Target-site resistance to pyrethroid insecticides in German populations of the cabbage stem flea beetle, Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae).

Cabbage stem flea beetle, Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae) is a major pest of winter oilseed rape in several European countries particularly attacking young emerging plants in autumn. Over the last several decades, pyrethroid insecticides have been foliarly applied to control flea beetle outbreaks. Recent control failures in northern Germany suggested pyrethroid resistance development in cabbage stem flea beetles, which were confirmed by resistance monitoring bioassays using lambda-cyhalothrin in an adult vial test. The purpose of this study was to investigate the presence of polymorphisms in the para-type voltage-gated sodium channel gene of P. chrysocephala known to be involved in knock-down resistance (kdr). By using a degenerate primer approach we PCR amplified part of the para-type sodium channel gene and identified in resistant flea beetles a single nucleotide polymorphism resulting in an L1014F (kdr) mutation within domain IIS6 of the channel protein, known as one of the chief pyrethroid target-site resistance mechanisms in several other pest insects. Twenty populations including four archived museum samples collected between 1945 and 1958 were analyzed using a newly developed pyrosequencing diagnostic assay. The assay revealed a kdr allele frequency of 90-100% in those flea beetle populations expressing high-level cross-resistance in discriminating dose bioassays against different pyrethroids such as lambda-cyhalothrin, tau-fluvalinate, etofenprox and bifenthrin. The presence of target-site resistance to pyrethroids in cabbage stem flea beetle is extremely worrying considering the lack of effective alternative modes of action to control this pest in Germany and other European countries, and is likely to result in major control problems once it expands to other geographies. The striking fact that cabbage stem flea beetle is next to pollen beetle, Meligethes aeneus the second coleopteran pest in European winter oilseed rape resisting pyrethroid treatments by expressing a target-site mutation, underpins the importance of diversity in available chemistry for resistance management tactics based on mode of action rotation in order to guarantee sustainable winter oilseed rape cultivation in Europe.