Incidence, Spread and Mechanisms of Pyrethroid Resistance in European Populations of the Cabbage Stem Flea Beetle, <i>Psylliodes chrysocephala</i> L. (Coleoptera: Chrysomelidae)

<div><p>Background</p><p>Cabbage stem flea beetle (CSFB), <i>Psylliodes chrysocephala</i> L. (Coleoptera: Chrysomelidae) is a major early season pest of oilseed rape throughout Europe. Pyrethroids have been used for controlling this pest by foliar application, but in recent years control failures have occurred, particularly in Germany due to the evolution of knock-down resistance (kdr). The purpose of this study was to investigate the incidence and spread of pyrethroid resistance in CSFB collected in Germany, Denmark and the United Kingdom during 2014. The level of pyrethroid resistance was measured in adult vial tests and linked to the presence of <i>kdr</i> genotypes.</p><p>Results</p><p>Although <i>kdr</i> (L1014F) genotypes are present in all three countries, marked differences in pyrethroid efficacy were found in adult vial tests. Whereas Danish CSFB samples were in general susceptible to recommended label rates, those collected in the UK mostly resist such rates to some extent. Moderately resistant and susceptible samples were found in Germany. Interestingly, some of the resistant samples from the UK did not carry the <i>kdr</i> allele, which is in contrast to German CSFB. Pre-treatment with PBO, prior to exposure to λ-cyhalothrin suggested involvement of metabolic resistance in UK samples.</p><p>Conclusion</p><p>Danish samples were mostly susceptible with very low resistance ratios, while most other samples showed reduced sensitivity in varying degrees. Likewise, there was a clear difference in the presence of the <i>kdr</i> mutation between the three countries. In the UK, the presence of <i>kdr</i> genotypes did not always correlate well with resistant phenotypes. This appears to be primarily conferred by a yet undisclosed, metabolic-based mechanism. Nevertheless our survey disclosed an alarming trend concerning the incidence and spread of CSFB resistance to pyrethroids, which is likely to have negative impacts on oilseed production in affected regions due to the lack of alternative modes of action for resistance management purposes.</p>