As nouns the difference between thiamethoxan and thiamethoxam
is that thiamethoxan is the insecticide (ez)-3-(2-chloro-1,3-thiazol-5-ylmethyl)-5-methyl-1,3,5-oxadiazinan-4-ylidene(nitro)amine while thiamethoxam is a particular neonicotinoid insecticide.