Mechanisms of pyrethroid resistance in clonal cultures of the pea aphid Acyrthosiphon pisum

The pea aphid Acyrthosiphon pisum is known as a worldwide sucking pest of leguminous plants. Although it needs to be kept under economic damage thresholds by the application of insecticides, no cases of insecticide resistance were yet described in the Arthropod Pesticide Resistance Database(https://www.pesticideresistance.org).

However, we collected and cloned a French, PYR-R1, and Italian, VR, population of A. pisum showing significantly lower susceptibility to pyrethroids when compared to a susceptible reference strain SUS-1. PYR-R1 and VR exhibit high pyrethroid resistance (RR >100-fold), but lack cross-resistance to etofenprox, a non-ester pyrethroid.

While no target site mutations were detected in the voltage-gated sodium channel, known as the pyrethroid target-site, the involvement of detoxification enzymes in the resistant strains was reflected by upregulations of carboxylesterase and cytochrome P450s monooxygenase (P450) activities. A deltamethrin metabolism study revealed significantly higher levels of 4’OH deltamethrin and 3-phenoxybenzoic acid in the resistant strains. The possible involvement of esterase and P450s in pyrethroid resistance was supported by RNAseq and RT-qPCR, that pictured high overexpression of CYP6CY12( >35-fold) and FE4_6921( >8-fold) in pyrethroid resistant strains compared to SUS-1. Functional evidence, obtained by recombinantly expressed CYP6CY12 gene, resulted in deltamethrin depletion and the formation of the major metabolite 4’OH deltamethrin. 

To conclude, we investigated the first validated case of pyrethroid field-resistance in pea aphids and identified overexpressed P450 and esterase genes mediating high metabolic pyrethroid resistance. The data presented will be important for future resistance monitoring studies