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 alpha-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 alpha-naphthyl-acetate, P-naphthyl-acetate, and para-nitrophenol acetate were substantially elevated in the GRMALRP, 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 SSS-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 (L9251) 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 L9251 and T929V were present in all GRMAL-RP males tested, at an approximately 1: 1 frequency, but never in combination in the same haplotype. (c) 2006 Elsevier Inc. All rights reserved.