Potential for pyrethroid resistance: Characterization of the Diaphorina citri Kuwayama voltage-gated sodium channel

Diaphorina citri, an insect pest of citrus and a vector of a disease-causing bacterium, ‘Candidatus’ Liberibacter asiaticus, was first discovered in Florida in 1998, and has since then spread through citrus groves across the state. The bacterium causes an incurable disease called Huanglongbing (HLB), the control of which relies heavily on the control of D. citri populations through the use of insecticides. In 2011 and 2012, state-wide screening for insecticide resistance revealed decreased susceptibility to two pyrethroids, fenpropathrin and zeta-cypermethrin. Anticipating that resistance against pyrethroids will continue to increase, and recognizing that target-site insensitivity is a common mechanism of resistance against this class of insecticides, we cloned and characterized the full length, D. citri voltage-gated sodium channel (VGSC) cDNA sequence. Using comparative sequence analyses of codon usage, we determined the likelihood of observing the reported amino acid changes that confer target-site insensitivity to pyrethroids in other insect species, and found that the most commonly reported amino acid change, L1014C/F/H/S/W, would require at least two nucleotide changes to occur. This is in contrast to the second most common change, M918I/L/T/V, which would require only one. Other potential amino acid changes, including several reported in pyrethroid resistant hemipterans, were found within the S4-S5 linker of domain II and the S6 helix of domain III, the region implicated in pyrethroid binding, which would require only one nucleotide change to confer a resistant phenotype. These results suggest that the D. citri VGSC holds potential for target-site insensitivity against pyrethroids.