Molecular characterization of pyrethroid resistance in field- collected populations of the pea aphid, Acyrthosiphon pisum

Acyrthosiphon pisum Harris (Hemiptera: Aphididae) is a globally distributed and economically important aphid pest of leguminous crops including peas, broad beans, and alfalfa. Infestations weaken plants directly by feeding, and indirectly, by the transmission of yield-affecting plant viruses. Although pea aphid control strategies largely rely on the application of insecticides, cases of insecticide resistance under applied conditions have not yet been described. Here we investigated the molecular basis of high levels of pyrethroid resistance (up to >180-fold) in two strains of A. pisum, PYR-R1 and VR, collected in the field in France and Italy, respectively. Foliar bioassays revealed significant synergistic effects between deltamethrin and piperonyl butoxide (a cytochrome P450 (P450) inhibitor), and, to a lesser extent, S,S,S-tributyl phosphorotrithioate (an esterase inhibitor). P450 and esterase activities in both resistant strains were significantly higher compared to an insecticide susceptible reference strain SUS-1. Target-site resistance mutations in the voltage-gated sodium channel known to confer pyrethroid resistance in other aphid pest species were not detected in either resistant strain. However, transcriptome profiling identified the P450 gene CYP6CY12 as highly over-expressed in the resistant PYR-R1 strain compared to the SUS-1 strain, and real-time quantitative PCR showed that this gene is also significantly overexpressed (>30-fold) in the VR strain. Functional expression of recombinant CYP6CY12 revealed its catalytic capacity to hydroxylate deltamethrin, thus confirming its causal role in pyrethroid resistance. Our study uncovered a P450-mediated mechanism of pyrethroid resistance in A. pisum for the first time and will inform future resistance management tactics to control this pest.