Current situation and forecasting of resistance evolution to lambda‐cyhalothrin in S panish medfly populations

BACKGROUND: The control of the Mediterranean fruit fly Ceratitis capitata (Wiedemann) in Spanish field populations mainly relies on the insecticides lambda-cyhalothrin and spinosad as bait sprays. However, their sustainable used is compromised by the development of lambda-cyhalothrin resistance and the detection of spinosad resistant alleles. In addition, the use of lure-and-kill traps covered with deltamethrin has increased in the last years. It is thus urgent to predict the impact that the combination of both pyrethroids will have in the evolution of lambda-cyhalothrin resistance and how they could be combined with spinosad so as to establish proper resistance management programs. RESULTS: Toxicity bioassays were performed to analyze the current levels of lambda-cyhalothrin resistance in field populations, proving that it has remained stable in the last decade. An evolutionary model was established to explore the weight of selected parameters in the evolution of lambda-cyhalothrin resistance in C. capitata and to forecast resistance development under different resistance management scenarios. Our results highlight the importance of fitness cost and inheritance to fit the experimental results. The analyses predicted that the rotation of lambda-cyhalothrin and spinosad, when deltamethrin traps are also deployed in the field, will slow down the evolution of resistance, especially when cross-resistance between both pyrethroids is considered. CONCLUSION: Lambda-cyhalothrin resistance has not increased in the last decade, probably due to the alternation of this insecticide with spinosad. Our modelling results indicate that the best option to avoid an increase in lambda-cyhalothrin resistant alleles, considering that deltamethrin use is growing, would be to continue combining their use with spinosad. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.