Susceptibility to thiamethoxam of Musca domestica from Danish livestock farms

BACKGROUND: Neonicotinoid baits are currently replacing anticholinesterase baits for control of adult houseflies (Musca domestica L.). Introduction of new insecticides includes evaluation of their cross-resistance potential, which was assessed for thiamethoxam in field populations from Denmark. RESULTS: In feeding bioassay with a susceptible strain, thiamethoxam LC50 at 72 h was 1.7 mu g thiamethoxam g(-1) sugar, making it 19-fold, 11-fold and threefold more toxic to houseflies than azamethiphos, methomyl and spinosad respectively. The field populations were 6-76-fold resistant to thiamethoxam. There was no correlation between the toxicities of thiamethoxam and spinosad, dimethoate, methomyl, bioresmethrin or azamethiphos. The toxicity in feeding bioassay at 72 h of imidacloprid in a susceptible strain was 32 mu g imidacloprid g(-1) sugar at LC50, making it 19-fold less toxic to houseflies than thiamethoxam. There was a strong significant correlation between the toxicities of thiamethoxam and imidacloprid in field populations. CONCLUSION: Neonicotinoid-resistant houseflies were present at a detectable and noticeable level before thiamethoxam and imidacloprid were introduced for housefly control in Denmark. The toxicity of thiamethoxam is explained by other parameters than the toxicities of spinosad, dimethoate, methomyl, bioresmethrin or azamethiphos. The cross-resistance between thiamethoxam and imidacloprid indicates a coincidence of mechanism of the toxicity and resistance in the field populations. (c) 2007 Society of Chemical Industry.