Benzoylurea selection in Plutella xylostella confers strong resistance and fitness costs linked to a mutation in chitinsynthase 1 (CHS1)

The lepidopteran species Plutella xylostella (diamondback moth) is one of the most destructive cosmopolitan pests of Brassicaceae. In many regions it has developed resistance against almost all classes of insecticides frequently applied to keep diamondback moth populations under economic damage thresholds. The latest chemical class of insecticides introduced for P. xylostella control is the diamides and only a few years after launch, high levels of resistance in a Philippine (Sudlon) strain of diamondback moth could be functionally linked to a ryanodine receptor (RyR) G4946E target-site mutation. Interestingly our cross-resistance studies revealed that this strain shows low, but consistent resistance to benzoylureas, a class of insecticides inhibiting chitin biosynthesis and discontinued more than two decades ago due to high resistance levels in many regions. Here, we report about the selection of the Philippine diamondback moth Sudlon strain with diflubenzuron resulting in high benzoylurea cross-resistance levels linked to a gradual homozygous fixation of a nonsynonymous mutation in chitinsynthase 1 (CHS1) over time. After several selection cycles this mutation could be fixed in addition to the G4946E RyR mutation conferring diamide resistance. We investigated the genetics of benzoylurea resistance and its fitness costs by the determination of life-table parameters in a comparative analysis with the parental strain. Furthermore, we isolated RNA from P. xylostella larval stages for a transcriptomic analysis using a comparative Illumina RNAseq approach to check for additional metabolic mechanisms of resistance induced by benzoylurea selection.