Monitoring the molecular mechanisms of insecticide resistance in Spodoptera frugiperda populations from Thailand

Spodoptera frugiperda is a major crop pest that invaded Thailand in 2018 which cause significant damage, particularly to maize. In recent years, a loss of efficacy of certain insecticides has been observed, suggesting the emergence of resistance. The aim of our study was to investigate the molecular mechanisms of resistance in S. frugiperda populations collected in several regions in Thailand. Analysis of the presence of mutations in insecticide targets revealed three mutations (M944L, F1005C and I1011M) in the voltage-dependent sodium channel, and the F290V mutation in acetylcholinesterase (AChE). These mutations can confer resistance to pyrethroids, organophosphates and carbamates, respectively. The impact of the F290V as well as two other AChE mutations have been functionally validated by inhibition studies with recombinantly expressed AChE-1 variants. The potential involvement of metabolic resistance was also investigated by monitoring the expression level of several cytochrome P450 genes. Most of the CYP9A subfamily P450s were overexpressed, regardless of the geographical origin of the populations, a worrying situation given the demonstrated ability of these P450s to metabolize insecticides belonging to different chemical classes including pyrethroids. Transcription factors such as CncC and Maf were also overexpressed in these field populations. All these results should be taken into consideration in order to propose effective and sustainable control of this pest in Thailand. Our results also demonstrated the need to develop monitoring strategies that take into account the two main resistance mechanisms, target modification and metabolic detoxification.