Identification of mutations in the para sodium channel of Bemisia tabaci from Crete, associated with resistance to pyrethroids

We investigated the mechanisms of resistance to α-cypermethrin in a Q biotype, highly resistant Bemisia tabaci strain (GRMAL-RP) isolated from Crete. Cytochrome P450-dependent monoxygenase activity with the substrate ethoxycoumarin, and carboxylesterase activity with the substrates α-naphthyl-acetate, β-naphthyl-acetate, and para-nitrophenol acetate were substantially elevated in the GRMAL-RP, compared to the susceptible SUD-S strain, while glutathione-S-transferase activity with the substrate 1-chloro-2,4-dinitrobenzene was not different. The metabolic inhibitors piperonyl butoxide and S,S,S-tributyl phosphorotrithioate synergised cypermethrin toxicity in the GRMAL-RP strain, however, mortality was still lower than that of the susceptible strain, indicating the presence of an additional resistance mechanism. Analysis of the sequence of the IIS4–IIS6 region of the para sodium channel gene of the GRMAL-RP strain revealed two amino acid replacements compared to that of the SUD-S susceptible strain. One is the leucine to isoleucine substitution at position 925 (L925I) previously implicated in B. tabaci pyrethroid resistance and the other is a novel kdr resistant mutation for B. tabaci, a threonine to valine substitution at position 929 (T929V). Genotype analysis showed that the L925I and T929V were present in all GRMAL-RP males tested, at an approximately 1:1 frequency, but never in combination in the same haplotype.