Spinosad resistance, esterase isoenzymes and temporal synergism in Frankliniella occidentalis (Pergande) in Australia

Temporal synergism with delayed release spinosad may provide a new technology to control spinosad-resistant Frankliniella occidentalis in the field. Here we show temporal synergism with PBO can improve control of resistant F. occidentalis (Oasis strain) with potential for increased efficacy with further formulation optimisation. [Display omitted] •Frankliniella occidentalis (WFT) is renowned for developing resistance to insecticides.•Spinosad-resistant WFT had ∼3-fold the esterase activity of susceptibles.•There was significant esterase binding to spinosad and PBO in spinosad-resistant WFT but not in susceptibles.•A 4h delayed release spinosad formulation with immediate PBO reduced WFT spinosad-resistance from 577 to 72-fold.•The spinosad formulation requires optimisation but has field potential to control spinosad resistant F. occidentalis. Spinosad has been widely used in Australia to control western flower thrips Frankliniella occidentalis (Pergande) but spinosad usefulness is now compromised by resistance. Here we studied a highly spinosad resistant strain of F. occidentalis to explore if esterases had a role in spinosad resistance. Enhanced esterase activity in pressured spinosad-resistant F. occidentalis was confirmed via PAGE electrophoresis and estimated to be approximately three times higher than that in a susceptible strain. Spinosad–esterase inhibition data in the resistant strain, showed a concentration effect with significant esterase–spinosad binding occurring at spinosad concentrations from 6.2× 10−7 to 1.5× 10−5M. Similarly, a spinosad–piperonyl butoxide (PBO) inhibition curve showed a concentration effect, with significant esterase–PBO binding occurring in the resistant strain at PBO concentrations between 3.3× 10−5M and 8.4× 10−4M. No binding of esterase to spinosad or PBO occurred in the susceptible strain. Results of bioassays in which spinosad resistant F. occidentalis were sprayed with a 4h delayed release formulation of cyclodextrin-complexed spinosad with immediately available PBO demonstrated that spinosad resistance was significantly reduced from 577 to 72-fold. With further development the PBO synergism of spinosad using a delayed release formulation, similar to that used here, may provide effective control for spinosad resistant F. occidentalis. Temporal synergism of spinosad may prove to be effective tactic for the control of spinosad resistant F. occidentalis where the main resistance mechanism involved has been confirmed to b