Spinosad resistance in female Musca domestica L. from a field-derived population.

BACKGROUND: Bait-formulated spinosad is currently being introduced for housefly (Musca domestica L.) control around the world. Spinosad resistance was evaluated in a multiresistant field population and strains derived from this by selection with insecticides. Constitutive and spinosad-induced expression levels of three cytochrome P450 genes, CYP6A1, CYP6D1 and CYP6D3, previously reported to be involved in insecticide resistance, were examined. RESULTS: In 2004 a baseline for spinosad toxicity of Danish houseflies where all field populations were considered to be susceptible was established. In the present study, females of a multiresistant field population 791a were, however, 27-fold spinosad resistant at LC(50), whereas 791a male houseflies were susceptible. Strain 791a was selected with spinosad, thiamethoxam, fipronil and imidacloprid, resulting in four strains with individual characteristics. Selection of 791a with spinosad did not alter spinosad resistance in either males or females, but counterselected against resistance to the insecticides thiamethoxam and imidacloprid targeting nicotinic acetylcholine receptors. A synergist study with piperonyl butoxide, as well as gene expression studies of CYP6A1, CYP6D1 and CYP6D3, indicated a partial involvement of cytochrome P450 genes in spinosad resistance. CONCLUSION: This study reports female-linked spinosad resistance in Danish houseflies. Negative cross-resistance was observed between spinosad and neonicotinoids in one multiresistant housefly strain. Spinosad resistance involved alterations of cytochrome P450 gene expression.

Low expression of nicotinic acetylcholine receptor subunit Mdα2 in neonicotinoid-resistant strains of Musca domestica L.

BACKGROUND: Neonicotinoid action as well as resistance involves interaction with nicotinic acetylcholine receptors (nAChRs). In the housefly, neonicotinoid resistance also involves cytochrome P450, as indicated by bioassay with synergist as well as altered expression. In bioassay, synergism was only partial and indicated possible target-site resistance. The nAChR α2 subunit is important in neonicotinoid toxicity to insects, and gene expression of the Mdα2 subunit was investigated in field populations and laboratory strains of neonicotinoid-resistant and insecticide-susceptible houseflies, Musca domestica L. The genomic sequence covering exon III-VII of Mdα2 was analysed for mutations. RESULTS: Gene expression profiling of Mdα2 revealed notable differences between neonicotinoid-resistant and insecticide-susceptible houseflies. On average, the neonicotinoid-resistant field population 766b and the imidacloprid selected strain 791imi had 60% lower copy numbers of Mdα2 compared with the susceptible reference strain. Sequencing of exon III-VII of the Mdα2, encoding acetylcholine binding-site regions and three out of four transmembrane domains, did not reveal any mutations explaining the increased neonicotinoid tolerance in the strains examined. CONCLUSION: Previous discoveries and the results of this study suggest that the neonicotinoid resistance mechanism in Danish houseflies involves both cytochrome P450 monooxygenase-mediated detoxification and reduced expression of the nAChR subunit α2.

Vitamin K requirement in Danish anticoagulant-resistant Norway rats (Rattus norvegicus).

Resistance to warfarin has been connected to an increase in dietary requirement for vitamin K in British strains of the Norway rat, Rattus norvegicus (Berk). This study examines vitamin K requirement of Danish anticoagulant-resistant Norway rats using a vitamin K deficient feeding test. Wild bromadiolone-resistant rats sampled from different localities in Denmark and rats from bromadiolone-resistant and susceptible laboratory strains were fed on a vitamin K deficient diet over a maximum period of 15 days. Development of vitamin K deficiency, measured as reduced blood-clotting capacity, took place in 43% of the Danish resistant rats and was independent of sex, treatment with supplementary vitamin K3 and sampling locality. Development of deficiency was slower for resistant rats that were supplemented with vitamin K3 prior to the feeding test, suggesting storage of the vitamin K in a vitamin body pool. Intraperitoneal administration of vitamin K1 revealed that 80 microg vitamin K1 kg(-1) bodyweight was sufficient to restore normal blood clotting activity in deficient rats, while 60 microg vitamin K1 kg(-1) bodyweight was insufficient. We conclude that vitamin K requirement is moderately increased in Danish homozygous resistant rats whereas heterozygous resistant rats only have a minor increase in vitamin K requirement compared with susceptible rats. We found no indication of different resistance types being present in our test material since vitamin K requirement was not different between rats from separate sampling localities.