F., Weber, E., Peffer, R., Noakes, J., and Robinson, M. (2005). Thiamethoxam induced mouse fiver tumors and their relevance to humans. Part 2 Species differences in response. Toxicol Sci 86, 48-55. 

Pastoor, T., Rose, P., Lloyd, S., Peffer, R., and Green, T. (2005). Case study Weight of evidence evaluation of the human health relevance of thiamethoxam-related mouse liver tumors. Toxicol Sci 86, 56-60. 

Recently, resistance to imidacloprid became a serious problem for crop protection. Comparative studies of other neonicotinoids revealed a high crossresistance to acetamiprid and thiamethoxam against imidacloprid-resistant strains [22]. However, bioassays exhibited that 4a has good activity against imidacloprid-resistant strains of brown planthopper (Table IV), showing very low cross-resistance to imidacloprid, as shown in Table V. 

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