Stability and potential mechanisms of fenpropathrin resistance in Asian citrus psyllid Diaphorina citri Kuwayama

Asian citrus psyllid, Diaphorina citri Kuwayama, populations exhibit varying levels of resistance to several insecticide classes, including pyrethroids. To investigate mechanisms underlying fenpropathrin resistance, we conducted artificial laboratory selection using insects collected from commercial citrus in Florida. After ten generations of selection, the LC₅₀ for fenpropathrin increased the associated resistance ratio increased by 96.67-fold. Then, we investigated stability and potential mechanisms underlying fenpropathrin resistance among crosses of D. citri populations with varying degrees of pyrethroid resistance. The stability of resistance was assessed with bimonthly toxicity bioassays in which five populations of D. citri were established with varying initial resistance (RR) to fenpropathrin ranging from fully susceptible (SS), cross (SR) to fully resistant (RR). We also assessed the expression of Voltage-Gated Sodium Channel (VGSC) gene levels in populations from crosses of field selected and laboratory susceptible (RS) D. citri populations after eight months without insecticide selection. There were no consistent changes in dosage-mortality responses of the susceptible strain after eight months without insecticide selection pressure. Fenpropathrin resistance declined the most in the cross population. Real-time quantitative PCR analysis using the laboratory susceptible population as a baseline revealed that levels of VGSC gene expression were significantly higher in the RR fully resistant populations after eight months of recovery than the other populations tested. The results suggest that fenpropathrin resistance is likely unstable under field conditions, when interbreeding with susceptible individuals is possible. Furthermore, target-site insensitivity may be one contributing mechanism to fenpropathrin resistance in D. citri.